[GWP-ASan] Only pack frames that are stored.

[hctim]

Summary

Backtrace() returns the number of frames that are *available*, rather
than the number of frames stored. When we pack, we supply the number of
frames we have stored. The number of available frames can exceed the
number of stored frames, leading to stack OOB read.

Fix up this problem.

[hctim]

Add a test to catch the old behaviour.

[hctim]

Don't know where the code changes went, let's try this!

[eugenis]

LGTM

compiler-rt/lib/gwp_asan/common.cpp

void AllocationMetadata::CallSiteInfo::RecordBacktrace(
     options::Backtrace_t Backtrace) {
   TraceSize = 0;
   if (!Backtrace)
     return;
 
   uintptr_t UncompressedBuffer[kMaxTraceLengthToCollect];
   size_t BacktraceLength =
       Backtrace(UncompressedBuffer, kMaxTraceLengthToCollect);
+  // Backtrace() returns the number of available frames, which may be greater
+  // than the number of frames in the buffer. In this case, we need to only pack
+  // the number of frames that are in the buffer.
+  if (BacktraceLength > kMaxTraceLengthToCollect)
+    BacktraceLength = kMaxTraceLengthToCollect;
   TraceSize =
       compression::pack(UncompressedBuffer, BacktraceLength, CompressedTrace,
                         AllocationMetadata::kStackFrameStorageBytes);
 }
 
 size_t AllocatorState::maximumAllocationSize() const { return PageSize; }
 
 uintptr_t AllocatorState::slotToAddr(size_t N) const {