[heap] Make ReadOnlySpace fully read-only

Up until now the first page of ReadOnlySpace was not read-only.
Changing this means that the space needs to be made writable before
PagedSpace's TearDown tries to free it.

Bug: v8:7464
Change-Id: I162b45b662a6e5ab61a35be280ef087375fdcc9b
Reviewed-on: https://chromium-review.googlesource.com/c/1454494Reviewed-by: Dan Elphick <delphick@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Cr-Commit-Position: refs/heads/master@{#59432}

src/heap/spaces.cc

@@ -3296,9 +3296,6 @@ void ReadOnlyPage::MakeHeaderRelocatable() {
 }
 
 void ReadOnlySpace::SetPermissionsForPages(PageAllocator::Permission access) {
-  const size_t page_size = MemoryAllocator::GetCommitPageSize();
-  const size_t area_start_offset =
-      RoundUp(MemoryChunkLayout::ObjectStartOffsetInDataPage(), page_size);
   MemoryAllocator* memory_allocator = heap()->memory_allocator();
   for (Page* p : *this) {
     ReadOnlyPage* page = static_cast<ReadOnlyPage*>(p);
@@ -3310,10 +3307,8 @@ void ReadOnlySpace::SetPermissionsForPages(PageAllocator::Permission access) {
     // page allocator manually.
     v8::PageAllocator* page_allocator =
         memory_allocator->page_allocator(page->executable());
-    // TODO(v8:7464): Map the whole space's memory read only (do not ignore the
-    // first page).
-    CHECK(SetPermissions(page_allocator, page->address() + area_start_offset,
-                         page->size() - area_start_offset, access));
+    CHECK(
+        SetPermissions(page_allocator, page->address(), page->size(), access));
   }
 }
 

src/heap/spaces.h

@@ -2973,6 +2973,10 @@ class ReadOnlySpace : public PagedSpace {
 
   explicit ReadOnlySpace(Heap* heap);
 
+  // TODO(v8:7464): Remove this once PagedSpace::TearDown no longer writes to
+  // memory_chunk_list_.
+  ~ReadOnlySpace() override { MarkAsReadWrite(); }
+
   bool writable() const { return !is_marked_read_only_; }
 
   void ClearStringPaddingIfNeeded();