[sanitizers] read/write page fault detection on mac.

Subscribers: kubabrecka

Differential Revision: http://reviews.llvm.org/D19495

llvm-svn: 267477

compiler-rt/lib/sanitizer_common/sanitizer_mac.cc

@@ -500,7 +500,8 @@ void LogFullErrorReport(const char *buffer) {
 }
 
 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) {