===============================================================================
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===============================================================================
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 IRIX6,
and only for O32 and N32 object styles.
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.
Please send all questions and comments to me, Don Hatch (hatch@sgi.com).
===============================================================================
Recent changes:
Wed Apr 30 15:50:26 PDT 1997
4/30/97
Fix bogus complaint of corruption when freeing memory
allocated with memalign.
Now recognizes MALLOC_SIZE_OF_INTEREST environment variable.
10/25/96
Fixed bug in histogram lookup where
frees were recorded as mallocs when stack trace depth = 0
4/6/96 Fixed install problem where the N32 .a's were
getting installed in /usr/lib instead of /usr/lib32,
clobbering the O32 versions.
3/29/96 Now recognizes environment variables MALLOC_TRACE
and MALLOC_TRACE_SIGNAL which can be used to set
and toggle malloc call tracing.
2/9/96 Installable images now contain a real N32 library
(the N64 version is still a stub). Stack tracing
is not attempted at all in the N32 version.
12/15/95
Installed images now include cshrc.dmalloc,
which is a file of example aliases to put in .cshrc
for turning dmalloc on and off.
Now recognizes environment variables MALLOC_RESET_SIGNAL,
MALLOC_CHECK_SIGNAL, MALLOC_INFO_SIGNAL which
can control a program's leak detection via signals;
see below.
11/23/95 Installable images now include N32 and N64 stubs
so that _RLD_LIST can include libdmalloc.so all the time.
Removed overly verbose output when mallocblksize
is called, to make inst and swmgr more friendly.
Added a workaround for bus errors in IRIX 6.2
that occur when accessing addresses near the end of the stack.
11/21/95 Now checks all amalloc arenas. (implements adelete as a no-op).
Fixed to work on the R8000.
4/13/95 The environment variable MALLOC_STACKTRASH can be used
to trash the stack on startup, to break programs that
depend on it being 0's; see below.
9/20/94 Now checks the malloc chain during execve() by default.
This option can be turned off or made more verbose
by using the MALLOC_CHECK_ATEXEC environment variable;
see below.
===============================================================================
WHAT LIBDMALLOC DOES
libdmalloc.a contains the following standard functions:
malloc
free
realloc
calloc
cfree
_execve
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() and execve(), 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 -lmpc -lexc -lmld -lmangle -init _malloc_init
(the -init part is not necessary, but it makes initialization
happen earlier, which can catch some bugs).
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_INFO_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_BAD_ASSERT (default unset)
If set, cause assert (cause a core dump) in the malloc_bad
function. Useful to find memory problems in long running code.
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_SIZE_OF_INTEREST (default=-1)
If set to a non-negative integer n, all mallocs of size n
will call the no-op function malloc_of_interest().
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_CHECK_ATEXEC (default=1)
If this variable is nonzero (as it is by default),
all malloced blocks will be checked for overflow and underflow
corruption during execve().
If set to a value >=2, a message will be printed
to stderr saying the name of the program being exec'ed.
If >= 3, the args will be printed too.
If >= 4, the args will be quoted.
MALLOC_CHECK_SIGNAL
If set, sending the process a SIGUSR2 signal (by default)
will cause a call to malloc_check().
If set to a value, the value will be used instead of SIGUSR2.
See also MALLOC_INFO_SIGNAL, MALLOC_RESET_SIGNAL, MALLOC_TRACE_SIGNAL.
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_INFO_SIGNAL
If set, sending the process a SIGUSR2 signal (by default)
will cause a call to malloc_check() and malloc_info(1,-1).
If set to a value, the value will be used instead of SIGUSR2.
See also MALLOC_CHECK_SIGNAL, MALLOC_RESET_SIGNAL, MALLOC_TRACE_SIGNAL.
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_RESET_SIGNAL
If set, sending the process a SIGUSR1 signal (by default)
will cause a call to malloc_check() and malloc_reset().
If set to a value, the value will be used instead of SIGUSR1.
See also MALLOC_CHECK_SIGNAL, MALLOC_INFO_SIGNAL, MALLOC_TRACE_SIGNAL.
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_STACKTRASH
If set, 4096 bytes (by default) of the stack will be set to 3's
(by default) to break programs that depend on it being 0's.
setenv MALLOC_STACKTRASH <n> will set <n> bytes to 3.
setenv MALLOC_STACKTRASH <n>,<v> will set <n> bytes to <v>.
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_TRACE (default=0)
If set, a message will be printed to stderr on every call to
malloc, free, realloc, amalloc, afree, and arealloc.
This can be changed at runtime; see MALLOC_TRACE_SIGNAL.
MALLOC_TRACE_SIGNAL
If set, sending the process a SIGUSR1 signal (by default)
will toggle the malloc_trace flag (see MALLOC_TRACE).
If set to a value, the value will be used instead of SIGUSR1.
See also MALLOC_RESET_SIGNAL, MALLOC_CHECK_SIGNAL, MALLOC_INFO_SIGNAL.
_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.
