[tsan] Fix the memcpy interceptor to be memmove compatible on OS X

[kubamracek]

Summary

On OS X, memcpy and memmove are actually aliases of the same implementation, which means the interceptor of memcpy is also invoked when memmove is called. The current implementation of the interceptor uses internal_memcpy to perform the actual memory operation, which can produce an incorrect result when memmove semantics are expected. Let's call REAL(memcpy) instead.

[dvyukov]

I suspect there were reasons to call internal_memcpy. For example, if we intercept memcpy from dynamic loader when nothing is initialized yet, REAL(memcpy) is NULL.
We generally try to move in the opposite direction -- the less we call out from runtime, the less problems we have.
Use internal_memmove, it is already implemented.

[kubamracek]

Updating patch to use internal_memmove and adding a comment.

compiler-rt/trunk/lib/tsan/rtl/tsan_interceptors.cc

 
 TSAN_INTERCEPTOR(void*, memcpy, void *dst, const void *src, uptr size) {
   // On FreeBSD we get here from libthr internals on thread initialization.
   if (!COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED) {
     SCOPED_TSAN_INTERCEPTOR(memcpy, dst, src, size);
     MemoryAccessRange(thr, pc, (uptr)dst, size, true);
     MemoryAccessRange(thr, pc, (uptr)src, size, false);
   }
-  return internal_memcpy(dst, src, size);
+  // On OS X, calling internal_memcpy here will cause memory corruptions,
+  // because memcpy and memmove are actually aliases of the same implementation.
+  // We need to use internal_memmove here.
+  return internal_memmove(dst, src, size);
 }
 
 TSAN_INTERCEPTOR(void*, memmove, void *dst, void *src, uptr n) {
   if (!COMMON_INTERCEPTOR_NOTHING_IS_INITIALIZED) {
     SCOPED_TSAN_INTERCEPTOR(memmove, dst, src, n);
     MemoryAccessRange(thr, pc, (uptr)dst, n, true);
     MemoryAccessRange(thr, pc, (uptr)src, n, false);
   }