Index: compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
===================================================================
--- compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
+++ compiler-rt/trunk/lib/sanitizer_common/sanitizer_mac.cc
@@ -506,7 +506,12 @@
 }
 
 SignalContext::WriteFlag SignalContext::GetWriteFlag(void *context) {
-  return UNKNOWN;  // FIXME: implement this.
+#if defined(__x86_64__) || defined(__i386__)
+  ucontext_t *ucontext = static_cast<ucontext_t*>(context);
+  return ucontext->uc_mcontext->__es.__err & 2 /*T_PF_WRITE*/ ? WRITE : READ;
+#else
+  return UNKNOWN;
+#endif
 }
 
 void GetPcSpBp(void *context, uptr *pc, uptr *sp, uptr *bp) {
Index: compiler-rt/trunk/test/asan/TestCases/Darwin/segv_read_write.c
===================================================================
--- compiler-rt/trunk/test/asan/TestCases/Darwin/segv_read_write.c
+++ compiler-rt/trunk/test/asan/TestCases/Darwin/segv_read_write.c
@@ -0,0 +1,26 @@
+// RUN: %clangxx_asan -std=c++11 -O0 %s -o %t
+// RUN: not %run %t       2>&1 | FileCheck %s --check-prefix=READ
+// RUN: not %run %t write 2>&1 | FileCheck %s --check-prefix=WRITE
+// REQUIRES: x86-target-arch
+
+#include <sys/mman.h>
+
+static volatile int sink;
+__attribute__((noinline)) void Read(int *ptr) { sink = *ptr; }
+__attribute__((noinline)) void Write(int *ptr) { *ptr = 0; }
+int main(int argc, char **argv) {
+  // Writes to shadow are detected as reads from shadow gap (because of how the
+  // shadow mapping works). This is kinda hard to fix. Test a random address in
+  // the application part of the address space.
+  void *volatile p =
+      mmap(nullptr, 4096, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
+  munmap(p, 4096);
+  if (argc == 1)
+    Read((int *)p);
+  else
+    Write((int *)p);
+}
+// READ: AddressSanitizer: SEGV on unknown address
+// 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.