deps: V8: backport 8ca9f77d0f7c

This fixes a crash when loading snapshots that contain empty
ArrayBuffer instances:

```js
const X = [];
X.push(new ArrayBuffer());
v8.startupSnapshot.setDeserializeMainFunction(() => {
  for (let i = 0; i < 1000000; i++) { // trigger GC
    X.push(new ArrayBuffer());
  }
})
```

Original commit message:

    [sandbox] Sandboxify JSArrayBuffer::extension external pointer

    Bug: chromium:1335043
    Change-Id: Id8e6883fc652b144f6380ff09b1c18e777e041c2
    Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3706626
    Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
    Reviewed-by: Igor Sheludko <ishell@chromium.org>
    Commit-Queue: Samuel Groß <saelo@chromium.org>
    Cr-Commit-Position: refs/heads/main@{#84544}

Refs: v8/v8@8ca9f77
PR-URL: #45871
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

common.gypi

@@ -36,7 +36,7 @@
 
     # Reset this number to 0 on major V8 upgrades.
     # Increment by one for each non-official patch applied to deps/v8.
-    'v8_embedder_string': '-node.6',
+    'v8_embedder_string': '-node.7',
 
     ##### V8 defaults for Node.js #####
 

deps/v8/include/v8-internal.h

@@ -398,7 +398,8 @@ constexpr uint64_t kAllExternalPointerTypeTags[] = {
   V(kWasmInternalFunctionCallTargetTag,     sandboxed, TAG(17)) \
   V(kWasmTypeInfoNativeTypeTag,             sandboxed, TAG(18)) \
   V(kWasmExportedFunctionDataSignatureTag,  sandboxed, TAG(19)) \
-  V(kWasmContinuationJmpbufTag,             sandboxed, TAG(20))
+  V(kWasmContinuationJmpbufTag,             sandboxed, TAG(20)) \
+  V(kArrayBufferExtensionTag,               sandboxed, TAG(21))
 
 // All external pointer tags.
 #define ALL_EXTERNAL_POINTER_TAGS(V) \

deps/v8/src/builtins/builtins-typed-array-gen.cc

@@ -70,8 +70,15 @@ TNode<JSArrayBuffer> TypedArrayBuiltinsAssembler::AllocateEmptyOnHeapBuffer(
                            UintPtrConstant(0));
   StoreSandboxedPointerToObject(buffer, JSArrayBuffer::kBackingStoreOffset,
                                 EmptyBackingStoreBufferConstant());
+#ifdef V8_COMPRESS_POINTERS
+  // When pointer compression is enabled, the extension slot contains a
+  // (lazily-initialized) external pointer handle.
+  StoreObjectFieldNoWriteBarrier(buffer, JSArrayBuffer::kExtensionOffset,
+                                 ExternalPointerHandleNullConstant());
+#else
   StoreObjectFieldNoWriteBarrier(buffer, JSArrayBuffer::kExtensionOffset,
                                  IntPtrConstant(0));
+#endif
   for (int offset = JSArrayBuffer::kHeaderSize;
        offset < JSArrayBuffer::kSizeWithEmbedderFields; offset += kTaggedSize) {
     // TODO(v8:10391, saelo): Handle external pointers in EmbedderDataSlot

deps/v8/src/compiler/code-assembler.h

@@ -553,6 +553,9 @@ class V8_EXPORT_PRIVATE CodeAssembler {
   TNode<BoolT> BoolConstant(bool value) {
     return value ? Int32TrueConstant() : Int32FalseConstant();
   }
+  TNode<ExternalPointerHandleT> ExternalPointerHandleNullConstant() {
+    return ReinterpretCast<ExternalPointerHandleT>(Uint32Constant(0));
+  }
 
   bool IsMapOffsetConstant(Node* node);
 

deps/v8/src/objects/js-array-buffer-inl.h

@@ -88,65 +88,60 @@ void JSArrayBuffer::SetBackingStoreRefForSerialization(uint32_t ref) {
 
 ArrayBufferExtension* JSArrayBuffer::extension() const {
 #if V8_COMPRESS_POINTERS
-    // With pointer compression the extension-field might not be
-    // pointer-aligned. However on ARM64 this field needs to be aligned to
-    // perform atomic operations on it. Therefore we split the pointer into two
-    // 32-bit words that we update atomically. We don't have an ABA problem here
-    // since there can never be an Attach() after Detach() (transitions only
-    // from NULL --> some ptr --> NULL).
-
-    // Synchronize with publishing release store of non-null extension
-    uint32_t lo = base::AsAtomic32::Acquire_Load(extension_lo());
-    if (lo & kUninitializedTagMask) return nullptr;
-
-    // Synchronize with release store of null extension
-    uint32_t hi = base::AsAtomic32::Acquire_Load(extension_hi());
-    uint32_t verify_lo = base::AsAtomic32::Relaxed_Load(extension_lo());
-    if (lo != verify_lo) return nullptr;
-
-    uintptr_t address = static_cast<uintptr_t>(lo);
-    address |= static_cast<uintptr_t>(hi) << 32;
-    return reinterpret_cast<ArrayBufferExtension*>(address);
+  // We need Acquire semantics here when loading the entry, see below.
+  // Consider adding respective external pointer accessors if non-relaxed
+  // ordering semantics are ever needed in other places as well.
+  Isolate* isolate = GetIsolateFromWritableObject(*this);
+  ExternalPointerHandle handle =
+      base::AsAtomic32::Acquire_Load(extension_handle_location());
+  return reinterpret_cast<ArrayBufferExtension*>(
+      isolate->external_pointer_table().Get(handle, kArrayBufferExtensionTag));
 #else
-    return base::AsAtomicPointer::Acquire_Load(extension_location());
-#endif
+  return base::AsAtomicPointer::Acquire_Load(extension_location());
+#endif  // V8_COMPRESS_POINTERS
 }
 
 void JSArrayBuffer::set_extension(ArrayBufferExtension* extension) {
 #if V8_COMPRESS_POINTERS
-    if (extension != nullptr) {
-      uintptr_t address = reinterpret_cast<uintptr_t>(extension);
-      base::AsAtomic32::Relaxed_Store(extension_hi(),
-                                      static_cast<uint32_t>(address >> 32));
-      base::AsAtomic32::Release_Store(extension_lo(),
-                                      static_cast<uint32_t>(address));
-    } else {
-      base::AsAtomic32::Relaxed_Store(extension_lo(),
-                                      0 | kUninitializedTagMask);
-      base::AsAtomic32::Release_Store(extension_hi(), 0);
-    }
+  if (extension != nullptr) {
+    Isolate* isolate = GetIsolateFromWritableObject(*this);
+    ExternalPointerTable& table = isolate->external_pointer_table();
+
+    // The external pointer handle for the extension is initialized lazily and
+    // so has to be zero here since, once set, the extension field can only be
+    // cleared, but not changed.
+    DCHECK_EQ(0, base::AsAtomic32::Relaxed_Load(extension_handle_location()));
+
+    // We need Release semantics here, see above.
+    ExternalPointerHandle handle = table.AllocateAndInitializeEntry(
+        isolate, reinterpret_cast<Address>(extension),
+        kArrayBufferExtensionTag);
+    base::AsAtomic32::Release_Store(extension_handle_location(), handle);
+  } else {
+    // This special handling of nullptr is required as it is used to initialize
+    // the slot, but is also beneficial when an ArrayBuffer is detached as it
+    // allows the external pointer table entry to be reclaimed while the
+    // ArrayBuffer is still alive.
+    base::AsAtomic32::Release_Store(extension_handle_location(),
+                                    kNullExternalPointerHandle);
+  }
 #else
-    base::AsAtomicPointer::Release_Store(extension_location(), extension);
-#endif
-    WriteBarrier::Marking(*this, extension);
-}
-
-ArrayBufferExtension** JSArrayBuffer::extension_location() const {
-  Address location = field_address(kExtensionOffset);
-  return reinterpret_cast<ArrayBufferExtension**>(location);
+  base::AsAtomicPointer::Release_Store(extension_location(), extension);
+#endif  // V8_COMPRESS_POINTERS
+  WriteBarrier::Marking(*this, extension);
 }
 
 #if V8_COMPRESS_POINTERS
-uint32_t* JSArrayBuffer::extension_lo() const {
+ExternalPointerHandle* JSArrayBuffer::extension_handle_location() const {
   Address location = field_address(kExtensionOffset);
-  return reinterpret_cast<uint32_t*>(location);
+  return reinterpret_cast<ExternalPointerHandle*>(location);
 }
-
-uint32_t* JSArrayBuffer::extension_hi() const {
-  Address location = field_address(kExtensionOffset) + sizeof(uint32_t);
-  return reinterpret_cast<uint32_t*>(location);
+#else
+ArrayBufferExtension** JSArrayBuffer::extension_location() const {
+  Address location = field_address(kExtensionOffset);
+  return reinterpret_cast<ArrayBufferExtension**>(location);
 }
-#endif
+#endif  // V8_COMPRESS_POINTERS
 
 void JSArrayBuffer::clear_padding() {
   if (FIELD_SIZE(kOptionalPaddingOffset) != 0) {

deps/v8/src/objects/js-array-buffer.h

@@ -168,14 +168,15 @@ class JSArrayBuffer
  private:
   void DetachInternal(bool force_for_wasm_memory, Isolate* isolate);
 
-  inline ArrayBufferExtension** extension_location() const;
-
 #if V8_COMPRESS_POINTERS
-  static const int kUninitializedTagMask = 1;
-
-  inline uint32_t* extension_lo() const;
-  inline uint32_t* extension_hi() const;
-#endif
+  // When pointer compression is enabled, the pointer to the extension is
+  // stored in the external pointer table and the object itself only contains a
+  // 32-bit external pointer handles. This simplifies alignment requirements
+  // and is also necessary for the sandbox.
+  inline ExternalPointerHandle* extension_handle_location() const;
+#else
+  inline ArrayBufferExtension** extension_location() const;
+#endif  // V8_COMPRESS_POINTERS
 
   TQ_OBJECT_CONSTRUCTORS(JSArrayBuffer)
 };

deps/v8/src/objects/js-array-buffer.tq

@@ -19,7 +19,7 @@ extern class JSArrayBuffer extends JSObjectWithEmbedderSlots {
   raw_max_byte_length: uintptr;
   // A SandboxedPtr if the sandbox is enabled
   backing_store: RawPtr;
-  extension: RawPtr;
+  extension: ExternalPointer;
   bit_field: JSArrayBufferFlags;
   // Pads header size to be a multiple of kTaggedSize.
   @if(TAGGED_SIZE_8_BYTES) optional_padding: uint32;

deps/v8/src/objects/objects-body-descriptors-inl.h

@@ -387,6 +387,8 @@ class JSArrayBuffer::BodyDescriptor final : public BodyDescriptorBase {
     // JSArrayBuffer instances contain raw data that the GC does not know about.
     IteratePointers(obj, kPropertiesOrHashOffset, kEndOfTaggedFieldsOffset, v);
     IterateJSObjectBodyImpl(map, obj, kHeaderSize, object_size, v);
+    v->VisitExternalPointer(map, obj.RawExternalPointerField(kExtensionOffset),
+                            kArrayBufferExtensionTag);
   }
 
   static inline int SizeOf(Map map, HeapObject object) {

deps/v8/src/sandbox/external-pointer-table.h

@@ -98,7 +98,7 @@ class V8_EXPORT_PRIVATE ExternalPointerTable {
   // returning the previous value. The same tag is applied both to decode the
   // previous value and encode the given value.
   //
-  // This method is atomic and can call be called from background threads.
+  // This method is atomic and can be called from background threads.
   inline Address Exchange(ExternalPointerHandle handle, Address value,
                           ExternalPointerTag tag);
 

deps/v8/src/snapshot/deserializer.cc

@@ -399,6 +399,7 @@ void Deserializer<Isolate>::PostProcessNewJSReceiver(Map map,
     auto buffer = JSArrayBuffer::cast(*obj);
     uint32_t store_index = buffer.GetBackingStoreRefForDeserialization();
     if (store_index == kEmptyBackingStoreRefSentinel) {
+      buffer.set_extension(nullptr);
       buffer.set_backing_store(main_thread_isolate(),
                                EmptyBackingStoreBuffer());
     } else {

deps/v8/test/cctest/heap/test-array-buffer-tracker.cc

@@ -218,7 +218,6 @@ TEST(ArrayBuffer_NonLivePromotion) {
   v8::Isolate* isolate = env->GetIsolate();
   Heap* heap = reinterpret_cast<Isolate*>(isolate)->heap();
 
-  JSArrayBuffer raw_ab;
   {
     v8::HandleScope handle_scope(isolate);
     Handle<FixedArray> root =
@@ -235,13 +234,12 @@ TEST(ArrayBuffer_NonLivePromotion) {
     CHECK(IsTracked(heap, JSArrayBuffer::cast(root->get(0))));
     heap::GcAndSweep(heap, NEW_SPACE);
     CHECK(IsTracked(heap, JSArrayBuffer::cast(root->get(0))));
-    raw_ab = JSArrayBuffer::cast(root->get(0));
+    ArrayBufferExtension* extension =
+        JSArrayBuffer::cast(root->get(0)).extension();
     root->set(0, ReadOnlyRoots(heap).undefined_value());
     heap::SimulateIncrementalMarking(heap, true);
-    // Prohibit page from being released.
-    Page::FromHeapObject(raw_ab)->MarkNeverEvacuate();
     heap::GcAndSweep(heap, OLD_SPACE);
-    CHECK(!IsTracked(heap, raw_ab));
+    CHECK(!IsTracked(heap, extension));
   }
 }