Index: lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- lib/sanitizer_common/sanitizer_mac.cc
+++ lib/sanitizer_common/sanitizer_mac.cc
@@ -500,7 +500,8 @@
 }
 
 SignalContext::WriteFlag SignalContext::GetWriteFlag(void *context) {
-  return UNKNOWN;  // FIXME: implement this.
+  ucontext_t *ucontext = static_cast<ucontext_t*>(context);
+  return ucontext->uc_mcontext->__es.__err & 2 /*T_PF_WRITE*/ ? WRITE : READ;
 }
 
 void GetPcSpBp(void *context, uptr *pc, uptr *sp, uptr *bp) {
Index: test/asan/TestCases/Posix/segv_read_write.c
===================================================================
--- test/asan/TestCases/Posix/segv_read_write.c
+++ test/asan/TestCases/Posix/segv_read_write.c
@@ -20,7 +20,7 @@
   else
     Write((int *)p);
 }
-// READ: AddressSanitizer: SEGV on unknown address
+// READ: AddressSanitizer: SEGV on unknown address11
 // READ: The signal is caused by a READ memory access.
 // WRITE: AddressSanitizer: SEGV on unknown address
 // WRITE: The signal is caused by a WRITE memory access.