Ensure safestack overflow test doesn't segfault

Summary:
In rL255491, the safestack overflow test was disabled for aarch64, since
it "is currently failing on an AArch64 buildbot with a segfault, but it
is currently passing on other configuration".

While testing on FreeBSD on x86, I also encountered a segfault.  This is
because the `fct()` function actually writes before and after `buffer`,
and on FreeBSD this crashes because `buffer` is usually allocated at the
end of a page.  That this runs correctly on Linux is probably just by
accident.

I propose to fix this by adding a pre and post buffer, to act as a
safety zone.  The pre and post buffers must be accessed in an 'unsafe'
way, otherwise -fsanitize=safestack will allocate them on the safe
stack, and they will not bookend `buffer` itself.  Therefore, I create
them large enough for `fct()`, and call it on both of them.

On FreeBSD, this makes the test run as expected, without segfaulting,
and I suppose this will also fix the segfault on AArch64.  I do not have
AArch64 testing capabilities, so if someone could try that out, I would
be much obliged.

Reviewers: pcc, kcc, zatrazz

Subscribers: llvm-commits, aemerson, emaste

Differential Revision: http://reviews.llvm.org/D15725

llvm-svn: 257106

0 files changed, 0 insertions, 0 deletions