[wasm] Add guard regions to end of WebAssembly.Memory buffers

src/base/platform/platform-posix.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Platform-specific code for POSIX goes here. This is not a platform on its
 // own, but contains the parts which are the same across the POSIX platforms
 // Linux, MacOS, FreeBSD, OpenBSD, NetBSD and QNX.
 
 #include <errno.h>
 #include <limits.h>
@@ skipping 111 matching lines @@
 // Create guard pages.
 void OS::Guard(void* address, const size_t size) {
 #if V8_OS_CYGWIN
   DWORD oldprotect;
   VirtualProtect(address, size, PAGE_NOACCESS, &oldprotect);
 #else
   mprotect(address, size, PROT_NONE);
 #endif
 }
 
+// Make a region of memory readable and writable.
+void OS::Unprotect(void* address, const size_t size) {

[Michael Lippautz, 2016/10/27 14:41:50]

Maybe we should have OS::Protect as a dual function to OS::Unprotect. 
You could keep OS::Guard to just call OS:Protect.

[Eric Holk, 2016/10/28 16:20:13]

I think I'd probably do OS::Protect and take flags for read, write or execute to
have it basically mirror mprotect or VirtualProtect.

+#if V8_OS_CYGWIN
+  DWORD oldprotect;
+  VirtualProtect(address, size, PAGE_READWRITE, &oldprotect);
+#else
+  mprotect(address, size, PROT_READ | PROT_WRITE);
+#endif
+}
 
 static LazyInstance<RandomNumberGenerator>::type
     platform_random_number_generator = LAZY_INSTANCE_INITIALIZER;
 
 
 void OS::Initialize(int64_t random_seed, bool hard_abort,
                     const char* const gc_fake_mmap) {
   if (random_seed) {
     platform_random_number_generator.Pointer()->SetSeed(random_seed);
   }
@@ skipping 600 matching lines @@
 
 void Thread::SetThreadLocal(LocalStorageKey key, void* value) {
   pthread_key_t pthread_key = LocalKeyToPthreadKey(key);
   int result = pthread_setspecific(pthread_key, value);
   DCHECK_EQ(0, result);
   USE(result);
 }
 
 }  // namespace base
 }  // namespace v8