<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>

1 files changed, 10 insertions, 3 deletions

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;
     }