Do checks on the fault address when we think it's an NPE.

bug:29321958
Change-Id: I28f4da56eb3e0b48721d3ac41114858bc80daadb
diff --git a/runtime/fault_handler.h b/runtime/fault_handler.h
index 625b1e8..56e0fb7 100644
--- a/runtime/fault_handler.h
+++ b/runtime/fault_handler.h
@@ -96,6 +96,14 @@
 
   bool Action(int sig, siginfo_t* siginfo, void* context) OVERRIDE;
 
+  static bool IsValidImplicitCheck(siginfo_t* siginfo) {
+    // Our implicit NPE checks always limit the range to a page.
+    // Note that the runtime will do more exhaustive checks (that we cannot
+    // reasonably do in signal processing code) based on the dex instruction
+    // faulting.
+    return CanDoImplicitNullCheckOn(reinterpret_cast<uintptr_t>(siginfo->si_addr));
+  }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(NullPointerHandler);
 };