[ASan] Minor fix: return to the last byte of the fake stack if we've raced by.

llvm-svn: 168043

compiler-rt/lib/asan/asan_thread.cc

@@ -143,9 +143,12 @@ const char *AsanThread::GetFrameNameByAddr(uptr addr, uptr *offset) {
   }
 
   if (shadow_ptr < shadow_bottom) {
+    // If we're one byte below the fake stack bottom, we've found the frame.
+    if (!is_fake_stack || (*shadow_bottom != kAsanStackAfterReturnLeftMagic)) {
       *offset = 0;
       return "UNKNOWN";
     }
+  }
 
   uptr* ptr = (uptr*)SHADOW_TO_MEM((uptr)(shadow_ptr + 1));
   CHECK((ptr[0] == kCurrentStackFrameMagic) ||