<https://webkit.org/b/120079> Flattening a dictionary can cause CopiedSpace corruption

Reviewed by Oliver Hunt.

When we flatten an object in dictionary mode, we compact its properties. If the object
had out-of-line storage in the form of a Butterfly prior to this compaction, and after
compaction its properties fit inline, the object's Structure "forgets" that the object
has a non-zero Butterfly pointer. During GC, we check the Butterfly and reportLiveBytes
with bytes = 0, which causes all sorts of badness in CopiedSpace.

Instead, after we flatten a dictionary, if properties fit inline we should clear the
Butterfly pointer so that the GC doesn't get confused later.

This patch does this clearing, and it also adds JSObject::checkStructure, which overrides
JSCell::checkStructure to add an ASSERT that makes sure that the Structure being assigned
agrees with the whether or not the object has a Butterfly. Also added an ASSERT to check
that the number of bytes reported to SlotVisitor::copyLater is non-zero.

* heap/SlotVisitorInlines.h:
(JSC::SlotVisitor::copyLater):
* runtime/JSObject.cpp:
(JSC::JSObject::notifyPresenceOfIndexedAccessors):
(JSC::JSObject::convertUndecidedToInt32):
(JSC::JSObject::convertUndecidedToDouble):
(JSC::JSObject::convertUndecidedToContiguous):
(JSC::JSObject::convertInt32ToDouble):
(JSC::JSObject::convertInt32ToContiguous):
(JSC::JSObject::genericConvertDoubleToContiguous):
(JSC::JSObject::switchToSlowPutArrayStorage):
(JSC::JSObject::setPrototype):
(JSC::JSObject::putDirectAccessor):
(JSC::JSObject::seal):
(JSC::JSObject::freeze):
(JSC::JSObject::preventExtensions):
(JSC::JSObject::reifyStaticFunctionsForDelete):
(JSC::JSObject::removeDirect):
* runtime/JSObject.h:
(JSC::JSObject::setButterfly):
(JSC::JSObject::putDirectInternal):
(JSC::JSObject::setStructure):
(JSC::JSObject::setStructureAndReallocateStorageIfNecessary):
* runtime/Structure.cpp:
(JSC::Structure::flattenDictionaryStructure):

Change-Id: Idfd8c22555f4373c1104316ff1ee28f5f84ef083
git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154366 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>

4 files changed, 32 insertions, 18 deletions

diff --git a/Source/JavaScriptCore/heap/SlotVisitorInlines.h b/Source/JavaScriptCore/heap/SlotVisitorInlines.h
index 4273a28d6..da338ce11 100644
--- a/Source/JavaScriptCore/heap/SlotVisitorInlines.h
+++ b/Source/JavaScriptCore/heap/SlotVisitorInlines.h
@@ -174,6 +174,7 @@ inline void SlotVisitor::donateAndDrain()
 
 inline void SlotVisitor::copyLater(JSCell* owner, void* ptr, size_t bytes)
 {
+    ASSERT(bytes);
     CopiedBlock* block = CopiedSpace::blockFor(ptr);
     if (block->isOversize()) {
         m_shared.m_copiedSpace->pin(block);
diff --git a/Source/JavaScriptCore/runtime/JSObject.cpp b/Source/JavaScriptCore/runtime/JSObject.cpp
index 01dc96333..5637e2090 100644
--- a/Source/JavaScriptCore/runtime/JSObject.cpp
+++ b/Source/JavaScriptCore/runtime/JSObject.cpp
@@ -595,7 +595,7 @@ void JSObject::notifyPresenceOfIndexedAccessors(VM& vm)
     if (mayInterceptIndexedAccesses())
         return;
     
-    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AddIndexedAccessors));
+    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AddIndexedAccessors), m_butterfly);
     
     if (!vm.prototypeMap.isPrototype(this))
         return;
@@ -681,7 +681,7 @@ ArrayStorage* JSObject::createInitialArrayStorage(VM& vm)
 ContiguousJSValues JSObject::convertUndecidedToInt32(VM& vm)
 {
     ASSERT(hasUndecided(structure()->indexingType()));
-    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateInt32));
+    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateInt32), m_butterfly);
     return m_butterfly->contiguousInt32();
 }
 
@@ -692,14 +692,14 @@ ContiguousDoubles JSObject::convertUndecidedToDouble(VM& vm)
     for (unsigned i = m_butterfly->vectorLength(); i--;)
         m_butterfly->contiguousDouble()[i] = QNaN;
     
-    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateDouble));
+    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateDouble), m_butterfly);
     return m_butterfly->contiguousDouble();
 }
 
 ContiguousJSValues JSObject::convertUndecidedToContiguous(VM& vm)
 {
     ASSERT(hasUndecided(structure()->indexingType()));
-    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateContiguous));
+    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateContiguous), m_butterfly);
     return m_butterfly->contiguous();
 }
 
@@ -765,7 +765,7 @@ ContiguousDoubles JSObject::convertInt32ToDouble(VM& vm)
         *currentAsDouble = v.asInt32();
     }
     
-    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateDouble));
+    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateDouble), m_butterfly);
     return m_butterfly->contiguousDouble();
 }
 
@@ -773,7 +773,7 @@ ContiguousJSValues JSObject::convertInt32ToContiguous(VM& vm)
 {
     ASSERT(hasInt32(structure()->indexingType()));
     
-    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateContiguous));
+    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateContiguous), m_butterfly);
     return m_butterfly->contiguous();
 }
 
@@ -831,7 +831,7 @@ ContiguousJSValues JSObject::genericConvertDoubleToContiguous(VM& vm)
         currentAsValue->setWithoutWriteBarrier(v);
     }
     
-    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateContiguous));
+    setStructure(vm, Structure::nonPropertyTransition(vm, structure(), AllocateContiguous), m_butterfly);
     return m_butterfly->contiguous();
 }
 
@@ -1129,7 +1129,7 @@ void JSObject::switchToSlowPutArrayStorage(VM& vm)
     case NonArrayWithArrayStorage:
     case ArrayWithArrayStorage: {
         Structure* newStructure = Structure::nonPropertyTransition(vm, structure(), SwitchToSlowPutArrayStorage);
-        setStructure(vm, newStructure);
+        setStructure(vm, newStructure, m_butterfly);
         break;
     }
         
@@ -1153,7 +1153,7 @@ void JSObject::setPrototype(VM& vm, JSValue prototype)
         vm.prototypeMap.addPrototype(asObject(prototype));
     
     Structure* newStructure = Structure::changePrototypeTransition(vm, structure(), prototype);
-    setStructure(vm, newStructure);
+    setStructure(vm, newStructure, m_butterfly);
     
     if (!newStructure->anyObjectInChainMayInterceptIndexedAccesses())
         return;
@@ -1213,7 +1213,7 @@ void JSObject::putDirectAccessor(ExecState* exec, PropertyName propertyName, JSV
     // getters and setters, though, we also need to change our Structure
     // if we override an existing non-getter or non-setter.
     if (slot.type() != PutPropertySlot::NewProperty)
-        setStructure(vm, Structure::attributeChangeTransition(vm, structure(), propertyName, attributes));
+        setStructure(vm, Structure::attributeChangeTransition(vm, structure(), propertyName, attributes), m_butterfly);
 
     if (attributes & ReadOnly)
         structure()->setContainsReadOnlyProperties();
@@ -1570,7 +1570,7 @@ void JSObject::seal(VM& vm)
     if (isSealed(vm))
         return;
     preventExtensions(vm);
-    setStructure(vm, Structure::sealTransition(vm, structure()));
+    setStructure(vm, Structure::sealTransition(vm, structure()), m_butterfly);
 }
 
 void JSObject::freeze(VM& vm)
@@ -1578,14 +1578,14 @@ void JSObject::freeze(VM& vm)
     if (isFrozen(vm))
         return;
     preventExtensions(vm);
-    setStructure(vm, Structure::freezeTransition(vm, structure()));
+    setStructure(vm, Structure::freezeTransition(vm, structure()), m_butterfly);
 }
 
 void JSObject::preventExtensions(VM& vm)
 {
     enterDictionaryIndexingMode(vm);
     if (isExtensible())
-        setStructure(vm, Structure::preventExtensionsTransition(vm, structure()));
+        setStructure(vm, Structure::preventExtensionsTransition(vm, structure()), m_butterfly);
 }
 
 // This presently will flatten to an uncachable dictionary; this is suitable
@@ -1603,7 +1603,7 @@ void JSObject::reifyStaticFunctionsForDelete(ExecState* exec)
     }
 
     if (!structure()->isUncacheableDictionary())
-        setStructure(vm, Structure::toUncacheableDictionaryTransition(vm, structure()));
+        setStructure(vm, Structure::toUncacheableDictionaryTransition(vm, structure()), m_butterfly);
 
     for (const ClassInfo* info = classInfo(); info; info = info->parentClass) {
         const HashTable* hashTable = info->propHashTable(globalObject()->globalExec());
@@ -1633,7 +1633,7 @@ bool JSObject::removeDirect(VM& vm, PropertyName propertyName)
         return true;
     }
 
-    setStructure(vm, Structure::removePropertyTransition(vm, structure(), propertyName, offset));
+    setStructure(vm, Structure::removePropertyTransition(vm, structure(), propertyName, offset), m_butterfly);
     if (offset == invalidOffset)
         return false;
     putDirectUndefined(offset);
diff --git a/Source/JavaScriptCore/runtime/JSObject.h b/Source/JavaScriptCore/runtime/JSObject.h
index c62dc2aec..7a78a46a6 100644
--- a/Source/JavaScriptCore/runtime/JSObject.h
+++ b/Source/JavaScriptCore/runtime/JSObject.h
@@ -595,6 +595,7 @@ public:
     void setButterfly(VM&, Butterfly*, Structure*);
     void setButterflyWithoutChangingStructure(Butterfly*); // You probably don't want to call this.
         
+    void setStructure(VM&, Structure*, Butterfly* = 0);
     void setStructureAndReallocateStorageIfNecessary(VM&, unsigned oldCapacity, Structure*);
     void setStructureAndReallocateStorageIfNecessary(VM&, Structure*);
 
@@ -1109,7 +1110,7 @@ inline void JSObject::setButterfly(VM& vm, Butterfly* butterfly, Structure* stru
 {
     ASSERT(structure);
     ASSERT(!butterfly == (!structure->outOfLineCapacity() && !hasIndexingHeader(structure->indexingType())));
-    setStructure(vm, structure);
+    setStructure(vm, structure, butterfly);
     m_butterfly = butterfly;
 }
 
@@ -1316,7 +1317,7 @@ inline bool JSObject::putDirectInternal(VM& vm, PropertyName propertyName, JSVal
                 return true;
             }
             // case (2) Despecify, fall through to (3).
-            setStructure(vm, Structure::despecifyFunctionTransition(vm, structure(), propertyName));
+            setStructure(vm, Structure::despecifyFunctionTransition(vm, structure(), propertyName), m_butterfly);
         }
 
         // case (3) set the slot, do the put, return.
@@ -1344,12 +1345,18 @@ inline bool JSObject::putDirectInternal(VM& vm, PropertyName propertyName, JSVal
     return true;
 }
 
+inline void JSObject::setStructure(VM& vm, Structure* structure, Butterfly* butterfly)
+{
+    JSCell::setStructure(vm, structure);
+    ASSERT_UNUSED(butterfly, !butterfly == !(structure->outOfLineCapacity() || hasIndexingHeader(structure->indexingType())));
+}
+
 inline void JSObject::setStructureAndReallocateStorageIfNecessary(VM& vm, unsigned oldCapacity, Structure* newStructure)
 {
     ASSERT(oldCapacity <= newStructure->outOfLineCapacity());
     
     if (oldCapacity == newStructure->outOfLineCapacity()) {
-        setStructure(vm, newStructure);
+        setStructure(vm, newStructure, m_butterfly);
         return;
     }
     
diff --git a/Source/JavaScriptCore/runtime/Structure.cpp b/Source/JavaScriptCore/runtime/Structure.cpp
index f551eaecc..950728cca 100644
--- a/Source/JavaScriptCore/runtime/Structure.cpp
+++ b/Source/JavaScriptCore/runtime/Structure.cpp
@@ -649,6 +649,12 @@ Structure* Structure::flattenDictionaryStructure(VM& vm, JSObject* object)
     }
 
     m_dictionaryKind = NoneDictionaryKind;
+
+    // If the object had a Butterfly but after flattening/compacting we no longer have need of it,
+    // we need to zero it out because the collector depends on the Structure to know the size for copying.
+    if (object->butterfly() && !this->outOfLineCapacity() && !hasIndexingHeader(this->indexingType()))
+        object->setButterfly(vm, 0, this);
+
     return this;
 }