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

src/base/platform/platform-posix.cc

Index: src/base/platform/platform-posix.cc
diff --git a/src/base/platform/platform-posix.cc b/src/base/platform/platform-posix.cc
index 3f4165de536c1f2047fc40dc248fd2158413598f..7a8ac973894f0fb539e928f7046e5780119d9bd7 100644
--- a/src/base/platform/platform-posix.cc
+++ b/src/base/platform/platform-posix.cc
@@ -99,6 +99,20 @@ intptr_t OS::CommitPageSize() {
   return page_size;
 }
 
+void* OS::AllocateGuarded(const size_t requested) {
+  size_t allocated = 0;
+  const bool is_executable = false;
+  void* mbase = OS::Allocate(requested, &allocated, is_executable);
+  if (allocated != requested) {

[jochen (gone - plz use gerrit), 2016/11/12 21:05:45]

why not allocated >= requested?

[Eric Holk, 2016/11/14 19:05:56]

The goal is to allocate all or none of the requested range. If that didn't
happen, we free what was allocated and return nullptr to indicate the failure. I
suppose `allocated < requested` would be okay, but I think it makes more sense
to require the exact amount.

That said, I noticed in the call to Free below we should probably use allocated
instead of requested as the size, so I will fix that.

+    OS::Free(mbase, requested);
+    return nullptr;
+  }
+  if (mbase == nullptr) {
+    return nullptr;
+  }
+  OS::Guard(mbase, requested);
+  return mbase;
+}
 
 void OS::Free(void* address, const size_t size) {
   // TODO(1240712): munmap has a return value which is ignored here.
@@ -129,6 +143,15 @@ void OS::Guard(void* address, const size_t size) {
 #endif
 }
 
+// Make a region of memory readable and writable.
+void OS::Unprotect(void* address, const size_t size) {
+#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;