Ensure safestack overflow test doesn't segfault

[dim]

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.

[dim]

Ping.

[zatrazz]

Another way would be to adjust fct to no pass the possible stack end by writing 6*sizeof(int) instead. I have no preference, so this change also LGTM.

I was not able to reproduce it yet on aarch64, but we can check on the faulty system (the buildbot) by removing the stable-runtime requirement.

[dim]

In D15725#319932, @zatrazz wrote:

Another way would be to adjust fct to no pass the possible stack end by writing 6*sizeof(int) instead. I have no preference, so this change also LGTM.

OK.

I was not able to reproduce it yet on aarch64, but we can check on the faulty system (the buildbot) by removing the stable-runtime requirement.

Shall I then remove the // REQUIRES: stable-runtime line from compiler-rt/trunk/test/safestack/overflow.c as part of this fix?

[dim]

I'll just commit it now, the test can be re-enabled at some later time.

compiler-rt/trunk/test/safestack/overflow.c

 __attribute__((noinline))
 void fct(volatile int *buffer)
 {
   memset(buffer - 1, 0, 7 * sizeof(int));
 }
 
 int main(int argc, char **argv)
 {
+  int prebuf[7];
   int value1 = 42;
   int buffer[5];
   int value2 = 42;
+  int postbuf[7];
+  fct(prebuf + 1);
+  fct(postbuf + 1);
   fct(buffer);
   return value1 != 42 || value2 != 42;
 }