Bug 1550037 - patch 2 - Ensure the font-list memory blocks passed to content processes are shared as readonly copies. r=jwatt

Differential Revision: https://phabricator.services.mozilla.com/D68779

--HG--
extra : moz-landing-system : lando

gfx/thebes/SharedFontList-impl.h

@@ -225,11 +225,12 @@ class FontList {
    */
   void ShareShmBlockToProcess(uint32_t aIndex, base::ProcessId aPid,
                               base::SharedMemoryHandle* aOut) {
-    if (aIndex >= mBlocks.Length()) {
+    MOZ_RELEASE_ASSERT(mReadOnlyShmems.Length() == mBlocks.Length());
+    if (aIndex >= mReadOnlyShmems.Length()) {
       // Block index out of range
       *aOut = base::SharedMemory::NULLHandle();
     }
-    if (!mBlocks[aIndex]->mShmem->ShareToProcess(aPid, aOut)) {
+    if (!mReadOnlyShmems[aIndex]->ShareToProcess(aPid, aOut)) {
       MOZ_CRASH("failed to share block");
     }
   }
@@ -325,6 +326,12 @@ class FontList {
    * added a block (or blocks) to the list, and we need to update!
    */
   nsTArray<mozilla::UniquePtr<ShmBlock>> mBlocks;
+
+  /**
+   * Auxiliary array, used only in the parent process; holds read-only copies
+   * of the shmem blocks; these are what will be shared to child processes.
+   */
+  nsTArray<mozilla::UniquePtr<base::SharedMemory>> mReadOnlyShmems;
 };
 
 }  // namespace fontlist

gfx/thebes/SharedFontList.cpp

@@ -520,7 +520,7 @@ FontList::~FontList() { DetachShmBlocks(); }
 bool FontList::AppendShmBlock() {
   MOZ_ASSERT(XRE_IsParentProcess());
   base::SharedMemory* newShm = new base::SharedMemory();
-  if (!newShm->Create(SHM_BLOCK_SIZE)) {
+  if (!newShm->CreateFreezeable(SHM_BLOCK_SIZE)) {
     MOZ_CRASH("failed to create shared memory");
     return false;
   }
@@ -541,6 +541,13 @@ bool FontList::AppendShmBlock() {
   mBlocks.AppendElement(block);
   GetHeader().mBlockCount.store(mBlocks.Length());
 
+  auto* readOnly = new base::SharedMemory();
+  if (!newShm->ReadOnlyCopy(readOnly)) {
+    MOZ_CRASH("failed to create read-only copy");
+    return false;
+  }
+  mReadOnlyShmems.AppendElement(readOnly);
+
   return true;
 }
 
@@ -548,7 +555,8 @@ void FontList::DetachShmBlocks() {
   for (auto& i : mBlocks) {
     i->mShmem = nullptr;
   }
-  mBlocks.SetLength(0);
+  mBlocks.Clear();
+  mReadOnlyShmems.Clear();
 }
 
 FontList::ShmBlock* FontList::GetBlockFromParent(uint32_t aIndex) {