Elia PintoRecent versions of Linux libc (later than 5.4.23) and glibc (2.x) include a malloc() implementation which is tunable via environment variables. When MALLOC_CHECK_ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of free() with the same argument, or overruns of a single byte (off-by-one bugs). When MALLOC_CHECK_ is set to 3, a diagnostic message is printed on stderr and the program is aborted.
Setting the MALLOC_PERTURB_ environment variable causes the malloc functions in libc to return memory which has been wiped and clear memory when it is returned. Of course this does not affect calloc which always does clear the memory.
The reason for this exercise is, of course, to find code which uses memory returned by malloc without initializing it and code which uses code after it is freed. valgrind can do this but it's costly to run. The MALLOC_PERTURB_ exchanges the ability to detect problems in 100% of the cases with speed.
The byte value used to initialize values returned by malloc is the byte value of the environment value. The value used to clear memory is the bitwise inverse. Setting MALLOC_PERTURB_ to zero disables the feature.
This technique can find hard to detect bugs. It is therefore suggested to always use this flag (at least temporarily) when testing out code or a new distribution.
But the test suite can use also valgrind(memcheck) via 'make valgrind' or 'make GIT_TEST_OPTS="--valgrind"'.
Memcheck wraps client calls to malloc(), and puts a "red zone" on each end of each block in order to detect access overruns. Memcheck already detects double free() (up to the limit of the buffer which remembers pending free()). Thus memcheck subsumes all the documented coverage of MALLOC_CHECK_.
If MALLOC_CHECK_ is set non-zero when running memcheck, then the overruns that might be detected by MALLOC_CHECK_ would be overruns on the wrapped blocks which include the red zones. Thus MALLOC_CHECK_ would be checking memcheck, and not the client. This is not useful, and actually is wasteful. The only possible [documented] advantage of using MALLOC_CHECK_ and memcheck together, would be if MALLOC_CHECK_ detected duplicate free() in more cases than memcheck because memcheck's buffer is too small.
Therefore we don't use MALLOC_CHECK_ and valgrind(memcheck) at the same time.
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com> --- This the third reroll of the original patch.
I redid the patch correcting the export command in a more portable way thanks to the Junio observation and not setting MALLOC_CHECK_ at the same time we are using valgrind. Added in the commit the reason, not so simple to find. Hope the better :=)
t/test-lib.sh | 9 +++++++++ 1 file changed, 9 insertions(+)
Show changes to t/test-lib.sh +9 −0
diff --git a/t/test-lib.sh b/t/test-lib.sh index 78c4286..f34b861 100644 --- a/t/test-lib.sh +++ b/t/test-lib.sh @@ -93,6 +93,15 @@ export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME export EDITOR +# Add libc MALLOC and MALLOC_PERTURB test +# only if we are not executing the test with valgrind +expr "$GIT_TEST_OPTS" : ".*\(--valgrind\)" >/dev/null || { + MALLOC_CHECK_=3 + export MALLOC_CHECK_ + MALLOC_PERTURB_="$( expr \( $$ % 255 \) + 1)" + export MALLOC_PERTURB_ +} + # Protect ourselves from common misconfiguration to export # CDPATH into the environment unset CDPATH
-- 1.7.10.4
