Landing: Write protect executable pages in the VM.

runtime/vm/freelist.h

Index: runtime/vm/freelist.h
diff --git a/runtime/vm/freelist.h b/runtime/vm/freelist.h
index 8ec1ac45e8d90c880ab6f18d94186d5509fa9c03..9bed00ee4dbe89d3288578c806f10f7d9d12c52b 100644
--- a/runtime/vm/freelist.h
+++ b/runtime/vm/freelist.h
@@ -19,9 +19,15 @@ namespace dart {
 // the element at the address following the next_ field.
 class FreeListElement {
  public:
+  // Maximum header size is three words (tags, next, and size).
+  static const intptr_t kHeaderSize = 3 * kWordSize;

[Ivan Posva, 2014/02/03 05:44:35]

This does not look and sound right. Especially how are you adding two-word sized
objects to the free list.

[Florian Schneider, 2014/02/10 11:56:17]

This is a little confusing, but should be right: The header can be 2 or 3 words.

The header of the remainder element has to be unprotected for the
SplitAndEnqueue operation. It does not hurt to unprotect always 3 words here:
The remainder element will be protected in full again afterwards.

+
   FreeListElement* next() const {
     return next_;
   }
+  uword next_address() const {
+    return reinterpret_cast<uword>(&next_);
+  }
 
   void set_next(FreeListElement* next) {
     next_ = next;
@@ -75,7 +81,7 @@ class FreeList {
   FreeList();
   ~FreeList();
 
-  uword TryAllocate(intptr_t size);
+  uword TryAllocate(intptr_t size, bool is_protected);
   void Free(uword addr, intptr_t size);
 
   void Reset();
@@ -92,7 +98,9 @@ class FreeList {
   void EnqueueElement(FreeListElement* element, intptr_t index);
   FreeListElement* DequeueElement(intptr_t index);
 
-  void SplitElementAfterAndEnqueue(FreeListElement* element, intptr_t size);
+  void SplitElementAfterAndEnqueue(FreeListElement* element,
+                                   intptr_t size,
+                                   bool is_protected);
 
   void PrintSmall() const;
   void PrintLarge() const;