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Date: Tue, 22 Apr 2014 10:43:59 -0500
From: "jfoug" <jfoug@....net>
To: <john-dev@...ts.openwall.com>
Subject: New functionality added to bleeding (and a few changes we as devs need to make)
There is a new memory debugging module that has recently been promoted into
jumbo-bleeding branch of JtR.
Any new .c files (with very few exceptions) should include this file:
#include "memdbg.h" as the LAST include. With this simple change, all
normal memory allocation and free calls 'can' be tracked, and upon program
exit, any non-freed items will be listed. Also during runtime, there are
checks for things like memory buffer overrun or underrun.
By DEFAULT (as the source comes from the git tree), all memory debugging
within memdbg will be turned off. To turn this on (there IS a performance
hit, and for some things, that hit can be a bit), simply edit the
memdbg_defines.h file. That file should be commented well enough to show
which lines to uncomment to make things work. I would recommend only
uncommenting MEMDBG_ON for most work. The MEMDBG_EXTRA_CHECKS does a lot
more, BUT it does not free memory (until program exit). There are formats
which allocate during their runtime. Those formats will run a system out of
memory very quickly, and should not be run using these extra checks.
There are also a few other macros to use. These are mostly used in 'main'
program files (i.e. a main that starts, parses command line, and exits, like
all of the *2john tools). In cases like this, there is another macro to add
to the end of the main() function. That is
MEMDBG_PROGRAM_EXIT_CHECKS(stderr) (or any file handle). This will be a
NOOP, if the MEMDBG_ON is not set. But if it is set, then it will be a
function that checks the state of memory, to make sure things were cleaned
up properly.
There are also more functions in there, which can be used. There are things
like MEMDBG_getSnapshot and MEMDBG_checkSnapshot allow you to set a check
around a function call, to see if something in it leaks. NOTE, this will
only work with NATIVE compiled code. You will not detect any leaks from any
included library calls, since the library was not built using memdbg. So
code like this
mem_handle = MEMDBG_getSnapshot(0);
call_Some_function();
// Do other code.
MEMDBG_getSnapshot(mem_handle);
There are also some functions which do NOT use memdbg logic no matter what.
These are:
libc_free() libc_malloc() and libc_calloc(). These will all through and not
use the memdbg functions at all. There are times when this MUST be done.
For instance, if a library call allocates memory which is defined by the
library as needing cleaned up by calling free to clean up that memory. This
is usually BAD practice for lib writers. They should provide a free function
to clean up their memory. This allows a library to be used, where the memory
allocator is NOT the default malloc/free (which we get with memdbg). Not
all libs are created that way, so if you call str=do_something(input); and
then str later needs to be freed using free(), then in jtr, we should use
the libc_free() call to free that memory.
I am sure there will be a question here and there about the memory debugging
code. It IS very helpful code. It has found quite a few leaks over the
past couple years. However, this has always been in a parallel branch by
itself, until magnum recently put this into the main bleeding trunk.
Again, the easiest way to go, is to simply ignore that it is there, BUT to
add a #include "memdbg.h" as the last include in your new .c files (and a
call to MEMDBG_PROGRAM_EXIT_CHECKS(stderr) to the end of a main() function).
That is all which is needed.
I originally wrote this back in the 90's. It has caught 1000's of memory
leaks, buffer over and under writes, etc. There were other 'tools' that
came out later which do much the same thing (bound checker, valgrind, etc).
The nice thing about this code, is you simply edit 1 line in a header,
rebuild, and get all (most) of the memory checking, without having to have
some external tool to do this checking. My original code was for C++. I
rewrote it to C, and put it in pub domain. Enjoy.
NOTE, memdbg_defines.h is in the .gitignore file. Any modifications you
make to your local copy will not be propagated back to the git repository.
Jim.
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