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Re: [PATCH][BZ #1349] Return requested size for malloc_usable_sizewhen MALLOC_CHECK_ > 0

On 08/31/2012 03:02 PM, Siddhesh Poyarekar wrote:

When MALLOC_CHECK_ is exported as a positive value (which I am doing
for this test), malloc_usable_size should return exactly 7, since the
malloc_check code adds a magic number at p[7] if p is the mem pointer.
Anything greater than that and the user may risk writing over the magic
value and tripping an assertion during free.

The "7" depends on malloc allocation granularity. On x86_64, the magic number is not 7, but 23, and malloc_usable_size(malloc(7)) == 24.

I'm not sure how to test this in a robust manner. Perhaps you could use a loop and look for a bumo in malloc_usable_size, and make sure that the size after the bump is not divisible by 4?

--
Florian Weimer / Red Hat Product Security Team

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