From 373cb301ad226ab33fdd65212ca0c5c78542f504 Mon Sep 17 00:00:00 2001
From: George Burgess IV <gbiv@google.com>
Date: Fri, 24 May 2019 13:45:36 -0700
Subject: [PATCH] memunreachable_test: don't use `write` to escape addresses

A recent change
(https://android-review.googlesource.com/c/platform/bionic/+/961601)
caused this test to fail. This change allowed LLVM to optimize what was
previously `__write_chk` into a standard call to `write`.

LLVM knows things about `write`. In particular, it knows that `write`
doesn't capture its buffer param, which `Ref` assumes isn't possible.

LLVM can't reason much about `volatile` operations; storing this to a
`volatile` ptr should be good enough. If not, there are a few other
tricks we can play here.

Bug: 133479661
Test: `atest
memunreachable_test:memunreachable_test.MemunreachableTest#stack --
--abi x86` on internal-master

Change-Id: I905e510210cafdf83fa1b50f2f090d4f8b9653da
---
 libmemunreachable/tests/MemUnreachable_test.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libmemunreachable/tests/MemUnreachable_test.cpp b/libmemunreachable/tests/MemUnreachable_test.cpp
index bba0c6d11..024b6bc21 100644
--- a/libmemunreachable/tests/MemUnreachable_test.cpp
+++ b/libmemunreachable/tests/MemUnreachable_test.cpp
@@ -47,7 +47,8 @@ class HiddenPointer {
 
 // Trick the compiler into thinking a value on the stack is still referenced.
 static void Ref(void** ptr) {
-  write(0, ptr, 0);
+  void** volatile storage;
+  storage = ptr;
 }
 
 class MemunreachableTest : public ::testing::Test {