[lsan][Fuchsia] Fix bounds checking for thread_local allocator cache when scanning TLS regions

[leonardchan]

Summary

When scanning over TLS regions, we attempt to check if one of the regions is one of the thread_local allocator caches which would be located in one of the TLS blocks pointer to by the DTV. This is to prevent marking a pointer that was allocated by the primary allocator (from a thread_local cache) as reachable. The check is a simple bounds check to see if the allocator cache is within the bounds of one of the TLS block we're iterating over, but it looks like the check for the end of the cache is slightly incorrect. Rather than checking end - cache_start <= cache_size, it should be something like cache_start + cache_size < end.

[leonardchan]

Prior to this, it looks like we'd unconditionally mark pointers in the allocator cache as reachable. So we may find more leaks after this.

[mcgrathr]

The pattern the code was using is the usual pattern to avoid potential overflow. It just should have been >= rather than <=. Even aside from the overflow avoidance, < end is not right but should be <= end AFAICT. But I think it's best to keep the overflow avoidance here and just fix the typo so it's <= end.

[leonardchan]

ping

[mcgrathr]

lgtm

compiler-rt/lib/lsan/lsan_common_fuchsia.cpp

void LockStuffAndStopTheWorld(StopTheWorldCallback callback,
   // variables as well as C11 tss_set and POSIX pthread_setspecific.
   auto tls = +[](void *chunk, size_t size, void *data) {
     auto params = static_cast<const Params *>(data);
     uptr begin = reinterpret_cast<uptr>(chunk);
     uptr end = begin + size;
     auto i = __sanitizer::InternalLowerBound(params->allocator_caches, begin);
     if (i < params->allocator_caches.size() &&
         params->allocator_caches[i] >= begin &&
-        end - params->allocator_caches[i] <= sizeof(AllocatorCache)) {
+        params->allocator_caches[i] + sizeof(AllocatorCache) < end) {
       // Split the range in two and omit the allocator cache within.
       ScanRangeForPointers(begin, params->allocator_caches[i],
                            &params->argument->frontier, "TLS", kReachable);
       uptr begin2 = params->allocator_caches[i] + sizeof(AllocatorCache);
       ScanRangeForPointers(begin2, end, &params->argument->frontier, "TLS",
                            kReachable);
     } else {
       ScanRangeForPointers(begin, end, &params->argument->frontier, "TLS",