[PATCH] malloc: additional unlink hardening for non-small bins [BZ #17344]

[Florian Weimer]

Chris Evans has suggested this as a way to stop certain exploits abusing 
malloc's metadata processing.  My patch is slightly different, the patch 
in the bug report has some whitespace changes and locking code which is 
not present in master.

Apparently, some downstreams compile malloc with asserts enabled, and 
they do not fire, so the validity of the additional checks has in effect 
already been tested in the field.

Tested on Fedora 20 x86_64, with no regressions.

-- 
Florian Weimer / Red Hat Product Security
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