Mailing List Archive

On Tue, Jan 30, 2007 at 05:38:10PM +0000, Daniel P. Berrange wrote:
> In updating Fedora 7 to use Xen 3.0.4 we encountered a problem with the
> use of pygrub - it would trigger a memory corruption report by glibc's
> free() routine having been given an invalid pointer. The pygrub process
> is thus terminated with extreme prejudice by glibc with SIGABRT
>
> After a little painful memory debugging in python I discovered that the
> fsimage python binding is mistakenly using PyMem_DEL instead of PyObject_DEL
> to deallocate its objects.
>
> PyMem_DEL simply ends up in a #define to free(). The memory associated with
> Python objects is not neccessarily allocated by malloc(), so calling free()
> is bogus. Python keeps an internal memory pool from which it allocates
> objects, so upon deallocation memory needs to be returned to this pool
> rather than free'd.
>
> As for why no one has hit this before I can only assume this is showing up
> now because of ever stricted glibc memory checking, internal changes in
> python 2.5 memory handling, or a combo of both + a little good/bad luck

Turns out this is a change in Python 2.5

http://docs.python.org/whatsnew/ports.html

"Note that this change means extension modules must be more careful when
allocating memory. Python's API has many different functions for allocating
memory that are grouped into families. For example, PyMem_Malloc(),
PyMem_Realloc(), and PyMem_Free() are one family that allocates raw memory,
while PyObject_Malloc(), PyObject_Realloc(), and PyObject_Free() are another
family that's supposed to be used for creating Python objects.

Previously these different families all reduced to the platform's malloc()
and free() functions. This meant it didn't matter if you got things wrong
and allocated memory with the PyMem function but freed it with the PyObject
function. With 2.5's changes to obmalloc, these families now do different
things and mismatches will probably result in a segfault. You should
carefully test your C extension modules with Python 2.5. "

I checked the rest of the python bindings in tools/ and didn't find any other
places where we'd obviously hit this problem

Regards,
Dan.
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