[compiler-rt] r221520 - [sanitizer] Never remove the last frame off the stack trace.

[Evgeniy Stepanov]

We remove frames that belong to the sanitizer runtime off the top of
the stack trace.

This situation, when the unwinder returned a single-frame stack trace,
was observed in an unusual environment where the CFI unwinder
misbehaved for a yet unknown reason. It involved building ASan runtime
with a custom build system (neither make or cmake), and I can't
reproduce the results in a test.

On Wed, Nov 12, 2014 at 1:46 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
> On Fri, Nov 7, 2014 at 4:03 AM, Evgeniy Stepanov <eugeni.stepanov at gmail.com>
> wrote:
>>
>> Author: eugenis
>> Date: Fri Nov  7 06:03:07 2014
>> New Revision: 221520
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=221520&view=rev
>> Log:
>> [sanitizer] Never remove the last frame off the stack trace.
>
>
> Test case? I'm also not quite sure what this is doing - why are we
> popping/ignoring frames exactly?
>
>>
>>
>> It can only make it worse.
>>
>> Modified:
>>
>> compiler-rt/trunk/lib/sanitizer_common/sanitizer_unwind_posix_libcdep.cc
>>
>> Modified:
>> compiler-rt/trunk/lib/sanitizer_common/sanitizer_unwind_posix_libcdep.cc
>> URL:
>> http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/sanitizer_common/sanitizer_unwind_posix_libcdep.cc?rev=221520&r1=221519&r2=221520&view=diff
>>
>> ==============================================================================
>> ---
>> compiler-rt/trunk/lib/sanitizer_common/sanitizer_unwind_posix_libcdep.cc
>> (original)
>> +++
>> compiler-rt/trunk/lib/sanitizer_common/sanitizer_unwind_posix_libcdep.cc Fri
>> Nov  7 06:03:07 2014
>> @@ -116,7 +116,7 @@ void BufferedStackTrace::SlowUnwindStack
>>    // We need to pop a few frames so that pc is on top.
>>    uptr to_pop = LocatePcInTrace(pc);
>>    // trace_buffer[0] belongs to the current function so we always pop it.
>> -  if (to_pop == 0)
>> +  if (to_pop == 0 && size > 1)
>>      to_pop = 1;
>>    PopStackFrames(to_pop);
>>    trace_buffer[0] = pc;
>>
>>
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>
>