[PATCH] libio vtable validation [BZ #20191]

[Florian Weimer]

This is my latest attempt at vtable verification for libio.  It uses 
Pedro's section-based approach to perform the validation of the vtable 
pointer.  This is a submission for patch review.  I'm reasonably happy 
with this patch and think it is close to something that can be committed.

The relevant ABI analysis is here:

   https://sourceware.org/glibc/wiki/LibioVtables

I believe I have added compatibility enablement to all relevant private 
constructor functions.

I verified that the apt-get binary from Debian woody i386 can still open 
files with the new glibc version.  (libstdc++ in Debian woody uses libio 
from glibc.)

Conversely, I checked that a few x86_64 binaries would not set the 
IO_accept_foreign_vtables flag when run.

The SHARED case in _IO_vtable_check uses the unprotected _dl_open_hook 
variable, which is somewhat risky.  This is essentially another vtable 
pointer which we need to protect anyway; I filed bug 20204 to cover 
that.  I will likely have time to implement the minimal solution based 
on pointer mangling (and address bug 20222) before the 2.24 freeze.

At a future point, we need to discuss if we can remove vtable 
compatibility from glibc releases for architectures which did not have a 
backend in GCC 2.95.  It seems that GCC 3.0 broke the C++ ABI in a way 
that is incompatible with glibc libio, so only GCC 2.95 could have 
produced a libstdc++ which uses external libio.

Florian
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