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Text File | 1994-08-02 | 18.2 KB | 359 lines

~4Dgifts/toolbox/public/libdmalloc README Libdmalloc is a debugging malloc library I wrote with ideas I stole from Conor P. Cahill's dbmalloc, Brandyn Webb's malloc-debug, Kipp Hickman's "leaky", Purify, and sgi's old dbx source. It is not a finished product. Currently the only environment in which it works is IRIX5 and maybe IRIX4 (the only problem with IRIX4 should be benign complaints regarding getwd). I believe it is MP-safe. It's very useful for finding memory corruption quickly in many different programs, since it has little overhead and does not require programs to be compiled in any special way. It uses the ability to plug in different a malloc library using DSOs. Please send all questions and comments to me, Don Hatch (hatch@sgi.com). =============================================================================== WHAT LIBDMALLOC DOES libdmalloc.a contains the following standard functions: malloc free realloc calloc cfree These are actually wrappers for the functions from libmalloc.a, which are also included in libdmalloc, but with disguised names mAlLoC, fReE, etc. The wrapper functions maintain malloc statistics, and do the following other good stuff: -- Initialize newly malloced memory to 1's, to break programs that depend on it being 0's -- Fill freed memory with 2's, to break programs that look at freed memory. -- free() and realloc() do sanity checks to make sure the area immediately surrounding the memory has not been modified (8 bytes or so on either side). If corruption is detected, it attempts to print a current stacktrace and also a stacktrace of the original malloc if possible. -- during exit(), all malloced memory is checked for corruption. =============================================================================== LINKING A PROGRAM WITH LIBDMALLOC There are two ways to link libdmalloc into your program. The first is simply to tell rld to do it at runtime: setenv _RLD_LIST /usr/tmp/libdmalloc.so:DEFAULT Then any non-setuid dynamic executable you run (use the 'file' program to determine whether an executable is dynamic or not) will use libdmalloc. [[ Hint: if you will want to run dbx on a stripped executable, dbx will work better (i.e. be able to make interactive calls) if your _RLD_LIST also contains crt1.so. You can make one of these as follows: ld -shared /usr/lib/crt1.o -o /usr/tmp/crt1.so setenv _RLD_LIST /usr/tmp/libdmalloc.so:/usr/tmp/crt1.so:DEFAULT ]] The above runtime-linking method will give the full memory-corruption detection power of libdmalloc, but the stack tracing routines will not work (and may core dump). So if you want to use libdmalloc to detect leaks, or if you want libdmalloc to give you a stack trace of the original call to malloc when corruption is detected, you will have to link the program with libdmalloc.a and its dependent libraries. The arguments to give the linker are: libdmalloc.a -lmld -lexc -lmpc -lmangle Try to link with .a's rather than .so's for the rest of the libraries wherever possible; the stack tracing routines really are not robust inside DSOs. =============================================================================== MEMORY CORRUPTION DETECTION CORE DUMPS THAT DON'T OCCUR WITH THE REGULAR MALLOC If this happens, it probably means the program is using uninitialized or freed memory; libdmalloc intentionally fills such regions with a fill pattern to break such programs. If you are running a debugger on a program linked with libdmalloc and you see the value "^A^A^A^A^A..." or 0x1010101 or 16843009 it usually means you are looking at an uninitialized malloced area; if you see "^B^B^B^B^B..." or 0x2020202 or 33686018, it usually means you are looking at an area that has been freed already. UNDERFLOWS AND OVERFLOWS When libdmalloc reports "overflow" corruption at a malloced address, it means data has been illegally written past the end of the array. Likewise "underflow" corruption means data has been illegally written in the region immediately preceding the array. libdmalloc checks at least the 8 bytes immediately preceding and following the malloced array for underflow and overflow corruption when the array is free()d or realloc()ed, and all malloced arrays are checked during exit(). When corruption is detected, error messages get sent to stderr. Error messages for overflows and underflows look something like this: % oawk /./ /dev/null oawk(24863): ERROR: underflow corruption detected during free at malloc block 0x100d8fb0 If you have compiled the program with libdmalloc.a and the offending mallocs and frees aren't called from within a DSO, you may be able to get libdmalloc to give you stack traces of the free() and the original malloc(): % unsetenv _RLD_LIST % setenv MALLOC_STACKTRACE_GET_DEPTH 10 % oawk /./ /dev/null oawk(24879): ERROR: underflow corruption detected during free at malloc block 0x100d8fb0 0 free() [dmalloc.c:892, 0x418000] 1 freetr() [b.c:99, 0x40ccd0] 2 freetr() [b.c:103, 0x40cd20] 3 freetr() [b.c:108, 0x40cd5c] 4 freetr() [b.c:103, 0x40cd20] 5 makedfa() [b.c:65, 0x40cae8] 6 yyparse() [awk.g.y:181, 0x40ac6c] 7 main() [main.c:132, 0x40e984] 8 __start() [crt1text.s:133, 0x409bb0] This block may have been allocated here: 0 add() [b.c:284, 0x40d598] 1 cfoll() [b.c:167, 0x40d07c] 2 cfoll() [b.c:173, 0x40d0d0] 3 cfoll() [b.c:177, 0x40d0ec] 4 cfoll() [b.c:173, 0x40d0d0] 5 makedfa() [b.c:62, 0x40cab8] 6 yyparse() [awk.g.y:181, 0x40ac6c] 7 main() [main.c:132, 0x40e984] 8 __start() [crt1text.s:133, 0x409bb0] If not (e.g. if unshared libraries are not available, or you don't want to recompile) you can probably still track down the original malloc in the debugger. -- To stop the program when corruption is detected, set a breakpoint in malloc_bad(). -- To stop the program when malloc is about to fail (return 0), set a breakpoint in malloc_failed(). -- To stop the program when malloc is about to return a particular address (say 0x100d8fb0, as in the above example) setenv MALLOC_BLOCK_OF_INTEREST 0x100d8fb0 and set a breakpoint in malloc_of_interest(). -- If while running a program in the debugger you want to know whether a given malloced array has overflowed or underflowed yet, call malloc_isgoodblock(addr). (See the above hint about using crt1.so to enable dbx to make interactive calls if you are debugging a stripped program). To check all malloced blocks at once, call malloc_check(); the returned value will be 0 for success, or -1 (with error messages to stderr) if corruption is detected. malloc_check() is called automatically during exit(). [[ Hint: if you are using the _RLD_LIST variable and the main program is stripped, dbx probably won't be able to find symbols in libdmalloc.so until the program is actually running. One way to do this is to set a breakpoint in getenv, run the program until it stops in getenv (dbx will seg fault, but don't worry, it's okay :-)), and then delete the breakpoint; then the desired symbols should be visible. ]] SUPPRESSING ERROR MESSAGES Certain known malloc overflow bugs always appear at a constant address in a program; the MALLOC_SUPPRESS environment variable can be used to suppress error messages about such bugs. For example, if you are tired of seeing the following messages: strings(24675): ERROR: overflow corruption detected during exit at malloc block 0x100010d0 (4 bytes) CC(24678): ERROR: overflow corruption detected during exit at malloc block 0x1001b9d8 (5 bytes) you can do the following: setenv MALLOC_SUPPRESS " \ CC:0x1001b9d8 \ /bin/CC:0x1001b9d8 \ /usr/bin/CC:0x1001b9d8 \ strings:0x100010d0 \ /bin/strings:0x100010d0 \ " Note that there is a separate entry for each likely value of argv[0]. Note also that libdmalloc may not be able to determine argv[0] if main() has overwritten it, so suppression may not always work. =============================================================================== LEAK DETECTION [[ XXX This section is slightly out-of-date; I haven't tried this stuff in a while, and there may be better ways to do things now. In particular, the new MALLOC_CHECK_ATEXIT environment variable is probably a good way to look for leaks during exit(). CaseVision probably does a better job of all this anyway. ]] libdmalloc also contains the additional functions and variables, defined in "dmalloc.h": void malloc_reset(); Sets all counts to zero. void malloc_info(int nonleaks_too, int stacktrace_print_depth); Prints out stacktraces of all leaks that have occurred since the last call to malloc_reset() (actually prints a histogram indexed by stack trace). If nonleaks_too is nonzero, the stacktraces of all mallocs and frees (not just leaks) will be printed. The stacktrace_print_depth argument specifies how much of each stacktrace you want to see; -1 means the entire trace (which is limited by the malloc_stacktrace_get_depth variable, see below). If the environment variable MALLOC_INFO_ATEXIT is set, then malloc_info(0,-1) will be called during exit(). void malloc_info_cleanup(); Frees all resources opened by malloc_info(). malloc_info() reads the symbol table of the executable object file (and all shared objects on which the executable depends) the first time it needs to look up a function name, file name and line number from a pc address; this is very time-consuming, so it leaves these files and symbol tables open. malloc_info_cleanup() attempts to close them and free the space. NOTE: as of this writing, libmld leaks like a sieve (see incident #176726) so malloc_info_cleanup may not be very effective. void malloc_failed(); This no-op function gets called whenever malloc fails for any reason. It exists solely for breakpoint debugging. int malloc_stacktrace_get_depth; This integer controls how deep a stacktrace to get and store at each malloc and free. Its initial value is 0, or the value of the environment variable MALLOC_STACKTRACE_GET_DEPTH if set. Applications may change this value at any time. There is a hard-coded max depth of 10 (to change this, recompile this library with -DMAX_STACKTRACE_DEPTH=20 or whatever). Values less than 0 or greater than the max depth are taken to mean the max depth. int malloc_fillarea; If nonzero, then newly malloced memory will be initialized to 1's and newly freed memory will be filled with 2's. The initial value of this variable is nonzero. I recommend not changing it. void malloc_init_function(); This is a no-op function that gets called at the beginning of the very first call to malloc(). It is designed to be overridden by applications if they wish to do things before the first malloc (putting code at the top of main() is not sufficient, since malloc can get called by pre-main initialization routines). For example, libmallocGadget's version of this function looks for the environment variables MALLOC_GADGET_STACKTRACE_GET_DEPTH and MALLOC_GADGET_M_KEEP and sets malloc_stacktrace_get_depth and mallopt() appropriately. Note that this overrides env MALLOC_STACKTRACE_GET_DEPTH; (i.e. MALLOC_STACKTRACE_GET_DEPTH has no effect when using the malloc gadget). =============================================================================== EXAMPLE OF FINDING LEAKS USING LIBDMALLOC AND DBX: Compile the program using the libraries specified above. % dbx leaky_program (dbx) when in main { assign malloc_stacktrace_get_depth = -1 } (make sure stacktraces are gathered on each malloc and free) (dbx) stop at 10 (break prior to suspected leak) (dbx) stop at 20 (break after suspected leak) (dbx) run Process 2916 (leaky_program) stopped at leaky_program.c:10 (dbx) ccall malloc_info(0,0) (make sure symbol table is loaded before reset, so it doesn't interfere with the statistics) (dbx) ccall malloc_reset() (reset all counts to 0) (dbx) cont Process 2916 (leaky_program) stopped at leaky_program.c:20 (dbx) ccall malloc_info(0,0) (print leaks since last reset) (dbx) ccall malloc_info(0,-1) (print leaks since last reset, with full stacktraces) (dbx) ccall malloc_info(1,0) (print all mallocs & frees since last reset) (dbx) ccall malloc_info(1,3) (print all mallocs & frees since last reset, with stacktraces shown to depth of 3) =============================================================================== ENVIRONMENT VARIABLES LIBDMALLOC LOOKS AT MALLOC_BLOCK_OF_INTEREST (default=0) If set to an address, libdmalloc will call the no-op function malloc_of_interest() during all mallocs which return the given address. This may be useful for finding the original malloc of a corrupt block in a deterministic program; simply setenv MALLOC_BLOCK_OF_INTEREST to be the address given in the corruption error message, and use the debugger to stop in malloc_of_interest(). MALLOC_CALL_OF_INTEREST (default=-1) If set to a non-negative integer n, the n'th call to malloc will call the no-op function malloc_of_interest(). This can be used if you wish to stop in the n'th call to malloc() in the debugger. MALLOC_CHECK_ATEXIT (default=1) If this variable is nonzero (as it is by default), all malloced blocks will be checked for overflow and underflow corruption during exit(). If set to a value >= 2, a message will be printed to stderr during this check, something like: ls(24659): Checking malloc chain at exit...done. This will keep you informed of which programs you are running are actually using libdmalloc. Keep in mind, however, that some programs (e.g. p_finalize and inst) will abort if they detect any stderr output from a child process. MALLOC_FILLAREA (default=1) If nonzero (as it is by default), malloc() and realloc() will fill all uninitialized bytes with the value of this variable, and free() and realloc() will fill all free'd bytes with 2's. MALLOC_INFO_ATEXIT (default=0) If this variable is set, malloc_info(0,-1) will be called during exit(). MALLOC_PROMPT_ON_STARTUP If set, during the first call to malloc(), the following message will be sent to /dev/tty: <commandname>(<process-id>): hit return to continue and one character will be read from /dev/tty before continuing. This may be useful for identifying "mystery programs" that are being run from other programs. MALLOC_STACKTRACE_GET_DEPTH (default=0) Initializes the value of the malloc_stacktrace_get_depth variable, which controls the depth of the stack trace to store during each malloc. See "malloc_stacktrace_get_depth" above for more info. MALLOC_SUPPRESS Use this variable to suppress messages coming from a particular program at a particular address. See "SUPPRESSING ERROR MESSAGES" above for details. _MALLOC_TRY_TO_PRINT_STACKTRACES If set, an attempt will be made to print stacktraces on error, even if _RLD_LIST is set. (Usually stack traces are not attempted if _RLD_LIST is set, since _RLD_LIST usually means libdmalloc.so is being used, which means stack tracing will probably core dump). _MALLOC_DONT_TRY_TO_PRINT_STACKTRACES If set, no attempt will be made to print stacktraces on error, even if _RLD_LIST is not set. (see _MALLOC_TRY_TO_PRINT_STACKTRACES). _STACKTRACE_ARGV0 If set, the function stacktrace_get_argv0() will return the value of this variable, rather than trying to get the value of argv[0] from the stack. This may be useful for programs whose main() overwrites argv.