Deprecate mcheck?

[Florian Weimer]

With -lmcheck, it is possible to add malloc checking hooks which wrap 
the allocations with an additional header.

The approach is not specific to our malloc implementation, it does not 
use internal implementation details (thus the separate header).  The 
current implementation is not thread-safe and may be incompatible with 
early allocations in C++ code.  With help from the malloc 
implementation, it should be possible to come up with something that is 
quite a bit more efficient.

It's also odd that this functionality is enabled with -lmcheck or an 
explicit function call, and not with LD_PRELOAD (where it completely 
reside if necessary).

It also seems to me that it can't be abused as a security feature.  The 
canaries are static, so the only secret is the heap randomization.  We 
also have something similar in glibc itself.

So I'd like to deprecate mcheck in 2.20, and remove -lmcheck in 2.21 and 
turn the functions into hidden NOPs.

-- 
Florian Weimer / Red Hat Product Security Team