--enable-stack-protector for glibc, v4, now with arm

[Nix]

On 3 Mar 2016, nix@esperi.org.uk said:

> I left that in specifically because I wasn't sure which way to go (we
> see it on all platforms, including SPARC and ARM). It is quite possibly
> desirable to de-PLTify this (because these calls are frequently made),
> but it's *also* quite possibly desirable not to do that, because, like
> malloc(), __stack_chk_fail() might reasonably be something that people
> might want to intercept, to change what happens on stack overrun.

"It's plausible" is of course code for "I have no idea and am guessing".

I just had a look on codesearch.debian.net. Overriders (ignoring other
libcs and early runtimes in things like openbios and grub which do
not link against a libc anyway):

physfs, on MacOS X only, because libssp is missing;
wine, overriding it with nothing just to placate the linker;
dmtcp, just prints an error and aborts, would be happy with ours;
       run in bizarre circumstances out of an mmap()ed memory region and
       also includes stubs for a lot of libgcc
condor, provides __stack_chk_fail itself iff glibc is old, so clearly
        *expects* glibc to provide it

*Nobody* overrides __stack_chk_fail with anything significant enough
that they'd be unhappy if we provided an implementation, so I'm going to
go with what you suggested and just de-PLTify it for local within-glibc
calls, leaving it unchanged for everything else.


I also note this code in the binutils ld testsuite:

    set flags "--defsym __stack_chk_fail=0"

which gives me confidence that my independent evolution of the same
horrendous hack in the librtld mapfile computation is not all that bad :)

-- 
NULL && (void)