[Python-Dev] Python under valgrind (original) (raw)
Hrvoje Niksic hrvoje.niksic at avl.com
Fri Nov 28 13:38:14 CET 2008
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A friend pointed out that running python under valgrind (simply "valgrind python") produces a lot of "invalid read" errors. Reading up on Misc/README.valgrind only seems to describe why "uninitialized reads" should occur, not invalid ones. For example:
$ valgrind python [... lots of output ...] ==31428== Invalid read of size 4 ==31428== at 0x808EBDF: PyObject_Free (in /usr/bin/python2.5) ==31428== by 0x810DD0A: (within /usr/bin/python2.5) ==31428== by 0x810DD34: PyNode_Free (in /usr/bin/python2.5) ==31428== by 0x80EDAD9: PyRun_InteractiveOneFlags (in /usr/bin/python2.5) ==31428== by 0x80EDDB7: PyRun_InteractiveLoopFlags (in /usr/bin/python2.5) ==31428== by 0x80EE515: PyRun_AnyFileExFlags (in /usr/bin/python2.5) ==31428== by 0x80595E6: Py_Main (in /usr/bin/python2.5) ==31428== by 0x8058961: main (in /usr/bin/python2.5) ==31428== Address 0x43bf010 is 3,112 bytes inside a block of size 6,016 free'd ==31428== at 0x4024B4A: free (vg_replace_malloc.c:323) ==31428== by 0x8059C07: (within /usr/bin/python2.5) ==31428== by 0x80EDAA5: PyRun_InteractiveOneFlags (in /usr/bin/python2.5) ...
valgrind claims that Python reads 4 bytes inside a block on which free() has already been called. Is valgrind wrong, or is Python really doing that? Googling revealed previous reports of this, normally answered by a reference to README.valgrind. But README.valgrind justifies reading from ununitialized memory, which doesn't help me understand how reading from the middle of a block of freed memory (more precisely, memory on which the libc free() has already been called) would be okay.
I suppose valgrind could be confused by PyFree's pool address validation that intentionally reads the memory just before the allocated block, and incorrectly attributes it to a previously allocated (and hence freed) block, but I can't prove that. Has anyone investigated this kind of valgrind report?
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