[PATCH v2] libio: use PTR_MANGLE/PTR_DEMANGLE for FILE vtables

[Florian Weimer]

On 10/01/2015 08:40 PM, Kees Cook wrote:
> Hi!
> 
> This is a resend (from 2011!) of my PTR_MANGLE series for the FILE vtable.
> It was previously discussed here:
> https://sourceware.org/ml/libc-alpha/2011-12/msg00073.html
> 
> It sounds like only very very old code was sharing FILE externally,
> so this appears safe now. "make xcheck" passes. If this is NOT okay for
> general use, I'd like to make it a configure option, since for modern
> glibc users, it works fine. (e.g. Chrome OS has been using this patch
> for 3 years now.)

This needs a configure option for now, but we can enable it for some
architectures per default.

At one point, someone needs to do the thankless job and unearth where
the cut-over point to modern libstdc++ was, and which architectures
actually need the vtable compatibility.  (If there was no GCC version
that could compile the relevant libstdc++ versions, then compatibility
is obviously unnecessary.)

Even better, if we can grab a few old binaries and they do not run on
current systems for other libstdc++-related reasons (or perhaps we
subtly broke the libio ABI over time), we might justify giving up
backwards compatibility in this area.