Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/sources/unix/volume25/msgd/mesg/mesg.c

C/C++ Source or Header | 1992-03-09 | 4.4 KB | 175 lines

/*
 * Copyright (c) 1987 Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms are permitted
 * provided that the above copyright notice and this paragraph are
 * duplicated in all such forms and that any documentation,
 * advertising materials, and other materials related to such
 * distribution and use acknowledge that the software was developed
 * by the University of California, Berkeley.  The name of the
 * University may not be used to endorse or promote products derived
 * from this software without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 */

#ifndef lint
char copyright[] =
"@(#) Copyright (c) 1987 Regents of the University of California.\n\
 All rights reserved.\n";
#endif /* not lint */

static char *sccsid = "@(#)mesg.c    4.2 (Berkeley) 10/18/80";
/*
 * mesg -- set current tty to accept or
 *    forbid write permission.
 *
 *    mesg [yn] "answerback message"
 *        y allow messages
 *        n forbid messages
 *        answerback is what the other guy gets told (Waterloo)
 *        needs to be setuid root to get rid of other people's
 *        old answerback message files
 *
 */

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <utmp.h>
#include <pwd.h>

#define ANSWERBACK_FMT    "/usr/tmp/mesg.%s"

main(argc, argv)
    char **argv;
{
    int uid, mode, r=0;
    char *tty, *p, *rindex(), *ttyname();
    FILE *fp;
    int i_own_answerback = 0;
    struct stat sbuf;
    struct passwd *pw;
    char answerback[200];

    (void) umask(022);    /* so we can read answerback file */
    tty = ttyname(2);
    if (tty == 0)
        error("Cannot determine name of current terminal", (char *)0);
    if ( (p=rindex(tty,'/')) != NULL )
        p++;
    else
        p = tty;
    /*
     * If this isn't a login session, use of mesg is meaningless.
     */
    uid = getuid();
    pw = getpwuid(uid);
    if (pw == NULL) {
        fprintf(stderr, "mesg: Can't find your uid (%d) in the password file\n", uid);
        exit(2);
    }
    if (!in_utmp(pw->pw_name, p)) {
        fprintf(stderr, "mesg: Current terminal session on %s is not a login session\n", tty);
        exit(2);
    }
    /* find name of answerback file */
    (void) sprintf( answerback, ANSWERBACK_FMT, p );
    /* Now make sure that if the answerback file exists, it belongs to
       the current user */
    if ( lstat( answerback, &sbuf ) == 0 ) {
        if (sbuf.st_uid == uid)
            i_own_answerback = 1;
        else
            (void) unlink(answerback);
    }
    if(stat(tty, &sbuf) < 0) error("cannot stat", tty);
    /*
     * must be root before here so we can get rid of other persons
     * answerback file since sticky bit is on tmp directory.
     * must be real user after here so he cant read/write *any* file
     * by creating a link in the tmp directory.
     */
    setuid(uid);
    if(argc < 2) {
        if(sbuf.st_mode & 020)
            printf( "is y\n" );
        else {    r=1;
            printf( "is n\n" );
        }
        if (i_own_answerback) {
            if ( (fp=fopen(answerback,"r")) != NULL ) {
                int c;
                while ( (c=getc(fp)) != EOF )
                    putc( c, stdout );
                (void) fclose( fp );
            } else
                error("Can't read answerback file", answerback);
        }
    } else {
        if (i_own_answerback)
            (void) unlink(answerback);
        if ( argv[1][0] == 'y' ) {
            mode = sbuf.st_mode | 020;
        } else if ( argv[1][0] == 'n' ) {
            mode = sbuf.st_mode & ~022;
            r = 1;
        } else
            error("usage: mesg [yn] [answerback message]", (char *)0);
        if (chmod(tty, mode) < 0)
            error("cannot change modes", tty);
        if ( argc > 2 ) {    /* write answerback message */
            int i;
            if ( (fp=fopen(answerback,"w")) == (FILE *)NULL )
                error("Couldn't write answerback file", answerback);
            for ( i=2; i<argc; i++ ) {
                if ( i>2 )
                    putc( ' ', fp );
                fputs( argv[i], fp );
            }
            putc( '\n', fp );
            (void) fclose( fp );
        }
    }
    exit(r);
}

error(s, arg)
char *s, *arg;
{
    fprintf(stderr,"mesg: %s", s);
    if (arg)
        perror(arg);
    else
        putc('\n', stderr);
    exit(2);
}

/*
 * Does the given (user,tty) pair appear in the utmp file?
 */
in_utmp(user, tty)
    char *user, *tty;
{
    FILE *fp;
    struct utmp utmp;

    if ((fp = fopen("/etc/utmp", "r")) == NULL)
        return 0;
    while (fread((char *)&utmp, sizeof utmp, 1, fp) == 1) {
        if (strncmp(user, utmp.ut_name, sizeof utmp.ut_name) == 0 &&
            strncmp(tty, utmp.ut_line, sizeof utmp.ut_line) == 0) {
#ifdef USER_PROCESS
            if ( utmp.ut_type != USER_PROCESS )
                continue;
#endif
            fclose(fp);
            return 1;
        }
    }
    fclose(fp);
    return 0;
}