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TRUSTED UNIX WORKING GROUP

(TRUSIX) RATIONALE FOR SELECTING ACCESS
CONTROL LIST FEATURES FOR THE UNIX SYSTEM

NCSC-TG₧020-A

Library No. 5-232,508

FOREWORD

The National Computer Security Center (NCSC) formed the Trusted UNIX
Working Group (TRUSIX) in 1987 to provide technical guidance to vendors
and evaluators involved in the development of Trusted Computer System
Evaluation Criteria (TCSEC) class B3 trusted UNIX* systems. The NCSC
specifically targeted the UNIX operating system for this guidance because
of its growing popularity among the government and vendor communities. By
addressing the class B3 issues, the NCSC believes that this information
will also help vendors understand how evaluation interpretations will be
made at the levels of trust below this class. TRUSIX is making no attempt
to address the entire spectrum of technical problems associated with the
development of division B systems; rather, the intent is to provide
examples of implementations of those security features discernible at the
user interface that will be acceptable at this level of trust.

TRUSIX is not intended to be a standards body. nor does it intend to
produce a de facto standard to compete against POSIX. Additionally, the
TRUSIX documents are not to be construed as supplementary requirements to
the TCSEC. The TCSEC is the only metric against which the trustworthiness
of an operating system will be evaluated.

This document, "Rationale for Selecting Access Control List (ACL) Features
for the UNIX System," is the first in a series of companion documents
being produced by TRUSIX. The guidelines described in this document
provide alternative methods for implementing ACLs in the UNIX system.

UNIX is a registered trademark of AT&T

Recommendations for revision to this guideline are encouraged and will be
reviewed periodically by the NCSC. Address all proposals for revision
through appropriate channels to:

     National Computer Security Center
     9800 Savage Road
     Fort George G. Meade. MD 20755-6000

Attention: Chief, Technical Guidelines Division

18 August 1989

     Patrick R. GALLAGHER JR
     Director
     National Computer Security Center

ACKNOWLEDGMENTS

Special recognition is extended to those members of the TRUSIX Working
Group who participated in the Access Control List Subcommittee. Members of
this subcommittee were: Craig Rubin. AT&T Bell Laboratories (Co-Chair);
Holly Traxler, National Computer Security Center (NCSC)/Institute for
Defense Analyses (IDA) (Co-Chair); Bruce Calkins. NCSC; and Casey
Schaufler9 Sun Microsystems. Recognition is also extended to the following
members of TRUSIX who provided input through discussion and comments:
Bernie Badger, Harris Corporation; Caralyn Crescenzi, NCSC; Cynthia
Irvine, Gemini Computers; Howard Israel, AT&T Bell Laboratories; Frank
Knowles. MITRE; James Menendez. NCSC; Dr. Eric Roskos, IDA; Rick
Siebenaler. NCSC, Lucy Stasiak, AT&T Bell Laboratories; Albert Tao, Gemini
Computers; Dr. Charles Testa, Infosystems Technology, Incorporated (IT!),
Mario Tinto, NCSC; Grant Wagner. NCSC; Larry Wehr, AT&T Bell Laboratories;
and Bruce D. Wilner. IT!.

Acknowledgment is also extended to the members of the POSIX P1003.6
Security Subcommittee and to those members of the computer security
community who contributed their time and expertise by actively
participating in the review of this document.

EXECUTIVE SUMMARY

The Trusted UNIX Working Group (TRUSIX) has examined the issues
surrounding implementation of access control lists (ACLs) in the UNIX
System and has identified a set of recommendations for implementors of ACL
features. These recommendations balance issues of compatibility with
existing applications, ease of use and acceptability to the end user9 and
architectural simplicity with the requirements for systems evaluated
according to the Trusted Computer System Evaluation Criteria (TCSEC). The
recommendations reflect the collected opinions and analyses of the
participating vendors, evaluators, and researchers regarding
implementation of ACL features.

The recommendations of TRUSIX with regard to ACLs are as follows:

     ₧ ACLs are required for files9 IPC objects, and UNIX system domain
     sockets.

     Access control for sockets that use name spaces other than those
     local to the UNIX system (UDP, TCP) must be addressed in the
     specification and evaluation of the system involved, and are neither
     explicitly recommended nor exempted.

     ₧ Access modes specifiable via ACLs should include read, write, and
     execute; other modes should be allowed to be added as desired, but no
     additional modes should be required to be supported.
     ₧ Each ACL entry should specify permissions for either a user or a
     group, but not both.
     ₧ Permissions granted by an ACL entry are masked by the group class
     file permission bits.
     ₧ Multiple concurrent groups should be supported. In addition, some
     method of group subsetting should be provided. It is recommended that
     this subsetting allow the user to become a member of only one group
     at login time, then to dynamically add groups to or delete groups
     from the working group set as required.
     ₧ A system-defined ordering of ACL evaluation that evaluates from
     most specific to least specific is recommended. Where multiple
     concur~nt groups are in use, and more than one matching group is
     found in the ACL, permissions granted by all matching groups should
     be ORed together.
     ₧ Modifications to mechanisms that change ownership, change the file
     permission bits, or access object attributes are not recommended.

Existing mechanisms for object access, inquiry, and deletion should not be
changed, and new parameters should not be added. Instead, new mechanisms
should be created that make use of existing ones.- The interface for
mechanisms that create objects should not be changed, except for the
possible creation of a default ACL.

     ₧ For the new mechanisms that are added to support ACL operations,
     get/set operations should be used. These operations should be
     implemented via a single system call with command arguments to
     specify the various operations. For commands at the user interface,
     the names getacl and setacl are recommended.
     ₧ Named ACLs need not be supported.
     ₧ Provision of default ACLs for file system objects is recommended,
     along with a user-specifiable mechanism for indicating whether or not
     they should be used.
     ₧ Provision of default ACLs for [PC objects is not recommended.
     ₧ Default ACLs should be provided on a per-directory basis.
     Newly-created subdirectories should inherit the default ACL of the
     parent directory.
     ₧ When a new object is created and ACL entries are attached via a
     default ACL. the file group-class permission bits are not affected
     unless an explicit mechanism is provided.

The preceding list summarizes the recommendations of the Trusted UNIX
Working Group. The main body of this document discusses the rationale for
these recommendations and gives further details of the recommendations
themselves. The appendix, the TRUSIX ACL Worked Example, gives an example
of how these recommendations might be implemented.

introduction

The intent of this document is to explore the issues involved in extending
the UNIX System discretionary access control (DAC) mechanism. DAC is a
means of controlling access to an object based on the identity of subjects
and/or groups to which they belong. The controls are discretionary in the
sense that they are chosen by the object owner.

The DAC mechanism employed in the current UNIX System was designed for
efficiency, flexibility. and ease of use. This mechanism allows and
encourages the sharing of information, but at a very coarse granularity,
via the use of file permission bits. File permission bits are associated
with three classes: owner (sometimes referred to as "user"), group. and
other. Access for each class is represented by a three-bit field allowing
for read, write, and execute-permissions.

Several methods exist for allowing discretionary access control on
objects. These methods include capabilities, profiles. access control
lists (ACLs), protection bits, and password DAC mechanisms. The intent was
to select a DAC mechanism with finer granularity than the current file
permission bits, while maximizing the compatibility with both the current
mechanism and POSIX P1003.1. Review of the

methods described in A Guide to Understanding Discretionary Access Control
in Trusted Systems(2], and of the desired outcome, point to the use of
ACLs. It should be noted that ACLs can be considered a straightforward
extension of the existing

     UNIX is a registered trademark of AT&T

UNIX system protection bits, since the protection bits may be interpreted
to be a limited form of an ACL, which always contains three entries. It
has been suggested that the fine granularity of coritrol provided by ACLs
may be simulated in UNIX systems by using the group mechanism. Groups are
lists of users which may be used to specify who may access a file. In the
worst case9 all possible combinations of users would have to be
represented in order to fully implement these lists. This corresponds to
(2'₧"M-1) groups, where M is the number of bits in the group-ID. Since the
number of possible combinations of users needed to implement this scheme
for N users is (2"N-1), the maximum number of users which could
effectively utilize such a system would be limited to the number of bits
in the group-ID. This number (often 16 or 32) is an unreasonably small
number for most UNIX systems and the management of the groups by users
would be difficult. Also, this scheme does not allow for individual users
in the lists to have different access rights. All users in the group would
be forced to have the access rights given by the file group class
permission bits. Some differences in access rights could be simulated by
using the file other class permission bits, but not with the same
functionality as provided by conventional ACLs.

The DAC features explored in this rationale are based on the DAC features
requested by customers, the class B3 DAC requirements described in the DoD
Trusted Computer Systems Evaluation Criteria [1] (TCSEC), and the DAC
mechanisms used in existing trusted systems (e.g., Multics). Based on
these inputs9 it has been determined that the current DAC mechanism in the
UNIX System is adequate for most needs and that the only enhancement
required is to allow reasonable, finer-grained control of objects. This
provides the capability to share or deny access to individually specified
users and/or groups and meets the class B3 requirements of the TCSEC.

The issues explored in this document will deal primarily with ACLs. Much
of the terminology has been adopted from the P1003.1 document and the
TCSEC; however, new terms will be defined when used. For most of the
issues identified, alternative solutions are given along with a
recommendation. Although an attempt was made to consider the' issues
independently, it should be noted that sole of the issues are actually
very dependent on each other and recommendations `made in some areas
greatly influenced later recommendations.

Goals

The primary goal in extending discretionary access control in the UNIX
system is to provide a finer graninlarity of control in specifying user
and/or group access to objects. This can be achieved through the addition
of access control lists. The following is a list of additional goals for
the extended DAC mechanism:

     ₧ The mechanism should provide compatibility with the existing
     (currently P1003.1) and emerging POSIX standards and with the current
     UNIX System DAC mechanism. In the unlikely event of a conflict
     between the current UNIX System DAC mechanism and POSIX, the POSIX
     interpretation will be used. In addition. the semantics of existing
     interfaces should be maintained.
     ₧ The following requirements for DAC in the TCSEC at class B3 should
     be fulfilled. "The TCB shall define and control access between named
     users and named objects (e.g., files and programs) in the ADP system.
     The enforcement mechanism (e .g., access control lists) shall allow
     users to specify and control sharing of those objects, and shall
     provide controls to limit propagation of access rights. The
     discretionary access control mechanism shall either by explicit user
     action or by default, provide that objects are protected from
     unauthorized access. These access controls shall be capable of
     specifying, for each named object, a list of named individuals and a
     list of groups of named individuals with their respective modes of
     access to that object. Furthermore. for each such named object, it
     shall be possible to specify a list of named individuals and a list
     of groups of named individuals for which no access to the object is
     to be given. Access permissions to an object by users not already
     possessing access permission shall only be assigned by authorized
     users."
     ₧ Reasonable vendor extensions to the DAC mechanism should not be
     precluded.

     For example, the specification of read, write and execute permissions
     should be supported. Other permissions should not be required nor
     should they be precluded as extensions.

     ₧ A minimum set of new interfaces and error codes should be provided.
     The new command interfaces provided for the user must be easy to use
     and the existing interfaces should continue to work as expected.
     ₧ Intermixing use of the existing and newly-defined DAC
     functions/commands should provide reasonable results. Security should
     be maximized by opting for more restrictive rather than less
     restrictive decisions ~h~ff₧' a choice must be made.
     ₧ When changing DAC on an object, at no time shall access be more
     permissive than either the initial or resulting access.

ACLs On Objects

A system can support several different types of objects, e.g., system
objects, public objects, named objects. System objects are entities
internal to the TCB (e.g., system data structures) not directly accessible
by the normal user and, as such, do not require discretionary access
control. Public objects are objects readable but unmodifiable to the
normal user (e.g., system clock)9 and thus also do not require
discretionary access control. Named objects are objects readable and
modifiable at the user interface (e.g., text files). The TCSEC class B3
requirement for DAC states that access control must be enforced on all
named objects in the system [1]. Although there may be some variance among
different UNIX system implementations, there are two common classes of
named objects that require ACLs. These classes are files (including
regular. directory. special. and named pipes), and named IPC objects
(including shared memory, message queues, semaphores, and sockets).

It is these classes of objects that will be protected by the discretionary
access control alternatives described later in the paper. It should be
pointed out, however, that discretionary access can not always be
completely determined solely by the file permission bits and the ACL
associated with the object. It is possible to have objects which have been
administratively configured for a specific access and thus not completely
affected by user DAC, e.g., a file system mounted read-only. There are
other instances where discretionary access of objects may be
time-dependent and thus not completely based on a current DAC setting.
Examples of this would be the inability to write a shared-text file while
it is being executed or trying to execute a file while it is open for
writing. These situations are acknowledged special cases and will not be
considered in the general discussion of determining effective
discretionary access.

ACLs On IPC Objects

IPC objects are named objects and are thus require ACLs at class B3. Note
that this does not include unnamed pipes which can only be used to connect
related processes. Although the semantics of IPC mechanisms are slightly
different from those of file system objects, a DAC scheme similar to that
used for file sky5~~~ objects should easily be adaptable to IPC objects.
For example, message queues utilize both a creator and an owner of an IPC
object and maintain creator and owner UIDs and G[Ds (cuid,uid, cgid,gid).
User access is checked against the cuid and the uid, and group access is
checked against the cgid and gid. This situation can easily be represented
with ACLs by using additional ACL entries to represent the creator U[D and
GID. Additionally, some access modes associated with file system objects,
such as execute, may not be applicable to IPC objects. This does not cause
a problem as long as the modes are a subset of those defined for file
system objects.

ACLs On Sockets

Sockets are named objects and would thus require ACLs at class 83. UNIX
system domain sockets use the file system name space for access control
decisions and currently have file permission bits associated with them.
Thus, domain sockets would also need to have ACLs associated with them.
Other types of sockets which use other name spaces (UDP. TCP) are
currently not protected with any type of access control. Since it is not
clear whether these types of sockets could currently be included in an
evaluated configuration, they will not be addressed at this time.

Additional Access Modes

Existing UNIX systems support three access modes: read, ivrite, and
execute/search. Additional access modes are conceivable, and may be
convenient to add while adding ACLs. Various possibilities include:

     ₧ read attributes of object
     ₧ write attributes of object
     ₧ append only to object
     ₧ truncate data of object
     ₧ delete object
     ₧ lock object
     ₧ restrict setuid execution of object
     ₧ restrict access of object based on time.

Note that this is not an' all-inclusive list.

In this and subsequent sections, alternative implementations of a given
topic are examined, followed by the TRUSIX recommendation.

Require Additional Access Modes

In this approach to handling additional access modes, new access modes
would be defined and required. This limits the availability of compliant
implementations and impacts compatibility.

Prohibit Additional Access Modes

In this approach, new access modes would explicitly not be allowed. Due to
loss of flexibility, compliance with this scheme would limit
implementation.

43 Allow Additional Access Modes (With Control)

In this approach, new access modes would not be defined. Instead. the
concept of and mechanism for adding new access modes would be defined.
This allows a vendor to produce whatever additional access modes are
desired. Since the mechanism for doing so is defined there is little
chance of collisions or contradictions. The mechanisms must be defined and
agreed upon by some regulating body which allocates access bits. Note no
such body currently exists which has been tasked to allocate access bits.

Allow Additional Access Modes (Without Control)

In this approach, additional access modes are neither defined nor
precluded. This method allows a vendor to produce whatever additional
access modes are desired, but there is no mechanism provided for adding
new modes. There would be no control on the access modes vendors might
add.

Recommendation

We recommend allowing additional access modes, without control. There
should be nothing precluding the addition of new access modes if desired.
However, since there is nothing currently in the POSIX P1003.1 standard
concerning additional access modes, no new access modes or mechanisms need
be defined.

ACL Entry Type And Format

The manner in which an ACL entry refers to a user or group of users is an
important factor in the usability of an ACL mechanism. The alternatives
are to have an ACL entry contain either a user or group in an entry. or to
have an ACL entry contain both a user and group. The issue is which of the
alternatives is more suitable to a system utilizing ACLs.

5.1 User And Group Entries

A user and group entry contains a reference to both a specific user and a
specific group together as a [UID,GID] pair. The UID-specific and
GID-specific entries can be represented as special "wildcard" cases
(denoted by *) meaning any user or group will match that entry. Using this
method, an ACL entry may refer to one user in a particular group
[UID,GID], one user in any group [UID,*], any user in a particular group
[*,GID], or any user in any group [*,*] which is equivalent to the file
other class permission bits. A typical ACL utilizing entries of this type
might look like the following:

user1.projA rw-

user2.projB r-

user3.* rwx

*.projA r-

*.* ---

Implementations of protected subsystems is the only clear example that
suggests using user and group ACL entries as a pair. Using the UNIX system
setgid-on-exec feature, it is possible to build protected subsystems.
Consider the following example which makes use of this feature.

A database of tapes is maintained in /etc/tapedata. The database
administrator (DBA) of the database wishes to produce a utility to control
access to this database. To begin with, there are some rules for dealing
with the database. Some users should have read and write access, others
just read access, and still others should have no access to the database.
Readers should only see data about their own tapes. In addition, since
other database utilities have poor error handling, all updates to the
database need to be made in the correct format.

The DBA has written a utility named tapedb which can read and update the
database. /etc/tapedata and tapedb both have the group tape associated
with them, and tapedb has the set-group-id bit on. The DBA has also
created an ACL for letc/tapedata which contains the following entries:

userl.tape r-

user2.tape r-

user3.tape rv-

user4.tape rv-

*.*

All users named in the ACL (in group tape) may read the database Only
user3 and user4 (in group tape) may update the database. If the only way
for a user to be a member of group tape is by executing tapedb, then the
DBA is satisfied that letc/tapedata is adequately protected.

While this example suggests a useful application of user and group ACL
entries, there are other ways to implement the example which do not
require this ACL entry type functionality. As described in the following
section, the same effect can be achieved through ACLs containing user or
group entries.`

Additionally, identification by a user and group pair is not used in a
UNIX System. In some systems, a user is identified by a userID,group~ID
pair. In Multics, for example, a user is identified by a user-ID,
project-ID pair. where a project-ID is equivalent to a group-ID on the
UNIX system. Userl in projA. on a Multics system, is distinct from userl
in projB. Since Multics users do not have the capability to change groups9
the only way for a user to be identified with another project would be to
log in with another group-ID. In UNIX systems, however. a user is really
only identified by the user-ID. Also, a user can easily change group-ID
through the neivgrp command or be associated with several groups at the
same time if using a system with multiple groups. Thus, controlling access
for a user while in a specific group is not as useful in a UNIX system.

User Or Group Entries

A user or group entry contains a reference to either a specific user or a
specific group, but only one at a time. Consider the following example,
where u indicates the user class, g indicates the group class, and o
indicates the other class:

u:userl rw-

u:user2 r-

u:user3 rw-

u:user4 rw-

g:projA r-

g:projs rw-

o: rw-

To address the protected subsystem implementation, consider again the tape
database example described in the previous section. Rather than
controlling access to the data, access can be controlled on two
subprograms; one which reads data, the other which updates data. The ACL
on the database, /etc/tapedata would be:

g:tapereaders r-

g:tapewriters rw-

o:

The user interface for access to the database is tapedb. The program
tapedb is not setgid, however, it invokes two other programs, tapedbread
and tapedb\vrite, that are setgid. Only users allowed to read the database
have execute permission on topedbread, while only those allowed to update
the database may execute tapedb_write. The ACL on tapedbread would be:

u:userl --x

u:user2 --x

u:user3 --x

u:user4 --x

o:

The ACL on tapedb_write would be:

u:user3 --x

u:user4 --x

o:

The program tapedb_read runs setgid to the group tapereaders, and the
program taped_vrite runs setgid to the group tapewriters.

Thus, the same protected subsystem can be provided through ACLs of type
user or group.

The main advantage of this scheme is that it provides more clarity for the
user. This is considered to be a very important advantage as a user's
understanding of such a mechanism is essential in promoting its correct
usage. Additionally, this scheme removes the need for wildcard specifiers,
thus eliminating the potential problems of picking an unused character as
a specifier.

Recommendation

User or group entries in ACLs are recommended. Since there is no clear
need for the user-group paired entry scheme and there are several
advantages to the user or group scheme, the user or group scheme is the
preferred alternative. Examples were examined which claimed to require the
use of user-group paired entries. One such example deals with protected
subsystems as described above. Protected subsystems, a useful and
important feature in a trusted system, can be implemented through other
means not requiring user-group paired entries. We have "found that this is
a limited class of applications and may be implemented with the user or
group scheme with minimal effort. For UNIX systems with multiple groups,
the user and group scheme becomes more difficult when determining access.
Additionally, the user or group scheme follows the idea in UNIX systems
that a user is only identified by user-ID and gives no special meaning to
what a user can do while only in a certain group. Finally, although
simplicity is a very subjective measure, in comparing the two alternatives
the advantage of simplicity outweighs the ability to specify both a user
and a group in a single entry.

Relationship Of ACL And File Permission Bits

ACLs expand upon the discretionary access control facility which is
already provided by the file permission bits. Although file permission
bits do not meet the TCSEC class B3 requirement for DAC, they are
sufficient for many uses and are the only mechanism available to existing
applications. Existing applications that are security-conscious use file
permission bits to control access. The relationship between the ACL and
the file permission bits is important to existing programs in order to
maintain compatibility. For example, use of chmod("object" 0) should
continue to work, denying subsequent opens to an object. The following
sections discuss possible approaches to handling the interaction of ACLs
with file permission bits. Any references to default ACLs will be fully
described in the Default ACLs section.

ACL Always Replaces File Permission Bits (Pure ACL)

In this approach, the file permission bits are no longer consulted for DAC
decisions.

Instead, each object always has an ACL and the ACL completely determines
access. Consider the following example illustrating this scheme. Assume
Userl and User2 are members of the group "GroupA" and User3 and User4 are
not.

file Owner/Group User2/GroupA

file permission bits: rwxr-x-x

ACL Entries:

User1 rwx

User2 r-

User3 rwx

User4 ---

In this example the file permission bits would have no effect on the
access control decision. User3 is able to read, write and execute the
file. User2 is able to read it, but not to execute or write to the file.
The file permission bits are completely ignored.

The resulting pure ACL system does not have to worry about interactions
between the ACL and the file permission bits, since the latter are not
used for access control decisions. A single, well defined access policy is
employed. Applications which should make use of DAC are forced to
understand the new rules. The major disadvantage of this scheme, however,
is that compatibility is lost. Every DAC cognizant progr&m, and that
should be every program that manipulates the discretionary access control
information on an object needs to be changed to understand ACLs.

Owner Selects ACL Or File Permission Bits

In this approach, either the file permission bits or the ACL are consulted
for the access control decision on a per object basis. The owner
determines whether the file permission bits or the ACL is used. The system
call chmod returns an indicative error if the object has an ACL, but
otherwise sets the file permission bits. Consider the two following
examples which illustrate this approach. Once again assume Userl and User2
are members of the group "GroupA" and User3 and User4 are not.

Example A (ACL selected):

file Owner/Group User2/GroupA

file Permission bits: rwxr-x-x

ACL Entries:

Userl rwx

User2 r-

User3 rvx

User4

Since there is an ACL on this file the access control is the same as in
the previous example.

Example B (file permission bits selected):

file Owner/Group User2/GroupA

file permission bits: rwxr-x-X

ACL Entries: NONE

Since there are no ACL entries on this file the access control ~r
determined by the permission bits. User2 (owner) has all access
permissions to the file. Userl (a user in GroupA) is allowed read and
execute access. User3 and User4 ("other" users) can only execute the file.

The resulting system behaves like a file permission bit based system if no
one ever sets ACLs and like the pure ACL system if a default ACL mechanism
is in use. Thus, either environment can be supported. The compatibility
issues raised in the previous section apply here as well. In addition, the
programs have to determine which access control mechanism applies to each
object created and set the DAC accordingly.

Independent ACL And File Permission Bits (AND)

In this approach, both the file permission bits and the ACL are consulted
for the discretionary access control decision on a per object basis.
Access is granted if and only if it is granted by both the ACL and the
file permission bits. Consider the following example9 which illustrates
this approach. For this example9 assume only User2 is in GroupA.

file Owner/Group User2/GroupA

file permission bits: rwxr-x-x

ACL Entries:

Userl rwx

User2 r-

User3 Tx

User4

In the example above, the file permission bits imply that Userl has
execute permission, whereas the permissions specified in the ACL imply
that Userl has full access. Without knowing which group User 1 is in, one
cannot predict whether or not Userl can read the file. If Userl is in
group GroupA, then Userl will have read and execute permissions. If Userl
is not in group GroupA, then only execute permission will be granted.
Similarly, without knowing User3"s group, one cannot predict whether or
not User3 has read access. User4 will have no possibility of access, due
to no permissions specified in the ACL entry. As the example illustrates,
there is no way to get a full ACL view with this scheme.

With this scheme, some compatibility is maintained. Calls to chmod have
the desired effect from the restrictive point of view. ACL entries can
further restrict access. Making use of the ACL as the effective access
control mechanism requires that the file permission bits be set wide-open
(i.e., read, write, and execute bits are set for user, group and other).
In situations where ACLs are not properly set, a new object will become
generally accessible. Likewise, if the ACL is removed then the object will
again be generally accessible. This scheme also allows for misleading
status information given to programs which only use the existing
mechanism.

Independent ACL And File Permission Bits (OR)

In this approach, both the file permission bits and the ACL are consulted
for the discretionary access control decision on a per object basis.
Access is granted if it is granted by either the ACL or the file
permission bits. The ACL is used to grant access beyond what is set in the
file permission bits.

Consider the following example illustrating this approach. Assume only
User2 is in GroupA.

file Owner/Group vser2/GroupA

file permission bits: rwxr-x-X

ACL EntrieS:

insert rwx

inser2 r-

inser3 rwx

User4

Userl, User2, and User3 have read, write, and execute access. User4 has
execute access.

Again, some compatibility is maintained. Calls to chmod have the desired
effect from the permissive point of view. The previous alternative's
problem of leaving the permission bits wide-open is thus avoided.

The problem with this scheme, however, is that a chmod call which would
deny all access (chmod("object", 0)) in a system without ACLs will not do
so here.

File Permission Bits Contained Within ACL

In this approach, only the ACL is consulted for discretionary access
control decisions. The file permission bits are replaced by three "base"
entries in the ACL. Calls to chmod modify the owner, group, and other
entries contained in the ACL.

Calls to stat read this information from the ACL.

In the following two examples assume the owner entry is evaluatefl before
additional user entries, and the group entry is evaluated before
additional gr1oup entries.

Example A:

file Owner/Group inser2/GroupA

file permission bits: rwxr-x-x

ACL Entries:

owner rwx

User1 rvx

User2 r-

User3 r-x

     User4

group r-x

other --x

In this example. it is not clear what permissions User2 is to be granted,
since a particular method for determining owner access has not been
specified for the case where an additional user entry also names the
owner. User2 could be granted read, write, and execute access as the
owner. read access only, as per the explicit entry for User2, or some
combination of the two (e.g., the AND or OR of the two). Userl, User3, and
User4 get their access from their ACL entries.

Example B: (After a chmod("object", 0))

     file Owner/Group inser2/GroupA
     file permission bits:

ACL Entries:

owner

User1 rwx

User2 r-

User3 r-x

     User4
     group -
     other

Changing the file permission bits to zero does not change the permissions
granted to Userl, User3, and User4, since their access is based on ACL
entries. User2's access may change depending on how owner access is
determined when additional user entries naming the owner also exist.

If no additional entries are added to the ACLs, this system looks like a
system without ACLs. The literal meaning of the file permission bits is
preserved in the ACL.

     ₧ As in the previous alternative, however, a chmod call which would
     deny all access (cbmod("object", 0)) in a system without ACLs will
     not do so here.

ACL Masked By File Permission/ Bits

In this approach, both the file permission bits and the ACL are used for
determining the discretionary access control decision. The access
indicated in the ACL entry is logically ANDed (masked) with one or more of
the file permission bit classes (file owner, file group, or file other
class) to determine the effective DAC permission.

Example:

file Owner/Group User2/GroupA

file permission bits: rwxr-x-x

ACL Entries:

User1 rwx

User2 r-

User3 rx

User4 ---

Assume that the group file permission bits are chosen as the mask, i.e.,
all ACL entries will be ANDed against the file group class permission
bits. User2, being the owner, gets read, write, and execute access to the
file. User3 is allowed read and execute access. Userl is allowed read and
execute access, the write access is disallowed by the file permission
bits. User4 is not allowed any access to the file. Calls to chmod have the
desired effect from the restrictive point of view but not necessarily from
the permissive point of view. Since the bits of the masked field will most
likely be set wide-open, the literal meaning of the field chosen for the
mask appears to be lost. The POSIX standard, however, allows for the
extended meaning of the group class permission. bits.

Recommendation

We recommend the AcL Masked By File Permission Bits approach. This is the
most reasonable approach when trying to balance security and
compatibility. The question of designating the masking field must still be
resolved. The file group class permission bits are the preferred masking
field, even though they encourage permissive default access by the owning
group. This choice must be made because the use of the file owner class
would cause compatibility problems in programs which attempt to establish
"owner-only" access, whereas the designation of the file other class could
leave objects open to attack were an ACL removed or never present. An
additional option of masking user entries with the file owner class
permission bits and group entries with the file group class permission
bits has the same disadvantages as masking against only the file owner
class. When masking against the file group class, the permissions indicate
the least upper bound of the permissions allowed for the ACL entries and
the user and other fields retain their previous semantics.

To summarize the approaches identified in this section:

The ACL Masked By File Permission Bits approach is a compromise for both
security and compatibility.

The Independent ACL And File Permission Bits (AND) approach suffers from
the serious flaw that the file permission bits must be set very
permissively in order to allow the ACL entries to predominate in the
discretionary access calculation. A simple mistake in setting the ACL
could grant object access to significantly more users than was intended.

The Independent ACL And File Permission Bits (OR) approach may require
that both ACL and the file permission bits be changed in order to deny a
particular access. Thus, existing programs could believe that they had
prevented access when they, in fact, had not. Similarly, in the File
Permission Bits Contained Within ACL approach, removing "other" permission
might not have the desired effect9 since, the owner, group, and other
entries may not be the only ones in the ACL. In neither case does a call
to chmod with a zero argument unequivocally revoke access from all users
as might be expected.

Whichever DAC scheme is ultimately selected, an appropriate balance must
be struck between the mutually conflicting concerns of compatibility and
security. In a DAC scheme where chmod cooperates with ACLs, chmod must not
grant inappropriate access or require unreasonable (i.e., permissive
public access) defaults.

Barring compatibility, the alternatives of ACLs replacing file permission
bits (Pure ACLs and On Demand) would be the most elegant way of enhancing
DAC for UNIX systems. By abandoning file permission bits, however, these
schemes have been rendered incompatible with existing systems. Thus, they
are not considered for a POSIX-compliant UNIX system DAC scheme.

Group Semantics

There are various ways of using the UNIX system group mechanism when
grouping system users. In designing ACLs it is important to understand the
possible semantics and provide enough flexibility to properly support
these semantics. Initially, there are no restrictions on how users can be
grouped. Various possibilities include:

     ₧ a shorthand way of referring to groups of subjects
     ₧ a method of grouping project work by group access rights
     ₧ privileged roles
     ₧ accountability (file ownership)

The issue arises. however, of how to deal with user membership when
considering these possible grouping mechanisms. For example, should a user
be permitted to be a member of more than one group at any given time? If
so. should there be a mechanism provided to allow the user to control
group membership? These issues will be addressed in the following
sections.

Single Group Membership

Under a single membership scheme, a user can only be a member of one
specific group at any given time. All discretionary access checks will be
made with respect to the user's OlD and a single GID. A user will only be
able to change his/her group through the use of the newgrp command. This
scheme is easy to implement and introduces no additional complexity with
respect to evaluating access within an ACL. Additionally. it would
certainly be acceptable in a class B3 system.

Multiple Concurrent Group Membership

Under a multiple concurrent group scheme, a user can be a member of more
than one group at the same time. This scheme introduces some complexity
when evaluating user access by allowing more than one ACL entry of equal
specificity to apply to a user simultaneously. For example, if a user is a
member of several groups at the same time and tries to access an object
with an ACL containing entries which match the user on more than one
group, what will the resulting access be? There are several ways of
determining the resulting access in such a case. These are discussed under
ACL Evaluation.

Another concern with the use of multiple concurrent groups is the
possibility of violating the least privilege principle. With multiple
concurrent groups if a user is in several groups at once, he/she is
granted access to all of those groups at all times rather than to just the
ones he/she needs at any given time. This could be contrary to the idea of
a user having a minimal set of privileges necessary to perform a
particular function at any given time.

It can be argued, however, that the least privilege requirement in the
TCSEC only applies to TCB architecture, making this issue irrelevant for
DAC. On the other hand there may be a problem with a system which
implements privileged roles through the group mechanism. The TCSEC class
B3 Trusted Facility Management requirement states that separate roles must
be assigned to operator and adinistrator functions and that each role be
restricted to performing only those functions necessary for that role.
Given a system, therefore, which uses the group mechanism to assign roles
and grant access basedon role identity to parts of the system which would
otherwise be inaccessible, it is clear that least privilege could be
violated through the use of multiple concurrent groups. The violation
would occur if the user who was a member of the group assigned to a
privileged role could also be a member of one or more additional groups.
Proper administration of these privileged groups, however. could still
allow for the use of multiple groups. but a subsetting capability, as
described in the next subsection9 would then be required.

Improperly controlled multiple concurrent groups with groups representing
privileged roles could therefore be a violation of the least privilege
principle. This would result in a failure to meet the class B3
requirements. This is only one specific implementation, however, and it is
certainly conceivable that multiple concurrent groups could be implemented
in such a way as to not be a violation of least privilege. The multiple
concurrent group scheme is currently a feature in some UNIX systems and is
thought to be an extremely useful and necessary feature to those who use
it.

Multiple concurrent groups would also be compatible with the POSIX
standard

Multiple Concurrent Groups With Subsetting

Another problem associated with multiple concurrent groups arises from the
fact that currently when a user logs on to a system he/she automatically
becomes a member of all of the groups that he/she is allowed membership
in. There is no way for the user to only be active in a subset of his/her
possible group set. Although there is no explicit requirement in the TCSEC
precluding this, the TCSEC does seem to imply that a user should by
default have a minimal amount of access rights atlogin.

There are several ways of approaching this problem; any of these methods
would be a possible and acceptable means of resolving this problem. First,
it is necessary to consider whether a user should be able to add or delete
groups from his/her group set and if so, with what restrictions. A user
should certainly not be allowed to add groups for which he/she is not
authorized. Therefore each user should have an "allowable group set" which
consists of all groups that user has been given authorization to be a
member of. Adding groups other than those which appear in this allowable
group set would be unacceptable.

There are at least two ways to allow a user to work with a subset of
his/her allowable group set. The first would be to keep the current scheme
where a user becomes a member of all of his/her groups at login, but
provide the user with a means (through some system call or command) to
drop specific groups if desired and work as a member of some subset of
his/her allowable group set. A command would allow a user the capability
but require an explicit action to do so. A system call, on the other hand,
would provide the means for restriction through a program which could be
set up to run automatically for the user. This would mean, however, that
the set of groups would either be hardcoded into the program or be set
through some type of configuration file. Another possible approach would
be to provide a mechanism that would cause a program's groups to be
restricted when that program is executed. Although this eliminates the
user having to remember to restrict his/her groups or having to hardcode a
group set into a program, it would add further complexity to the system.

Recommendation

We recommend that the multiple concurrent group capability be provided
along with some method of subsetting. The preferred method would be to
only allow the user to become a member of one group at login and provide
him/her with a means of dynamically adding/deleting to his/her working
group set. This recommendation, of course, may conflict with
implementations which use the group mechanism for privilege roles.

ACL Evaluation

This issue deals with how an ACL is evaluated to determine access rights
of a subject to a particular object. There are several possible ordering
methods for ACL evaluation, as well as several different ways to evaluate
multiple group entries. Two levels of ordering must actually be considered
when deriving an ACL evaluation scheme; the ordering of the classes (user,
group, other), and then the ordering of the entries within each class.

Ordering Of Classes

It would certainly be possible to specify an ordering of any combination
of the the three classes, user, group, and other. However, since both the
POSIX standard and all current UNIX systems specify a "user, then group,
then other" ordering, (or most-to-least specific), when evaluating access
with permissiori bits, this ordering should be maintained for ACLs as
well.

The method of evaluating an ACL in a most-to-least specific manner can be
described as follows. The owner identity of the object is first checked
against the effective identity of the subject. If there is a match the
search stops. Next, a check is made against the owning group identity of
the object and the effective group of the subject. If there is a match and
the subject does not have multiple groups, the search stops. Otherwise the
rest of the group entries are searched next. If the subject has multiple
groups, the group entries are evaluated as presented in the Multiple Group
Evaluation section, otherwise they are searched in order as the user
entries are. Finally, if no user or group entries were found to match the
effective identity of the subject, access is determined based on the other
entry.

For the following discussion on the ordering of ACL entries9 it will be
assumed that the classes will be ordered and follow this most-to-least
specific regime.

User-Defined Ordering

In this method, entries are considered according to the ordering given by
the user. The first entry as specified by the user is considered first,
the second entry next, and soon.

As long as the "user, then group, then other" order is followed, the only
security relevant problem with this method occurs when evaluating group
entries with multiple groups. If a user is a member of multiple groups and
matches more than one of the group entries, the resulting access may be
dependent upon the ordering of the group entries. See the Multiple Group
Evaluation section for various possibilities.

Unless all matching group entries are considered when determining access,
the burden is placed on the user to correctly order the group entries.
This method may appear to be more convenient for users, however, it may
require the user to have extensive knowledge of group membership.
Additionally, it does not allow for very efficient access evaluation as
discussed in the following section.

System-Defined Ordering

In this method, entries are considered according to a system-defined
ordering. Although the user does not have the flexibility of choosing an
arbitrary order of entries, a system-defined ordering gives consistency to
ACLs throughout the system and may also allow for quicker access
determination.

The system may use any of a variety of ordering methods, two of which are
alphabetical ordering by user or group name and numeric ordering by user
or group ID. An ordering of lowest to highest UID or GID, or vice-versa,
is recommended as it provides an efficient way to check for redundant
entries. Redundant entries should not be allowed in an ACL.

It is important to mention that actual sorting need not be done by the
kernel itself as long as the kernel enforces the specified ordering. In
other words, the sorting can be achieved through the use of library
routines. The ACL commands would automatically use the library sorting
routines and users would also be encouraged to do so when writing their
own programs which manipulate ACLs. When an ACL is passed to the kernel,
the kernel verifies that the entries are sorted or else a failure will
occur. In this manner, efficiency is achieved while still enforcing a
system-defined ordering.

This alternative is simple. reduces the possibility of user error, and
allows for more efficient access determination.

Multiple Group Evaluation

When a subject is a member of multiple groups, there are several ways the
group entries may be evaluated, regardless of the ordering of the entries.
The following methods may be used to evaluate access when multiple groups
are used:

The first entry which matches one of the subject's groups might be used to
determine access. While this is an efficient method, it does not take
notice of the possibility of other groups granting access.

The entry which matches one of the subject's groups and grants the least
access might be used. This method does not recognize the possibility that
all the groups together might grant or deny the desired access.

The entry which matches one of the subject's groups and grants the most
access might be used. This method also does not recognize the possibility
that all the groups together might grant or deny access.

ANDing the permissions of all the entries which match groups of the
subject is another possible method. This approach may be considered too
restrictive, since even one entry which grants access may be overruled by
other entries which deny access.

ORing the permissions of all the entries which match groups of the subject
is also a possibility. This method may be considered too permissive, since
the maximum permissions allowed by all the matching entries taken together
is the result.

However, the same effect can be achieved currently, through the user
simply invoking the newgrp command to change to the group with the
d1esired access or by opening the same file twice from two different
groups which together provide the desired access.

Recommendation

A system-defined ordering which evaluates ACLs entries from most-to-least
specific is recommended. Since multiple groups were designed to be
permissive and permissive results can be achieved through other means
anyway, the method which ORs the permissions of all matching group entries
is recommended for systems implementing multiple groups.

Concern has been expressed that this scheme violates the wording in the
TCSEC, for DAC at class B3. The TCSEC states: Furthermore, for each such
named object, it shall be possible to specify a list of named individuals
and a list of groups of named individuals for tvhich no access to the
object is to be given. The ORing of groups, however, does not present a
conflict with the class B3 DAC requirement, as it still allows the user to
specify groups that shall have no access.

DAC Compatibility

Designing an ACL mechanism requires that attention be given to the use of
system calls which check or modify the existing DAC mechanisms, and to the
additional use of ACL mechanisms in system calls. The classes of DAC
mechanisms which return or change the value of the discretionary access
control information are those mechanisms which: change ownership of an
object9 change the file permission bits. create objects, access object
attributes. and access object data. Each of these classes will now be
examined and a determination will be made of what changes, if any9 are
required for inclusion in a system with ACLs. For each class, we provide
alternative solutions and identify the preferred choice.

Changing Ownership Of An Object

Mechanisms which change ownership of an object (e.g., chown, msgctl,
semctl, shmctl) could create a new user or group entry for the object
owner or group, with the same access permissions as the original entry for
the object owner or group. The original entry would become an additional
user or group entry. The problem with this alternative is that by leaving
the original entry for the object owner or group behind as an additional
user or group entry, the mechanism will always create an ACL for an object
which did not have one to begin with.

The preferred alternative is for these calls to suffer no additional side
effects due to the presence of ACLs. This can be achieved by not storing
explicit IDs in the owner and owning group ACL entries. An advantage of
this altern'ative is that the ACL entries for object owner and object
owning group can be readily distinguished syntactically from the other
user and group entries.

Changing The File Permission Bits

Mechanisms which change the file permission bits (e.g., chmod, msgctl,
semctl, shmctl) might be changed so that they fail, or partially fail,
when presented with an object that has an ACL.

Complete failure is a poor alternative since these mechanisms change the
file mode, not just the file permission bits. For example, a program
should be able to do a legitimate operation such as changing the setgid
bit on any file. Partial failure means that these mechanisms would make
the requested changes but return an error value different from -1. This is
a poor alternative for two reasons: it does not make good sense to succeed
while returning failure. and programs often do not differentiate between
error return values.

Other alternatives attempt to minimize surprises to the caller by changing
ACL entries. The first of these alternatives is to mask the access
permissions in all the object's additional entries. Access permissions for
entries with specific user and specific group are ANDed with the supplied
user and group access permissions. Access permissions for entries with
only a specific user are ANDed with supplied permissions for the user, and
permissions for entries with only a specific group are ANDed with supplied
permissions for the group. While this meets POSIX requirements, programs
that wish to change only the file mode (non-access) bits will have the
masking occur as an undesirable side effect. Another alternative is to
disable the additional entries. This implicitly requires a new mechanism
to enable entries that have been disabled. POSIX requirements are also
satisfied by this alternative, but the same problems exist as in the
previous alternative; programs using these mechanisms to change the
non-access file mode bits will have entries disabled as an undesirable
side effect. Still another alternative is to delete the additional
entries. This has similar advantages and disadvantages as ACL entry
disabling. It is simpler since there is no need for an ACL entry enabling
mechanism. Information given by the user, however, is deleted without
warning.

The preferred method is to make no changes to these mechanisms. The
mechanisms will affect only file permission bits and ACL entries for the
object owner or group. While this does not provide non-ACL cognizant
programs with expected results for operations on objects with ACLs, it is
not perceived as a serious problem. This alternative is consistent with
the preferred alternative for mechanisms which access object attributes as
well (see below).

Creating Objects

Mechanisms which create or truncate objects (e.g., creat, open, mkfifo,
mkdir, msgget, semget, shmget) should work as they currently do, except
that they may create an ACL as part of the default ACL mechanism. Please
refer to the section on default ACLs for more information. Note that
default protection on newly-created objects will be accomplished via the
umask and/or default ACLs.

It may also be desirable to add other types of ACL features to mechanisms.
For example, one might wish to add the capability during file creation to
adopt a specific ACL. For changes of this type, parameters of existing
mechanisms should not be changed9 and new parameters should not be added.
New mechanisms should be created which make use of existing ones. For
example. creat may need to be modified to take ACLs into account, but the
parameter list should not change. Instead of adding an ACL parameter to
creat. a new system call (i.e.. with some other name) should be used.
which takes the ACL as a parameter and then uses creat.

Accessing Object Attributes

Mechanisms which access object attributes (e.g. stat, msgctl, semctl,
shmctl) could be modified to fail when applied to an object with an ACL.
This is an unacceptable alternative since these mechanisms return more
information than simply the file mode. Thus. non-functionality would
require a new mechanism to return the additional information for objects
with ACLs.

Another alternative is to find all the entries in the ACL that apply to
the user-ID and group-ID of the subject, just like a permissive access
check. Then OR all the associated permissions together, and return the
results in the appropriate file permission bits (user, group, and other).
While this alternative integrates the idea of ACLs into mechanisms that
access object attributes9 the context of the mechanisms affects the result
returned to the point where the meaning of what the mechanisms return is
somewhat clouded.

The preferred alternative is to make no changes to these mechanisms. The
mechanisms will continue to return the file permission bits as if ACLs did
not exist. Another mechanism must then be used to find out if the file has
an ACL, and if so, what its entries are. While this alternative does not
provide all information to subjects that don't know about ACLs, it does
not change the current behavior of these mechanisms.

Accessing Object Data

There are a number of system calls which will need to have AFL
functionality added to them (i.e., for access checking). These calls
include all those taking file system object names as parameters, as well
as those IPC mechanisms which perform access checks. Examples of some of
these calls are: open, msgsnd, msgrcv, semop, and shmat.

It is also important for portability that programs use the available
access control mechanisms in an appropriate manner, so that the security
policy is interpreted correctly. For instance, at the system call level,
the permission information returned by the use of stat may not be
sufficient to determine allowed access; other information such as ACL
contents may have to be evaluated as well.

Recommendation

The following is a summary of the preferred alternatives stated in this
section. Regarding compatibility with existing DAC mechanisms that either
1) change ownership or group of an object. 2) change file permission bits,
or 3) access object attributes should remain unchanged and not affect an
existing ACL on the object or create an ACL where one did not exist
before.

Regarding the addition of ACL functionality, existing mechanisms should
not be changed, and new parameters should not be added. Instead, new
mechanisms should be created which make use of existing ones.

ACL System Calls And Commands

This issue addresses what the naming conventions and functionality for ACL
system calls and commands should be.

For system calls, there are at least two alternative types of designs.
Each depends on how the ACL is viewed. In one approach, the ACL is a
series of independent records which can be individually manipulated using
calls similar to open, read, write. and close. This approach has a nice
parallel to the way files are read and written, but may be viewed as
overly complicated given the relative infrequency of ACL modification. In
the other approach. the ACL is considered a single unit and is not changed
record-by-record, but instead always manipulated as a whole. This approach
uses a "get" and "set" concept for ACL operations, where an ACL, as a
whole, is retrieved, modified locally, and then replaced [3]. This
approach is simple and reflects the growing trend towards get/set type
operations.

It may also be reasonable to extend the "get" and "set" concept to apply
to default ACLs as well as to the ACL associated with an object. This is a
natural extension of the way ACLs would be manipulated, and default ACL
operations may be easily added to the recommended system call interface
described below,;

'

There are also two possible methods for implementing these calls. One
option is to use separate system calls for each of the ACL operations
(i.e., getacl, setacl). The other option is to have one ACL system call
that can be invoked with a number of command arguments indicating the
desired ACL operation [3]. An example of a useful additional command
argument is one that would return the number of entries in the ACL. This
method conserves the number of system calls, and provides the flexibility
to add ACL commands via command arguments. Additionally, using this
method, designers are free to implement library functions based on the
system call with particular command flags.

For commands, the same issues apply as for system calls. In a system with
ACLs9 however, there will be a need for commands to not only manipulate
ACLs. but also to show and manipulate all discretionary access control
information. These commands should include. at a minimum:

     ₧ command(s) to retrieve and set file permission and mode bits (Is,
     chmod)
     ₧ command(s) to retrieve and set ACL information (new)
     ₧ command(s) to retrieve effective discretionary access to files
     (new)

In addition, there may be useful features to add to existing utilities
(e.g., the ability to find a file according to its ACL [12]) so that they
might be able to conform to the enhanced DAC mechanisms.

Recommendation

For the ACL system call interface, get/set ACL type operations should be
used9 and should be implemented with a unified system call with command
arguments used to implement the various operations. For commands. the
names getacl and setacl are recommended since they follow from the get/set
concept.

Named ACLs

A named ACL, as described in A Guide to Understanding Discretionary Access
Control in Trusted Systems [2], is an ACL that can be shared or referred
to by name. They may be implemented in one of two ways; either as a
template copied into a user's ACL or shared through a pointer from the
user's ACL space (shared ACL). A change to a shared ACL results in a
change to the discretionary access on all objects using this ACL. This
result may be considered to be a side-effect or a desired feature
depending on the circumstance. Additionally, it may be difficult to
determine which objects are sharing a specific named ACL, and a user may
mistakenly grant access to an object that was not intended. Another
problem with named ACLs is that as objects they may themselves be required
to contain discretionary access controls. This suggests the idea of
recursive ACLs, a situation to be avoided.

Recommendation

Named ACLs need not be supported. but a system that does should be no less
secure or less flexible than one that does not. Absolute Siexibility of
ACLs can be achieved, however, through the use of default ACLs as
discussed in the following section. There is no strong case one way or the
other for named ACLs. There are advantages and disadvantages to both
alternatives and it would really depend on the environment as to whether
named ACLs would be of any benefit.

Default ACLs

When considering ACLs, an issue arises as to whether a predesignated set
of ACL entries should be assigned to an object automatically at the time
of creation. The following alternatives present the possible ways to
address this issue.

No Default ACLs

In this approach, no ACL is assigned at object creation time. The process
umask will limit the file permission bits, as it currently does, to
provide some default protection on an object.

While this alternative maintains compatibility with existing programs, it
is not a very practical solution. Depending on the relationship of the
file permission bits and the ACL, the absence of default ACLs may not make
sense. For instance, in a pure ACL implementation, the absence of default
ACLs would result in no initial protection on newly created files.
Additionally, this alternative would not encourage the use of ACLs by new
programs, and would prevent ACL creation by old programs. ACLs could not
propagate through the system and hence their usability would be lost.

Require Default ACLs

In this approach, an ACL would always be assigned at object creation time.
This would allow for initial finer grained control on an object.

Requiring default ACLs may cause incompatibilities for an old program that
only looks at the file permission bits when it creates an object. Also,
for many users, the umask may be a sufficient tool for limiting the
permissions on an object when it is created. The main advantage of
requiring default ACLs is that the usability of ACLs is greatly improved.
Additionally, since an ACL is associated with an object in a single atomic
operation, the possibility of a temporarily insecure state is avoided.

Provide Default ACLs

A mechanism is provided to put default ACLs on new objects. However9 not
all new objects need to have default ACLs. This alternative allows
specification of a default `ACL, giving a finer granularity of access
control than that provided by the file permission bits, and, at the same
time allows, where desired, compatibility with existing programs.

Recommendation

Providing default ACLs and mechanisms to specify whether or not to use
them is the best solution. This allows both classes of users, those who
want default ACLs and those who do not (even those who want no ACLs at
all), the flexibility to specify the scheme that they find most
appropriate. Although in many cases the process umask would be sufficient
to assign default permissions, systems and/or users making explicit use of
ACLs will desire default ACLs. The default ACL scheme used should be
straightforward to the user and should sensibly interact with the existing
DAC mechanisms, including the ilmask mechanism. Note that even if an
object is created with no default ACL, ACL entries may still be added to
the object.

This section has really only addressed default ACLs on file system
objects. IPC objects are not part of the file system name space, and
therefore require further consideration. IPC objects are relatively short
lived. and are generally not manipulated by users at the command level as
are files. Based on these characteristics default ACLs on IPC objects are
probably not needed, and their use is not recommended.

Location Of Default ACLs

Consider the following possibilities for the origination of the default
ACL.

System Wide

In this approach, one specific default ACL is assigned to any object
created on the system by any subject. This is a very inflexible solution
and misses the intent that discretionary access be set at the discretion
of the user.

Per Process

In this approach, each user process defines a default ACL, similar to the
umask currently used. This is a somewhat restrictive approach since this
allows the user to set only a single set of defaults for all files
created. It is likely that a user will wish to associate different default
ACLs with files created for different projects. Additionally, the default
ACL entries would have to be stored in the process area. The amount of
process space required to hold the entries would vary based on the number
of entries.

Per GID Of Created File

A default ACL could be associated with each GID. If GIDs are viewed as
project identifiers, the effect is to associate a unique default ACL
within each project subtree of the file system hierarchy. Further, in some
UNIX Systems, where GIDs propagate to newly created objects based on the
GID of the creating directory (rather than upon that of the creating
subject), default protection very naturally distributes across the file
system. However this variant imposes a somewhat restrictive viewpoint on
the utility of groups.

Per Directory

This approach would allow the object's default ACL to originate from the
containing directory of the object. A directory would contain both an ACL
to be used for access checking and a default ACL to be used when a new
object is created in the directory. All objects created in the directory
would be assigned the default ACL. Newly created subdirectories would
inherit the default ACL of the parent directory. In this manner, the
default will propagate down through the file system structure resulting in
much duplication of ACLs, possibly using much space. However, the
utilization of such space is a small price to pay for enhanced security
and usability, so the default should probably continue to propagate until
the user takes some explicit action to stop the propagation.

Recommendation

A user typically arranges objects per directory representing project work
or areas of interest. Since it is desirable, then, for similar objects to
contain the same ACL, the per-directory approach becomes the preferred
mechanism. Newly-created subdirectories should inherit the default ACL of
the parent directory, so that defaults are propagated down the file
system, unless explicitly turned off.

Default ACL Entries At File Creation

Currently, when a file is created a user can specify its initial
pe'rmissions, however the access can be further restricted by the umask
mechanism. The uniask specifies the default protection bit settings when a
file is created. Any bits set in the umask will be cleared in the bit
settings on the newly created file. It is important, then, to consider how
the default permission bit settings should interact with the entries in a
default ACL.

Consider the following options in the context of masking the ACL entries
by the file group class permission bits as recommended in the ACL
Evaluation section. Also note that these options are discussed with
respect; to the ACL entry types as described in the ACL Entry Type and
Format section. Additional mechanisms in the ACL which allow direct
modification of the file group class permission bits at file creation are
not precluded.

OR File Group Class Permission Bits

Add the default entries to the file and change the file group class
permission bits to reflect the maximum permissions allowed in the ACL.
This could result in more permission than was specified in the creation
call. It is not reasonable to assume that the default permission bit
settings can be ignored and completely overridden by the ACL. For example,
if a default entry exists for user "fred" with the specified permissions
of "rwx" but the file is not executable, then this permission should not
be given.

AND File Group Class Permission Bits

Add the default entries to the file but change the permissions of the ACL
entries so that they are no greater than the file group class permission
bits. This is a reasonable alternative, but it may present a compatibility
problem for some applications. An example of this problem would be when a
C compiler creates a file. The file would not originally be created with
execute permission, therefore no ACL entries on the file (which were
default entries copied from the directory) would have execute permission.
The last step for the compiler would be to make the file executable,
however at this point, execute permission' which may have been specified
in the default ACL entry is lost.

No Change To File Group Class Permission Bits

Add the default entries to the file but do not change the file group class
permission bits. This may result in ACL entries which are restricted by
the file group class permission bits.

Recommendation

The No Change To File Group Class Permission Bits is recommended since it
is a reasonable alternative which does not present any problems of
compatibility for some applications.

Summary

This document has provided an analysis of key issues involved in extending
the discretionary access control in the UNIX system. For oach of the
issues identified, the paper has suggested alternative solutions,
discussed the pros and cons of each, and then provided a recommendation.

The following is a review of some of the important recommendations
presented in the paper. An access control list mechanism was chosen to
extend the current DAC mechanism. When considering the types of access
provided in the UNIX system, additional access modes need not be defined,
however they should also not be precluded. The recommended ACL entry type
was that of user or group entries. The main advantages of this solution
are conformance with the UNIX system method of identification through
either the user-ID or the group-ID, and simplicity for the user. The
method in which file protection bits and ACLs interact is a very important
and complex issue given the conflicting goals of security and
compatibility. The recommendation of masking the ACL entries by the group
field of the protection bits was chosen as the most accommodating solution
considering these goals. A system defined ordering of the ACL entries was
preferred and it was recommended that the access allowed for a user in
multiple groups should be the sum of all access allowed for each group
represented in the ACL. Considering other multiple group issues9 it was
recommended to provide the multiple concurrent group capability along with
some method of subsetting. It was also recommended that default ACLs be
provided and that they originate from the parent directory of the newly
created object.

It is important to note that although these and other specific
recommendations were given, it is certainly possible to design an
acceptable class B3, POSIX-compliant UNIX system following some of the
other alternatives. In fact, there are issues where the recommended
solution may not be superior to another alternative and the designer
should consider his/her own specific requirements when making a choice in
those areas. It must also be pointed out that building a system following
all the recommendations presented in this paper will not guarantee a full
class B3 system. There are many additional class B3 requirements that go
beyond the interface specification.

APPENDIX: Worked Example

Introduction and Overview

This worked example describes one particular Implementation following the
recommendations in the TRUSIX rationale.

Discretionary Access Control

Discretionary access control (DAC) provides for the controlled sharing of
objects (e.g., files, IPC objects) between subjects (e.g., processes).
With discretionary access control, the owner of an object can grant
permissions to other users. The discretionary access control mechanism
uses object owner, object group, file permission bits (nine permission
bits) and the access control list (ACL) of an object to determine the
discretionary access to the object.

This document will detail the DAC interfaces and their run-time behavior.

The goals of this ACL mechanism were:

     ₧ compatibility with the current UNIX System DAC mechanism and POSIX

P1003.1

     ₧ user command interfaces that are easy to use and understand
     ₧ adhere to the "principle of least astonishment"
     ₧ interfaces should continue to work as expected
     ₧ chmod 000 file - no access to file
     ₧ chmod 700 file - only owner access to file
     ₧ chmod 444 file - denies write and execute access to file

In addition, intermixing use of the existing and new DAC commands should
give reasonable results. For instance chmod should not fail due to ACLs,
and when chmod x file is executed (x is an octal permission) Is -l
displays x as the permissions. The current output of Is -l displays the
file permission bits as a constant width set of nine characters:

rwxrwxrwx

However, an ACL, which consists of one or more user entries, one or more
group entries, one class entry, and one other entry, is not a constant
length (in the following example, indicates zero or more occurrences of
the preceding entry type):

     # file: filename
     # owner: uid
     #group:gid
     user::rwx
     user:uid:rwx

*

     group::rwx
     group:gid:rwx

*

     class:rwx
     other:rwx

The file permission bits shown by the Is command have the following
meaning: (note the following "class" definitions are from the IEEE POSIX
Std 1003.1-1988):

     1. the first 3 bits (high order) represent the file owner class and
     define the permissions for the object owner,
     2. the middle 3 bits (commonly called the group permission bits),
     represent the file group class. This class includes the owning group
     of the file and will be extended to include additional user and
     additional group ACL entries.
     3. the last 3 bits (low order) represent the file other class and
     define the permissions for other (those that did not fall into 1 or 2
     above).

These nine bits indicate the maximum discretionary permissions for an
object. The actual permissions may always be less than indicated. For
instance, the permission may indicate write access on an object by a
specific subject, but the file system may be mounted read only. If an ACL
mechanism is used these bits will continue to indicate the maximum
discretionary permissions for the object and the ACL may further restrict
permissions.

There is a direct mapping between the ACL and the file permission bits.
Specifically, the file owner class permission bits will always be equal to
the permissions of the ACL entry for the object owner (they may be the
same bits depending upon the implementation). Additionally, the file other
class permission bits will always be equal to the ACL other entry
permissions. And the file group class permission bits will always be equal
to the ACL class entry permissions. Typically, the file group class
permission bits are set to the maximum permissions allowed to the
additional user entries, the owning group entry, and the additional group
entries.

Whenever a file is created on a file system that supports ACLs, the ACL
will contain a user entry for the object owner, a group entry for the
object owning group, a class entry for the file group class permissions,
and an other entry for the rest of the world. For compatibility with the
current mechanism, if the ACL contains no additional user or additional
group entries, the permissions in the group entry for the object owning
group and the class entry must be the same.

Use of Access Control Lists

The use of DAC with ACLs will be explained by comparing it to how a user
of a non-ACL supporting UNIX System (as currently exists) would use DAC.
To use the current DAC mechanism a user usually first executes Is -l and
based on the output decides what the permissions must be changed to. in
order to allow the desired access (for example the user may want to make
the file executable, or only allow the owner to have write permission).

EXAMPLE:

$ ls -l foo

     ₧ rw-rw-rw- 1 craig demo 53 Mar 6 17:37 foo S chmod 600 foo $ ls -l
     foo
     ₧ rw 1 craig demo 53 Mar 617:37 foo

In the new DAC mechanism, using a pure ACL, there will be two new commands
getacl and setacl (there will be a new function, acl, for which these
commands provide a user interface). The getacl command will be used to
display the ACL and the setacl command will be used to change the ACL.

These commands will be used in much the same way that Is and chmod are
used. A user would first execyte getacl to look at the ACL and then use
setacl to make the desired changes. Because the ACL is not a fixed size,
it may be difficult to manipulate. In order to simplify the use of ACLs
the following example shows how the ACL may be easily manipulated using a
text editor to give greater flexibility (note that changes may also be
specified on the setacl command line).

EXAMPLE:

     #the output of getacl is redirected to the file tmp
     $ getacl bar > tmp
     #the file tmp is edited and the line in italics is inserted
     $ vi tmp
     # file: bar
     # owner: craig
     # group: demo
     user::rw-
     group::rw-
     group:guest:r-
     class:rw-
     other:rw-
     #setacl is executed and the contents of the file tmp become the new
     ACL for bar
     $ setacl -f tmp bar
     #the output from getacl for the file bar is displayed
     $ getacl bar
     # file: bar
     # owner: craig
     group: demo
     user::rw-
     group::rw-
     group guest :r-
     class:rw-
     other:rw-

Structure of Access Control Lists

The ACL consists of the following types of entries, which must be in the
following order:

     1. user entry - This type of entry contains a user ID and the
     permissions associated with it. There must always exist one entry of
     th1is type, which will represent the object owner, and will be
     denoted by a null (unspecified) user ID. There may be additional user
     entries specified; however, no two additional user entries will have
     the same user ID and there may not be any additional entries with a
     null user ID. The term "additional user entries" will be used to
     indicate all user entries except the entry for the object owner.
     2. group entry - This type of entry contains a group ID and the
     permissions associated with it. There must always exist one entry of
     this type, which will represent the object owning group, and will be
     denoted by a null (unspecified) group ID. There may be additional
     group entries specified; however. no two additional group entries may
     have the same group ID and there may not be any additional entries
     with a null group ID. The term "additional group entries" will be
     used to indicate all group entries except the entry for the object
     owning group.
     3. class entry - This type of entry contains the maximum permissions
     granted to the file group class. There is exactly one of these
     entries in an ACL.
     4. other entry - This type of entry contains the permissions granted
     to a subject if none of the above entries have been matched. There is
     exactly one of these entries in an ACL.
     5. default entry - This type of entry may only exist on a directory.
     These entries are similar to the entries described above, except that
     they are never used in an access check, but are used to indicate the
     non-default ACL entries that should be added to a file created within
     the directory. Default entries are optional9 but no two default
     entries may have the same type and ID.

Within each category the entries must be ordered as follows:

Entries in the user category shall be sorted numerically by user ID from
lowest to highest, except for the object owner entry9 which always
precedes all other user entries.

Entries in the group category shall be sorted numerically by group ID from
lowest to highest, except for the object owning group entry, which always
precedes all other group entries.

Entries in the default:user category shall be sorted numerically by user
ID from lowest to highest, except for the default object owner entry,
which always precedes all other default user entries. Entries in the
default:group category shall be sorted numerically by group ID from lowest
to highest, except for the default object owning group entry, which always
precedes all other default group entries. The proper ordering of entries
required by the acl function can be obtained by the use of the aclsort
function. ACL entries given as input to the setacl command need not be
sorted; the sorting will be performed by the setacl command.

The permissions that may be specified in an ACL entry are read(R),
write(w), and execute/search(x). When the setacl command is executed, the
file owner class permission bits will be set to the permissions specified
for the owner and the file other class permission bits will be set to the
permissions specified for other. As an option, the file group class
permission bits will be manipulated such that they reflect the maximum
permission that the ACL permits to members of the file group class (any
ACL entry other than the object owner or other). Otherwise, the file group
class permission bits will be set to the permissions specified by the
class entry. Therefore, if the file group class only allows read
permission then additional user entries and any group entries in the ACL
will not grant write or execute permission.

This ACL scheme supports finer discretionary access controls than the
current mechanism, while maintaining compatibility with the current
permissions mechanism. The DAC information may be changed in one atomic
operation, avoiding the possibility of an intermediate insecure state.
Finer controls can be specified via the ACL, including explicit
specification of users disallowed any access to the object. Additionally,
the file permission bits provide a summary of all access rights.

Rationale: The ACL scheme described here will allow entries to be either
permissive or restrictive. In general, an entry that results in less
permission than the file other class permissions would grant would be
considered restrictive. An entry that results in more permission than the
file other class permissions would grant would be considered permissive.
In the event that a file with an ACL is exported to a non-ACL system, the
loss of permissive entries would not present a security problem; however,
the absence of support for restrictive entries may allow a process to have
permission that it would not have been granted on a system with ACLs. This
behavior must be described in the documentation.

Discretionary Access Check Algorithm

A process may request read, write, or execute/search access permissions to
a file. Each access mode is logically checked separately using the
following algorithm. The process request is granted if all individually
requested modes are granted. Otherwise, the access request is denied.

Note, this is a logical description of the access check. The physical code
sequence may be different for better performance.

Discretionary Access Check Algorithm:

     I. File Owner Class: If the effective user ID of the process matches
     the user ID of the owner of the file, the process is in the file
     owner class. If the requested access mode bit is set in the file
     owner class permission bits, this access mode is granted. Otherwise,
     access is denied.

Note, the user ACL entry for the object owner matches the file owner class
permission bits.

     II. File Group Class: If the process is not in she- file owner class
     and if the effective user ID of the process matches the user ID of an
     additional user ACL entry or the effective group ID or any of the
     supplementary group IDs of the process matches the group ID of any
     group ACL entry9 the process is in the file group class. If the
     process matched an additional user ACL entry, only that entry is used
     as the matching ACL entry; otherwise, the matching group ACL entry or
     entries are used. If the requested access mode bit is set in the file
     group class permission bits and is set in a matching ACL entry, this
     access mode is granted. Otherwise, access is denied.
     Note, the permissions of the additional user or group ACL entries
     further restrict the access specified by the file group class
     permission bits. Also, the class ACL entry matches the file group
     class permission bits.
     III. File Other Class: If the process is not in the file owner class
     or file group class, the process is in the file other class. If the
     requested access mode bit is set in the file other class permission
     bits, this access mode is granted. Otherwise9 access is denied.

     Note, the other ACL entry matches the file other class permission
     bits.

The following examples show ACL use and the results of applying current
and new DAC commands.

EXAMPLE 1:

     $create file foo
     $ > foo
     #execute ls -l and getacl on the file foo
     $ Is -l foo
          ₧ rw-r-r-- 1 craig demo 0 Mar 620:27 foo

     $ getacl foo
     $ file: foo
     $ owner: Craig
     $ group: demo
     user::rw-
     group::r-
     class:r-
     other:r-

EXAMPLE 2:

#execute getacl and Is -I on the file, run.sh, with added ACL entries

S Is -I run.sh

     ₧ rwxr-xr-x+ 1 Craig demo 73 Mar 620:27 run.sh
          $ getacl run.sh
          # file: run.sh
          # owner: Craig
          # group: demo
          user::rwx
          user:fred:r-x
          user:larry:--x
          group::r-x
          group guest:---
          class:r-x
          other:r-x

EXAMPLE 3:

#use the chmod command on a file with an ACL

#use getacl to report both the ACL entries and the effective permissions

     $ chmod 644 run.sh
     S Is -l run.sh
          ₧ rw-r-r--+ 1 craig demo 73 Mar 620:27 run.sh
     $ getacl run.sh
     # file: run.sh
     # owner: craig
     #group: demo
     user::rw-

user:fred:r-x #effective:r-

user:larry:--x #effective:---

group::r-x #effective:r-

     group guest:---
     class:r- -
     other:r-

File Object Creation

When a new object (regular files, special files, directories, named pipes)
is created in

the file system, there are several important attributes that must be
initialized. These are the user ID of the owner of the file, the group ID
associated with the file, the file permission bits, and the ACL.

The user ID of the file is set to the effective user ID of the invoking
process. The group ID of the file depends upon the mode of the containing
directory. If the S_ISGID bit is not set on the directory, the group ID of
the file is set to the effective group ID of the invoking process. If the
S_ISGID bit is set on the directory, the group ID of the file is set to
the group ID of the containing directory.

Each function that creates a new file supplies an initial value for the
file permission bits. This initial value is then merged with the file mode
creation mask (umask) of the invoking process and with any default ACL
entries of the containing directory to form the file permission bits and
ACL of the new file.

Although in many cases the process umask is sufficient to assign default
permissions, users making explicit use of ACLs may desire default ACLs.
The default ACL scheme must sensibly interact with the existing DAC
mechanism, including umask. The default ACL entries specify permissions
for users and/or groups and/or others, that will be assigned to a new
file. These default ACL entries are associated with a directory. Note, an
ACL on a directory may contain entries that control access to the
directory and entries (defaults) used for new file creation in that
directory.

The process of creating the file permission bits and the ACL for the new
file is called "ACL Merge". First, any mode parameter is transformed into
the equivalent ACL form. For example, the mode 0664 is equivalent to
user::rw-, group::rw-, class:rw-, other:r--. Also, the complement of the
umask is used to obtain the equivalent ACL. Thus, the umask 022 is
equivalent to user::rwx, group::r-x, class:r-x, other:r-x.

Two ACLs are merged by first logically sorting both ACLs into one ACL.
Then any pair of matching entries are replaced with an entry that has
permissions formed by ANDing the matched entries. Thus a permission is in
the merged entry only if it was previously in both entries.

The first ACL merge is with the initial mode from the file creation
function and with the process file mode creation mask. The second ACL
merge is with any default entries from the containing directory. The
result is the ACL for the new file. The file permission bits are then set
from the user, class, and other ACL entries. Note, this may be different
from the setacl command with the -r option since this merge does not set
the file group class permission bits to the maximum permission of the file
group class entries.

Finally, if the new object is a directory, then any default entries from
the containing directory are copied to the new ACL. That is, the default
ACL entries of the new directory are the same as the default ACL entries
of the containing directory.

An example of the ACL merge operation is shown in the following figure:

000666::rw

user::rw-

creat("file", 0666) .. . .. . group::rw-

     class:rw-
     other:rw- ACL Merge

Operation

umask 002 user::rwx

group::rwx

class:rwx

other:r-w

     0664
     user::rw-
     group::rw-
     class :rw-
          other:r-

Directory .... user:gamma:r--

default ACL group::r-

entries .. .. . group:alpha:rw-

qroup:beta:--- 0664

user::rw-

user:gamma:r-

     group::r-
     group:alpha:rw-
     group:beta:---
     class:rw-
     other:r-

IPC Object Creation

When an IPC object is created (by shmget for shared memory, by semget for
semaphores, by msgget for messages), its cuid and uid will be set equal to
the effective user ID of the invoking process and its cgid and gid will be
set equal to the effective group ID of the invoking process. The initial
permissions are set equal to the specified permissions in the flag
argument to the *get calls (shmflg, semflg, and msgflg, respectively).
Note that default ACLs do not apply to IPC~' objects, although ACLs may be
added explicitly to an IPC object via the acllpc call.

Compatibility Requirements

A user will generally use the current DAC commands (Is and chmod) or the
new DAC commands (getacl and setacl). However, the use of these commands
are likely to still be inter-mixed, and they must all give correct
information.

The entire interface to the current discretionary access control
information must continue to function as it currently does. For example,
chmod must still be able to modify the file permission bits and Is must
still be able to report them.

Note that although Is will still report these permissions. they will not
be the only permissions evaluated during an access check. The output of Is
will continue to be the maximum permission that may be granted, but there
may be additional discretionary access control information (ACL entries)
that was added to the object. In order to indicate that additional entries
exist, Is-I will display the character "+" to the right of the current
permissions display if an ACL is present. Therefore, when additional
discretionary access control information has been added, in the form of
ACL entries (as shown in the examples on previous pages), a user will need
to use the newly provided command, getacl, to get a full view of the
current discretionary access controls in effect. Although chmod will still
modify the file permission bits, it will not change any additional
discretionary access control information (i .e., ACL entries for
additional users and additional groups) added to the object. To change
these additional entries if they exist, the user will need to use the
setacl command.

When the owner of an object is changed, the result will be identical to
the current behavior. If the owner is changed to a user ID for which an
additional user entry already exists in the ACL, the additional user entry
is not changed but the user entry for the object owner will take
precedence during an access check. When the group of an object is changed.
the result will be identical to the current behavior. If the group is
changed to a group ID for which an additional group entry already exists
in the ACL, the additional group entry is not changed but the group entry
for the object owning group will take precedence during an access check
(except in the case of multiple concurrent groups, where all group entries
are given equal treatment).

When the ACL contains no additional user or additional group entries, the
permissions in the group entry for the object owning group and in the
class entry must be the same. This behavior is the same as the current
mechanism since the file permission bits can only specify at most three
different permissions.

Documentation Requirements

The ACL mechanism and its proper use must be fully described in the
Trusted Facility Manual and manual pages must be created for the Security
Features User's Guide and Security Features Programmer's Guide for all new
commands and functions.

Commands and Functions

setacl Command

DESCRIPION: The setacl command will support the changing of discretionary

permission information associated with a file. It will allow the file
owner or a process with appropriate permission or appropriate privilege to
perform the following functions:

     1. replace an entire ACL, including the default ACL entries on a
     directory,
     2. add, change, or delete an ACL or default ACL entry or entries.

This command gives the user an interface to a pure ACL mechanism, allowing
a finer granularity for file access.

Note that this command only supports the file system objects: e.g.,
regular files, special files, directories, and named pipes. For
simplicity, these objects are referred to as "files".

SYNOPSIS:

setacl [-r] [ -rn (u[ser]::operm Jperm[,]]

     [rn[ser]:uid:operm perm[,...]]
     [g[roup]::operm perm[9]]
     [g[roup]:gid:operm perm[,...]]
     [c[lass]:operm perm[,]]
     [o[ther]:operm perm[,]]
     [d[efault]:u[ser]::operm perm]
     [d[efault]:u[ser]:uid:operm perm[....]]
     [d[efault]:g[roup]::operm perm]
     [d[efault]:g[roup]:gid:operm perm[....]]
     [d[efault]:c[lass]:operm perm]
     [d[efault]:o[ther]:operm perm]

     ₧ d [u[ser]:uid[,...]][g[roup]:gid[,...]] [d [efault]:u[ser]:[,...]]
          [d[efault]:u[ser]:uid[,...]] [d[efault]:g[roup]:[,...]]
          [d[efault]:g[roup]:gid[,...]] [d[efault]:c[lass]:[,...]]
          [d[efault]:o[ther]:[,...]]]

file

or

setacl [-r] -s u[ser]::operm perm[,]

     [u[ser]:uid:operm perm[,...]]
     g[roup]::operm perm[,]
     [g[roup]:gid:operm perm[,...]]
     c [lass]:operm perm [,]
     o[ther]:operm perm[,]
     [d[efault]:u[ser]::operm perm]
     [dl[ef&Wt]:u[ser]:uid:operm perm[,...]]
     [d[efault]:g[roup]::operm perm]
     [d[efault]:g[roup]:gid:operm perm[,...]]
     [d[efault]:c[lass]:operm perm]
     [d[efault]:o[ther]:operm perm]

file...

or

setacl [-r] -f acljile file

where:

operm = octal representation of permissions

(Note: for an ACL entry one octal digit is required)

perm = a permissions string composed of the

characters r (read). w (write), x (execute/search), or - (no permission).
The permission string must be at least 1 character and no more than 3
characters. The characters r. w, and x may only be in the string at most
once. The characters may be in any order within the string.

uid = user identity (i.e., login name or user ID) gid = group identity
(i.e., group name or group ID)

When the -f option is specified, it will take the access control
information stored in the file acljile and assign it to the file file. See
the PROCESSING section below for further information on the format of the
file acljile.

PROCESSING: A unique ACL will exist for each file on the system. There are
four

types of ACL entries, consisting of user, group, class, and other. The
user entry for the file owner, the group entry for the file owning group,
the class entry for the file group class, and the entry for other must
always be in the ACL.

     1. user entry - This type of entry contains a user ID and the
     associated permissions that will be granted to the user. There must
     always exist one entry of this type, which will represent the file
     owner, and will be denoted by a null (unspecified) user ID. There may
     be additional user entries specified; however each entry must specify
     a unique user ID and there may not be any additional entries with a
     null user ID. If there is a user entry with a user ID equal to the
     file owner the file owner entry will take precedence when an access
     check is performed.
     2. group entry - This type of entry contains a group ID and the
     associated permissions that will be granted to the group. There must
     always exist one entry of this type, which will represent the file
     owning group, and will be denoted by a null (unspecified) group ID.
     There may be additional group entries specified; however, each entry
     must have a unique group ID and there may not be any additional
     entries with a null group ID.
     3. class entry - This type of entry contains the maximum permissions
     for the file group class. There is exactly one of these entries in an
     ACL.
     4. other entry - This type of entry contains the permissions granted
     to a subject if none of the above entries have been matched. There is
     exactly one of these entries in an ACL.

When the setacl command is used to change the ACL. it may result in
changes to the file permission bits. Specifically. when the user ACL entry
for the file owner is modified the file owner class permission bits will
be modified. When the class ACL entry is modified, the file group class
permission bits will be modified. When the other ACL entry is modified the
file other class permission bits will be modified.

When the additional user entries or additional group entries of the ACL
are modified, the file group class permission bits may also need to be
modified to reflect the maximum permission allowed by these entries.

The -r, recalculate, option will result in the permissions specified in
the class entry being ignored and replaced by the maximum permission
needed for the file group class. For example, if there are no additional
user entries or additional group entries, the permission of the group
entry for the file owning group is used for the class entry.

A directory may contain default ACL entries. These entries may be of the
type default:user. default:group, default:class, or default:other. For
default:user entries, if no user ID is specified, this entry will apply to
the file owner permissions. Additional default:user entries must have a
unique user ID specified. For default:group entries, if no group ID is
specified, this entry will apply to the file owning group permissions.
Additional default:group entries must have a unique group ID specified. If
there are no additional default:user entries or additional default:group
entries, then the permissions of the default group and the default class
must be the same.

If a file is created in a directory which contains default ACL entries the
entries will be added to the newly created file. Note that the default
permissions specified for the file owner class, file group class, and file
other class will be constrained by the umask and the mode( specified in
the file creation call. If default ACL entries are specified for a file
which is not a directory the command will fail (11), see ERRORS AND
RETURNS.

With no options and arguments (1), see ERRORS AND RETURNS. If the MAC or
DAC check fails when a request is made to modify the ACL (2), see ERRORS
AND RETURNS. If the file named file does not exist (6), see ERRORS AND
RETURNS.

If options are specified, the validity of the option-arguments will be
checked. If an invalid option is specified (3a), see ERRORS AND RETURNS.
The arguments must be processed in the order specified (e.g., if the
modify option is specified with a user, followed by the delete option with
ihe same user, the entry will be deleted).

For the -m. -s, and -d options, if uid is not a valid login name or a
valid user ID (3b), or if gid is not a valid group name or a valid group
ID (3c), or if a specified perrn is not r. w. x, -, or a specified operm
is not an octal digit (3d), see ERRORS AND RETURNS.

The -m option is used to add a new ACL entry or change an existing ACL
entry.

If an entry already exists for the specified uid or gid. the specified
permissions (perm operm) will replace the current permissions. If an entry
does not exist for the specified uid or gid, an entry will be created.
Note that an entry with no permissions will result in the specified uid or
gid being denied access (any permissions) to the file. To specify no
access in an entry being modified or added, either 0 should be specified
for operm or - should be specified for perm.

The -s option is used to replace the ACL information on a file. The effect
of using this option is that all entries are removed, and replaced by the
newly specified ACL. If -s is specified with -d, -f, or -m (ST, see ERRORS
AND RETURNS. There must be exactly one user entry specified for the file
owner, exactly one group entry specified for the file owning group,
exactly one class entry specified for the file group class, and exactly
one other entry specified. If there is no user entry specified for the
file owner. or no group entry specified for the file owning group9 or no
class entry specified for the file group class, or no other entry
specified (8), see ERRORS AND RETURNS. There may be additional user ACL
entries and additional group ACL entries specified. If duplicate entries
are specified (9), see ERRORS AND RETURNS.

The -d option is used to delete an existing entry from the ACL. If a
matching entry is not found (4a), see ERRORS AND RETURNS. Othdrwise, the
matching entry will be deleted. The user entry for the file owner, the
group entry for the file owning group, the class entry, and the other
entry may not be deleted from the ACL. If an attempt is made to delete one
of these entries (4b), see ERRORS AND RETURNS.

(Note: deleting an entry may have different effects than removing all the
specified permissions for an entry. If an entry is deleted and a search is
later done for the user or group identity that appeared in the entry, this
identity might match another entry and then be given the permissions
specified in this other entry. If the original entry remained with no
permissions and a search was done for this identity. the search might
match this entry and the subject would be denied access.)

The -f option is used to assign the ACL information contained in the file
named acljile to the specified file(s). If -f is specified with -d. -s. or
-m (5), see ERRORS AND RETURNS. If the file named acljile does not exist
(6), see ERRORS AND RETURNS. The file named acljile must be readable by
the invoking subject. If it is not readable (2), see ERRORS AND RETURNS.
If the entire file named aclJile contains correct external
representation(s) for ACL entries, the ACL for the specified file(s) will
be (removed and) replaced with the ACL whose external representation is
contained in the file named acljile. Each external representation of an
ACL entry, contained in the file named acljile, must be on a separate line
and must be in the following format:

     u[ser]::operm perm
     [u[ser]:uid:operm perm]
     g[roup]::operm perm
     [g[roup]:gid:operm perm]
     c[lass]:operm perm
     o[ther]:operm perm
     [d[efault]:u[ser]::operm perm]
     [d[efault]:u[ser]:uid:operm perm[,...]]
     [d[efault]:g[roup]::operm perm]
     [d[efault]:g[roup]:gid:operm perm[,...]]
     [d[efarnlt]:c[lass]:operm perm]
     [d[efault]:o[ther]:operm perm]

The entries are not required to be in any specific order within the file.
There must be exactly one user entry specified for the file owner, exactly
one group entry specified for the file owning group, exactly one class
entry specified for the file group class, and exactly one other entry
specified. If not, see ERRORS AND RETURNS. There may be additional user
ACL entries and additional group ACL entries specified. If duplicate
entries are specified (9), see ERRORS AND RETURNS.` Validity checks are
performed on all entries. If an invalid entry isencountered (7); see
ERRORS AND RETURNS. If the exact problem can bedetermined an additional
message may be displayed (3b)(3c)(3d), see ERRORS

AND RETURNS.

The character "#" will be used to indicate a comment. All characters
starting with the #. to the end of the line will be ignored. Note that
this includes any effective permissions (#effective:rwx) displayed by
getacl. This command may be executed on a file system that does not
support ACLs. If ACL entries are specified which do not map into the base
permissions (10), see ERRORS AND RETURNS, otherwise the base permissions
will be set.

ERRORS AND RETURNS: Following is a list of error conditions and the

corresponding error message that should be output when this condition
occurs.

usage: setacl [-r] [ -m [rn(ser]::operm perm[.]]

     [rn[ser]:uid:operm perm[,...J]
     [g[roup]::operm perm[9]]
     [g(roup]:gid:operm perm[....]]
     [c[Iass]:operm perm[₧]]
     [o[ther]:operm perm[,]]
     [d(efault]:u[ser]::operm perm]
     (d(efarnlt]:u[ser]:uid:operm perm(9...]]
     [d[efault]:g[roup]::operm perm]
     [d[efault]:g[roup]:gid:operm perm[,...]]
     [d[efault]:c[lass]:operm perm]
     [d[efault]:o[ther]:operm perm]

     ₧ d [rn[ser]:uid(,...]][g[roup]:gid(,...]] [d [e?ault]:u(ser]:] fd
     [efault]:u[ser]:uid[,...]] [d[efault]:g[roupj:[,...]]

     [d[efault]:g(roup]:gid] [d[efarnlt]:o[ther]:]]

file...

or

setacl [-r] -s rn(ser]::operm perm[,]

     [rn[ser']:uid:operm perm[,...]]
     g[roup]::operm perm(.]
     [g[roup]:gid:operm perm[,...]]
     c[lass]:operm perm[,]
     o[ther]:operm perm[,]
     [d[efault]:u[ser]::operm perm]
     [d[efault]:u[ser]:uid:operm perm[9...]]
     [d[efault]:g[roup]::operm perm]
     [d[efault]:g[roup]:gid:operm perm[....]]
     [d[efault]:c[lass]:operm perm]
     [d[efault]:o[ther]:operm \perm]

file

or

setacl [-r] -f acljile file

(1) No options or arguments:

     UX:setacl: ERROR: incorrect usage
     usage:...

(2) If MAC or DAC check fails on the specified file:

UX:setacl: ERROR: permission denied for `jilename"

(3) invalid option-arguments:

     (a) incorrect/unknown option specified:

     UX:setacl: ERROR: illegal option-"-option"
          usage:

     (b) invalid user ID:

UX:setacl: ERROR: unknown user-id "uid"

     (c) invalid group ID:

     UX:setacl: ERROR: unknown group-id "gid" (dJ invalid permission:

UX:setacl: ERROR: unknown permission "permission"

usage:...

(4) invalid attempt to delete an ACL entry:

     (a) attempt to delete a non-existent entry from an ACL:
     UX:setacl: ERROR: matching entry not found in ACL
     (b) attempt to delete file owner, file owning group. class. or other
     ACL entries:
     UX:setacl: ERROR: file owner, file group, "class". and "other"
     entries
     may not be deleted

(5) the options specified are mutually exclusive:

UX:setacl: ERROR: incompatible options specified

usage:

(6) filename does not exist:

UX:setacl: ERROR: file `jile,name" not found

(7) an invalid ACL entry encountered in the file acljile:

UX:setacl: ERROR: "acljile", line line; invalid ACL entry

(8) required entry for file owner, file owning group, class, or
other missing:

UX:setacl: ERROR: required entry for file owner, file group, "class", or
"other" not specified usage:

(9) duplicate ACL entries specified:

UX:setacl: ERROR: duplicate entries: "acl,entry"

(10) the file system does not have ACLs, and additional entries
are specified:

UX:setacl: ERROR: only file owner, file group, "class" or "other"

entries may be specified

(11) the specified file is not a directory, and default e('ntries
have been specified:

UX:setacl: ERROR: default ACL entries may only be set on directories

OUTPIIT: None

getacl Command

DEsCRII'TION: The getacl command will support (he displaying of
discretionary

information associated with a file. It will allow the file owner or a
process with appropriate permission or appropriate privilege to perform
the following functions:

     1. display the owner, group, and ACL for the specified file(s),
     2. display the default ACL for a directory.

Note that this command only supports the file system objects: e.g.,
regular files, special files, directories. and named pipes. For
simplicity, these objects are referred to as "files".

SYNOPSIS:

getacl [-ad] file

PROCESSING: With no arguments (1), see ERRORS AND RETURNS. If MAC or

DAC check fails when a request is made to display the ACL information (2),
see ERRORS AND RETU'RNS. With invalid options (3), see ERRORS AND RETURNS.
If the file named file does not exist (4), see ERRORS AND REtuRNS.

With the -a option specified, the filename, owner, group, and the ACL of
the file will be displayed. With the -d option specified, the filename,
owner, group, and the default ACL of the file will be displayed, if it
exists. If the specified file does not support default ACLs (e.g., it is
not a directory) only the filename, owner, and group will be displayed.
With no option specified, both the ACL and the default ACL (if it exists)
of the file will be displayed. This command may be executed on a file
system that does not support ACLs.

It will report the ACL based on the base permission bits.

ERRORS AND RETURNS: Following is a list of error conditions and the

corresponding error message that should be output when [his condition
occurs.

usage: getacl [-ad] file

(1) No arguments:

     UX:getacl: ERROR: incorrect usage
     usage:...

(2) If MAC or DAC check fails when a request is made to display
the ACL information:

UX:getacl: ERROR: permission denied for `file" - (3) incorrect/unknown
option specified:

     UX:getacl: ERROR: illegal option-"-optio,t"
          usage:...

(4) file does not exist:

UX:setacl: ERROR: file `file" not found

OUTPUT: When an ACL is displayed, the external representation of the ACL
will

     be as follows:
     # file: filename
     # owner: uid
     # group: gid
     user::perm
     inser:uid:perm
     group::perm
     group:gid:perm
     class:perm
     other:perm
     default :user: :perm
     default :user:uid:perm
     defai,lt :group: :perm
     default :group :gid :perm
     default :class :perm
     default :other :perm

The ACL entries will be displayed in the order listed above (the user
entry for the file owner, followed by zero or more additional user
entries, followed by the group entry [or the file owning group, followed
by zero or more additional group entries, followed by the class entry for
the file group class, followed by the entry for other). When the specified
file is a directory the entries described above may be followed by default
entries (the default user entry for the file owner, followed by zero or
more additional d~efault:user entries, followed by the default:group entry
for the file owning group, followed by zero or more additional
defarnlt:group entries, followed by the default:class entry for the file
group class, followed by the entry for default:other). Note that these
default ACL entries are never used in an access check. If more than one
file is specified, a blank line will be displayed before the ACL of the
next file is displayed.

The first line displays the name of the file, next the file owner, and
then the file owning group. The user entry without a user ID indicates the
permissions -that will be granted to the owner of the file. The additional
user entries indicate the permissions that will be granted to the
specified user. The group entry without a group indicates the permissions
that will be granted to the group of the file. The additional group
entries indicate the permissions that will be granted to the specified
group. The class entry indicates the permissions that will be granted to
the file group class. The other entry indicates the permissions that will
be granted to others.

The default entries (default user, default group, default:class, and
default:other) may only exist for directories, and indicate the default
user, group, class, and other entries respectively that will be merged
with the ACL for a new file created within the directory.

The uid is a login name, or a user ID (only if there is no login name
associated with the user ID); gid is a group name. or a group ID (only if
there is no group name associated with the group ID); and perm is a three
character string composed of the letters representing the separate
discretionary access controls, r (read). w (write), x (execute/search), or
the character -. The perm will be displayed in the following order: rwx.
If a permission is not granted by this ACL entry. the placeholder. "-",
will appear. For example. if the user does not have write permission. but
does have read and execute permission, r-x will be output.

The file group class permission bits constrain the ACL (represent the most
access that any entry in the ACL may have). If a user executes the chmod
command and changes the file group class permission bits this may change
the permissions that would be granted based on the ACL alone. This
behavior is necessary for the save-restore model (all permissions are
temporarily removed via chmod 000 file and then restored) to work
correctly. In order to indicate that the file permission bits are more
restrictive than an ACL entry, getacl will display the ACL entry as
described above with an additional tab followed by a sharp sign and the
effective permissions.

Note that output from getacl will be in the correct format for input to
setacl. Therefore, if the output is redirected into a file (e.g., getacl
junk > entries), this file can be used as input to setacl (e.g., setacl -f
entries junk.new). In this way, a user can easily assign one file's ACL
information to another file.

EXAMPLES:

1) File with several ACL entries:

     #file:fred
     #owner:craig
     # group: demo
     user::rwx
     user:spy:---
     user:larry:rw-
     group::r-
     class:rw-
     other:---

2) Same file. after a "chmod 700 fred":

     #file:fred
     $ owner: craig
     $ group: demo
     user::rwx
     user:spy:---

user:larry:rw- #effective:---

group::r-- $effective:---

     class:---
     other:---

3) Directory with ACL entries including default ACL entries:

     $ file: foodir
     $ owner: craig
     $ group: demo
     user::rwx
     user:spy:---
     user:larry:rwx
     group::r-x
     class:rwx
     other:r-
     default:user::rwx
     default user :larry :rwx
     default:rnser:worm:- --
     default :group:demo:r -
     default:other:---

A.23 4

DESCRIPTION: The act call will support the getting and setting of
discretionary

     ₧ permission information associated with a file. It will allow the
     file owner or a process with appropriate permission or appropriate
     privilege to perform the following functions:
          1. get or set a file's ACL information in an atomic operation.
          2. return the number of entries contained in an file's ACL.

Note that this call only supports the file system objects: e.g., regular
files, special files, directories, and named pipes. For simplicity, these
objects are referred to as "files".

SYNOPSIS:

#include <tbd.h>

int acl(const char *path, int cmd. int nentries, struct acl "aclbufp)

Three values for cmd will be supported: ACLSET, ACLGET, and ACL_CNT. The
value of nentries is the number of ACL entries that can fit in the
user-supplied ACL buffer for an ACL_GET or the number actually present for
an ACL_SET; and aclbufp is a pointer to the user-supplied buffer of ACL
entry structures. The buffer will consist of an array of four (USEROBJ,
GROUPOBJ, CLASSOBJ, and OTHEROBJ entries are required) or more occurrences
of the following structure:

     struct acl [
     intatype;
     uidt aid;
     ushort a_perm;

Twelve values of arype will be supported to specify the type of entry:
(six for access checking and six for defaults), USEROBJ, USER, GROUPOBJ,
GROUP, CLASSoBJ, OTHERoBJ, DEF_USER_OBJ, DEF&SER, DEFGROUPoBJ, DEFGROUP,
DEFCLASSoBJ, and DEFoTHERoBJ. When arype is USER or DEF,USER, aid will be
a user id, and when atype is GROUP or DEFGROUP, aid will be a group id.
When atype is USERoBJ, GROUPoBJ, CLASSoBJ, OTHER_OBJ, DEF_USERoBJ,
DEFGROUPoB,l, DEFCLASSoBJ, or DEF_OTHER_OBJ, aid will not be used. The
permissions for the entry will be contained in a~erm.

     ₧ 56 -

PROCESSING: When the specified cmd is ACL,CNT, the return value from the
call

will be the number of ACL entries for the filename pointed to by path. The
values of nentries and ac'bitfp will be ignored. ff the user does not pass
the DAC and MAC checks to see the ACL. the act call will fail (see ERRORS
AND RETURNS).

When the specified cmd is ACL_GET. the ACL information for the filename
pointed to by path will be retrieved and the ACL entries will be placed in
the buffer pointed to by actbttjp. The value of nentries is the number of
entries that can be held in the allocated buffer. If the number of ACL
entries in the ACL is greater than the value of ttentries (that is. the
buffer space allocated to hold the files ACL entries is less than nentries
times the size of an entry), the acl call will fail (see ERRORS AND
RETURNS). On success. the return value from this call will be the number
of ACL entries retrieved. On any error. the contents of the acl structures
pointed to by actbufp are indeterminate. If the user does not pass the DAC
and MAC checks to see the ACL, the act call will fail (see ERRORS AND
RETURNS).

When the specified cmd is ACL_SET, ACL entries currently in the buffer
pointed to by actbufp, for the filename pointed to by path, will be set if
all required checks are passed. The contents of nentries shall be the
number of ACL entries in the buffer, pointed to by aclbufp, to be copied.
On success. the return value from this call will be 0. If the invoking
user does not pass the DAC and MAC checks to set an ACL, the act call will
fail (see ERRORS AND RETURNS). If an error occurs, either due to DAC and
MAC checks or the validation check listed below. there will be no change
to the current ACL information. Before the ACL entries are actually set,
validation checks will be performed to determine that the ACL entries are
in the following order:

     a) a user entry for the file owner (USEROBJ),
     b) additional user entries (USER),
     c) a group entry for the file owning group (GROUPOBJ),
     d) additional group entries (GROUP),
     e) a class entry for the file group class (CLASSOBJ),
     f) an entry for other (OTHEROBJ),
     g) default user entry for the file owner (DEFLySEROBJ),
     h) default additional user entries (DEFLSER),
     i) default group entry for the file owning group (DEF,GROUPOBJ),
     j) default additional group entries (DEFGROUP),
     k) default class entry for file group class (DEFCLASSOIBJ).
     l) default entry for other (DEFOTHEROBJ),

The entry in classes a), c), e), and f) must always exist. The entry for
classes a), c)9 e), f), g), i), k), and l) do not use the aid field.
Classes b) and h) may contain zero or more entries and the entries must be
sorted by uid (lowest to highest). Classes d) and j) may contain zero or
more entries and the entries must be sorted by gid (lowest to highest).
(this ordering should be done with the aclsort function).

Class g). h), i), j), k) and l) entries are only applicable for
directories. If an attempt is made to set default ACL entries on a file
that is not a directory, the call will fail (see ERRORS AND RETURNS).

Validation of the ACL will be performed. If entries containing duplicate
uids or gids are found, or there is not exactly one user entry specified
for the file owner, one group entry specified for the file owning group.
one class entry specified for the file group class, and one other entry
specified, or there are no additional user and group entries and the
permissions of the class entry are not equal to the permissions of the
group entry, or there are no additional default:user and default:group
entries and the permissions of the default:class entry is not equal to the
permissions of the default:group entry, the call will fail (see ERRORS AND
RETURNS).

The file owner class permission bits will be changed, such that they are
equal to the permissions specified for the user entry of the file owner in
the ACL. The file group class permission bits will be changed, such that
they are equal to the permissions specified for the class ACL entry. The
file other class permission bits will be changed, such that they are equal
to the permissions specified for the other ACL entry.

This function may be executed on a file system that d~s not support ACLs.
With ACLGET as the cmd it will report the ACL& based on the file
permission bits. With ACLSET as the cmd, if ACL entries are specified
which do not map into the file permission bits, see ERRORS AND RETURNS,
otherwise the file permission bits will be set.

A design may constrain the maximum number of ACL entries that are

written, with a system-wide tunable parameter, aclmax. If the number of

ACL entries exceeds the value of aclmax the function will fail (see ERRORS
AND RETURNS).

ERRORS Ah₧D RETURNS: If the act call is unsuccessful, a value of -I will
be

returned and errno will be set to indicate the error. Only
implementation-independent errnos are presented.

Under the following conditions, the function act will fail and will set
errno to the specified value (note: unless otherwise stated. the errno
applies to ACLCNT, ACLGET, and ACLSET):

ENOTDIR if a component of the path prefix is not a directory

ENOTDIR if an attempt is made to set a default ACL on a file type

other than a directory

ENOENT if a component of the pathname should exist but does not

EACCES if the DAC and/or MAC check fails

EINVAL if cmd is not ACLCNT, ACLGET, or ACLSET

EINVAL if cmd is ACLSET and the ACL entries do not pass the

validation check

ENOSPC if cmd is ACLGET and the space required for the

file's ACL entries exceeds nentries

ENOSPC if cmd is ACL,SET and there is insufficient space

in the file system to store the ACL

EINVAL if the number of acl entries exceeds the value of aclmax

ENOSYS if the file system type does not support ACLs, and

additional entries are specified

aclsort Function

DESCRIPTION: The aclsort function will take as input a buffer containing
ACL

entries (including default ACL entries) and sort them into the correct
order to be accepted by the act or the aclipc function. It will optionally
calculate the maximum permissions needed for the object group class and
set the class ACL entry.

SYNOPSIS:

$include <tbd.h> int aclsort(int nentries, int calclass, struct acl
*aclbufp) Where the value of nentries is the number of ACL entries, the
value of calclass if non-zero indicates to recalculate the class entry,
and aclbufp is a pointer to ACL entry structures.

PROCESSING: A call to aclsort will result in the contents of the buffer
being sorted

in the following order:

     a) a user entry for the object owner,
     b) additional user entries.
     c) a group entry for the object owning group,
     d) additional group entries,
     e) a class entry for the file group class,
     f) an entry for other,
     g) default user entry for the object owner.
     h) default additional user entries,
     i) default group entry for the object owning group.
     j) default additional group entries,
     k) default class entry for the file group class,
     l) default entry for other.

Classes a), c), e), and f) must each have exactly one entry, if not, see
ERRORS AND RETURNS. Classes g), i), k), and l) must have zero or one
entry, if not, see ERRORS AND RETURNS. Entries will be sorted in
increasing order, by user ID in classes b) and h), and by group ID in
classes d) and j). Following sorting, a check will be performed to verify
that no duplicate entries (more than one entry containing the same user ID
or the same group ID) exist. If duplicate entries are found, see ERRORS
AND RETURNS. If there are no entries in classes b) and d). the function
will set the permission field, a~er'n. in the class entry e) to that of
the group entry c). If there are entries in classes b) or d) and the
calclass argument is non-zero, the function will set the permission field,
a~erm, of the class entry to the maximum permission of the entries in the
file group class. Otherwise, the class entry permissions will remain
unchanged.

If there are no entries in classes h) and j), the function will set the
permissions in the default class entry k) to that of the default entry i).
Upon success. aclsort will return the value 0.

ERRORS AND RETURhS: If the aclsort function is unsuccessful due to
duplicate

entries, the return value will be the position (entry number) of the first
duplicate entry. If there is less than one user entry for the object
owner, group entry for the object owning group, class entry for the file
group class. or other entry specified, a value of -1 will be returned. If
there is more than one user entry for the object owner. group entry for
the object owning group, class entry for the file group class. or other
entry specified, they will be treated as duplicate entries, and the return
value will be the position of the duplicate entry.

If the aclsort function is unsuccessful for any other reason, a value of
-1 will be returned.

chmod Function

DESCRIPTION: The chmod function supports the following functionality:

     1. it allows a subject to change the file mode. including the
     permissions for the file owner class. the file group class, and the
     file other class of a file. Note that the chmod command will not
     require any modifications.

SYNOPSIS: No change.

PROCESSING: Any permissions changes made with the chmod command or
function

will update the file permission bits. This includes changing the file
owner ACL entry. the class ACL entry, and the other ACL entry if the
corresponding group(s) of bits are changed by this call. Any additional
ACL entries will not be affected. Note. the permissions granted by such
additional entries are constrained by the file group class permission
bits. If no additional user and no additional group entries exist, the
file group class permission bits will also represent the permissions for
the owning group of the file.

ERRORS AND RETURNS: No change.

OUTPUT: No change.

chown Function

DESCRIPTION: The choivn function supports the following functionality:

     1. it allows a subject to change the owner and/or group of a file.

Note that the chotin system call/command and the chgrp command will not
require any modifications.

SYNOPSIS: No change.

PROCESSING: When the owner of a file is changed, the result will be
identical to

the current behavior. If the owner is changed to a user ID. for which an
additional user entry already exists in the ACL, the additional user entry
is not changed but the user entry for the file owner will take precedence
during an access check. When the group of a file is changed. the result
will be identical to the current behavior. lithe group is changed to a
group ID, for which an additional group entry already exists in the ACL,
the additional group entry is not changed but the group entry for the file
owning group will take precedence during an access check (except in the
case of multiple concurrent groups, where all group entries are given
equal treatment).

ERRORS AND RETURNS: No change.

OUTPUT: No change.

EXAMPLES: The following examples illustrate the operation of the cho\L'n
function.

For each example, there is a "before" state showing the output of getacl,
the chown function that is executed, and the "after" state output.

     EXAMPLE 1:
     BEFORE:
     $ file: filel
     $ owner: larry
     $ group: guest
     user::rwx
     group::r-
     class:r- -
     other:---
     CALL: chown(filel, lisa, demo)

     AFTER:
     $file:filel
     $ owner: lisa
     $ group: demo
     user::rwx
     group::r-
     class:r-
     other:---
     EXAMPLE 2:
     BEFORE:
     $ file: file2
     $ owner: tarry
     $ group: guest
     user::rwx
     user:fred:r-
     group::r-
     group:dev:r-
     class:r-
     other:- -

CALL: chown(file2. lisa demo)

     AFTER:
     $ file: file2
     $ owner: lisa
     $ group: demo
     user::rwx
     user:fred:r- -
     group::r-
     group:dev:r-
     class:r-
     other:---
     EXAMPLE 3:
     BEFORE:
     $ file: file3
     $ owner: larry
     $ group: guest
     user::rwx
     user:lisa:r-
     user:fred:r-
     group::r-
     group:dev:r-
     group:demo:r-
     class:r-
     other:-- -
     CALL: chown(file3. lisa, demo)

     AFTER:
     $ file: file3
     $ owner: lisa
     $ group: demo
     user::rwx
     user:lisa:r-
     user:fred:r-
     group::r-
     group:dev:r-
     group:demo:r-
     class:r-
     other:---

Note in EXAMPLE 3, a user entry contains a user ID that is the same as the
file owner. In this case the file owner entry takes precedence. Also in
EXAMPLE 3, a group entry contains a group ID that is the same as the
owning group of the file. If multiple concurrent groups are not being
used, the object owning group entry takes precedence.

aclipc Function

DESCRIPTION: The actipc call will support the getting and setting of
discretionary

`permission information associated with an IPC o6ject. It will allow the
object owner or a process with appropriate permission or appropriate
privilege to perform the following functions:

     1. get or set an IPC object's ACL information in an atomic operation.
     2. return the number of entries contained in an IPC object's ACL.

Note that this call only supports the IPC objects: e.g., shared memory
segments. semaphores. and message queues. For simplicity, these objects
are referred to as "IPC objects" in the remainder of this description.

SYNOPSIS:

#include <tbd.h> int aclipc(int type, int id, int cmd, int nentries,
struct acl *aclbufp) Three values for type will be supported: IPCSHM,
IPCSEM, and IPCMSG. If type is IPC_$HM, id must be a valid shmid returned
by shmget. If type is IPC,SEM. id must be a valid semid returned by
semget. If type is IPC,MSG, id must be a valid msgid returned by msgget.
Three values for cmd will be supported: ACLSET. ACLGET, and ACL_CNT. The
value of nentries is the number of ACL entries that can fit in the
user-supplied ACL buffer for an ACLGET or the number actually present for
an ACLSET; and aclbufp is a pointer to the user-supplied buffer of ACL
entry structures. The buffer will consist of an array of four (USEROBJ,
GROUPOBJ, CLASSOBJ, and OTHEROBJ entries are required) or more occurrences
of the following structure:

     struct acl (
     intatype;
     uidt aid;
     ushort aperm;

Six values of ajype will be supported to specify the type of entry:

USERoBJ, USER, GROUPOBJ, GROUP, CLASSOBJ, and OTHEROBJ. When ajype is
USER, aid will be a user id, and when ajype is GROUP, aid will be a group
id. When ajype is USEROBJ, GROUPOBJ, CLASSOBJ, or OTHEROBJ, aid will not
be used. The permissions for the entry will be contained in ajerm.

PROCESSING: When the specified cmd is ACLCNT, the return value from the
call

will be the number of ACL entries for the IPC object specified by type and
id. The values of net!tries and aclbufp will be ignored. If the invoking
user does not pass the DAC or MAC checks to see the ACL. the aclipc call
will fail (see ERRORS AND RETURNS).

When the specified cmd is ACLGET the ACL information for the IPC object
specified by type and id will be retrieved and the ACL entries will be
placed in the buffer pointed to by aclbufp. The value of i;entries is the
number of entries that can be held in the buffer. If the number of ACL
entries in the ACL is greater than the value of nentries (the buffer space
allocated to hold the file's ACL entries is less than nentries times the
size of an entry), the aclipc call will fail (see ERRORS AND RETURNS). On
success. the return value from this call will be the number of ACL entries
retrieved. On any error, the contents of the acl structures pointed to by
aclbiifp are indeterminate. If the user does not pass the DAC and MAC
checks to see the ACL, the aclipc call will fail (see ERRORS AND RETURNS).

When the specified cmd is ACLSET, ACL entries currently in the buffer,
pointed to by aclbufp, for the IPC object specified by type and id, will
be set if all required checks are passed. The contents of nentries shall
be the number of ACL entries in the buffer pointed to by aclbufp to be
copied. On success, the return value from this call will be 0. If the
invoking subject does not pass the DAC and MAC checks to set an ACL the
aclipc call will fail (see ERRORS AND RETURNS). If an error occurs, either
due to DAC or MAC checks or the validation check listed below, there will
be no change to the current ACL information. Before the ACL entries are
actually set, validation checks will be performed to determine that the
ACL entries are in the following order:

     a) a user entry for the IPC object owner (USEROBJ),
     b) additional user entries (USER),
     c) a group entry for the IPC object owning group (GR,OUPOBJ),
     d) additional group entries (GROUP),
     e) a class entry for the IPC group class (CLASSOBJ),
     f) an entry for other (OTHEROBJ).

     The entries in class a), c), e), and f) must always exist. The entry
     for class a),

c), e), and f) do not use the aid field. Class b) may contain zero or more
entries and the entries must be sorted by uld (lowest to highest). Class
d) may contain zero or more entries and the entries must be sorted by gid
(lowest to - highest). (this ordering should be done with the aclsort
function). Validation of the ACL will be performed. If entries containing
duplicate ttids or gids are found. or there is not exactly; one user entry
for the object owner. one group entry for the object owning group. one
class entry for the IPC group class, or one other entry specified. or
there are no additional user and group entries and the permissions of the
class entry are not equal to the permissions of the group entry, the call
will fail (see ERRORS AND RETL'RNS). The IPC owner permission bits will be
changed, such that they are equal to the permissions specified for the
user entry of the object owner in the ACL. The IPC group class permission
bits will be changed, such that they are equal to the permissions
specified for the class ACL entry. The IPC other class permission bits
will be changed, such that they are equal to the permissions specified for
the other ACL entry.

A design may constrain the maximum number of ACL entries that are written,
with a system-wide tunable parameter, aclmax. If the number of ACL entries
exceeds the value of aclmax the function will fail (see ERRORS AND
RETURNS).

ERRORS AND RETURNS: If the aclipc call is unsuccessful, a value of -I will
be

returned and errno will be set to indicate the error. Only
implementation-independent errnos are presented.

Under the following conditions, the function aclipc will fail and will set
errno to the specified value (note: if cmd is unspecified, the errno
applies to ACLCNT, ACLGET, and ACLSET):

EINVAL if type is not IPC,SHM, IPC,SEM, or IPC,MSG

EINVAL if the value of id is (1) not a valid messagequeueldentifier and

the type was IPCMSG, (2) not a valid semapho~e1dentifier and

the type was IPCSEM, or (3) not a valid sharedmemory,dentifier

and the type was IPCSHM

EINVAL if cmd is not ACLCNT, ACL_GET, or ACLSET

EINVAL if cmd is ACLSET and the ACL entries do not pass

the validation check

EACCES if the DAC and/or MAC check fails

ENOSPC if cmd is ACLGET and the space required for the

IPC's object ACL entries exceeds nentries

ENOMEM if cmd is ACL_SET and there is insufficient

space to store the ACL

EINVAL if the number of acl entries exceeds the value of aclmax

shmctl, semctl, & msgctl Functions

DESCRIPTION: The shmctl. semctl, and msgctl functions support the
following

functionality:

     1. they allow a subject to change the user ID. group ID, and
     permissions on IPC objects.

SYNOPSIS: No change.

PROCESSING: No change.

ERRORS AND RETURNS: No change.

REFERENCES

[1] Department of Defense Trusted Computer Systems Evaluation Criteria.
DoD 5200.28-STD, December 1985.

[2] National Computer Security Center, A Guide to Understanding
Discretioiiary Access Control in Trusted Systems, NCSC-TG-003 Version-I,
September 1987. [3] UNIX System Access Control List Proposal, C. Rubin,
AT&T. May 15. 1988. [4] Adding Access Control Lists To UNIX, A.
Silverstein, B. McMahon. G. Nuss, Hewlett-Packard Co.. March 12. 1988.

[5] Discretionary Access Control System Functions, D. H. Steves, IBM,
March 14, 1988.

[6] P1003 .6 Security Extension Proposal: Discretionary Access Control
Semantics, W. Olin Sibert. Oxford Systems Inc., May 18. 1988.

[7] PlOO3.6 Supplementary Document: Discretionary Access Control..
Problems in P1003.1 Draft 12 , W. Olin Sibert, Oxford Systems Inc.. May
18, 1988. [8] P1003.6 Supplementary Document: Comments on Hewlett-Packard
ACL Proposal, W. Olin Sibert. Oxford Systems Inc., May 18,1988.

[9] Extending The UNIX Protection Model with Access Control Lists, G.
Fernandez, L. Allen, Apollo Computer Inc., June 1988.

[10] On Incorporating Access Control Lists into the UNIX Operating System
S. M. Kramer, SecureWare Inc., June 1988.

[11] Trusted UNIX Discretionary Ac~ss and Privilege Control Mechanisms,
B.D.

Wilner, Infosystems Technology Inc.. June 2,1988.

(12] Access Control List Design, Hewlett Packard, October 21, 1988.

[13] Proposal for Adding Access Control Lists to POSIX, P. B. Flinn,
SecureWare Inc., July 25, 1988.

[14] Discretionary Access Control Proposal, H. L. Hall, Digital Equipment
Corporation, Oct. 1988.

[15] Portable Operating System Interface for Computer Environments IEEE
Std. 1003.1-1988