Improve and fix the stack-protector tests.

Bug: http://b/26888853

(cherry picked from commit fc69a8ad5f0d9b63de48e3858fb4811ede7ac64e)

Change-Id: Ibc431076000b9a8db46f68f858480045b03b6e79
diff --git a/tests/stack_protector_test_helper.cpp b/tests/stack_protector_test_helper.cpp
new file mode 100644
index 0000000..34f3c77
--- /dev/null
+++ b/tests/stack_protector_test_helper.cpp
@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2012 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Deliberately overwrite the stack canary.
+__attribute__((noinline)) void modify_stack_protector_test() {
+  char buf[128];
+  // We can't use memset here because it's fortified, and we want to test
+  // the line of defense *after* that.
+  char* p = buf;
+  while ((p - buf) < static_cast<int>(sizeof(buf) + sizeof(void*))) *p++ = '\0';
+}