Disable SharedArrayBuffers to mitigate Spectre.  Based on:

  https://trac.webkit.org/changeset/226386/webkit

Backported to webkitgtk-2.18.4 by Mark H Weaver <mhw@netris.org>


--- webkitgtk-2.18.4/Source/JavaScriptCore/runtime/JSGlobalObject.h.orig	2017-12-19 02:23:07.000000000 -0500
+++ webkitgtk-2.18.4/Source/JavaScriptCore/runtime/JSGlobalObject.h	2018-01-06 19:28:55.985066986 -0500
@@ -338,8 +338,10 @@
     WriteBarrier<Structure> m_moduleLoaderStructure;
     WriteBarrier<JSArrayBufferPrototype> m_arrayBufferPrototype;
     WriteBarrier<Structure> m_arrayBufferStructure;
+#if ENABLE(SHARED_ARRAY_BUFFER)
     WriteBarrier<JSArrayBufferPrototype> m_sharedArrayBufferPrototype;
     WriteBarrier<Structure> m_sharedArrayBufferStructure;
+#endif
 
 #define DEFINE_STORAGE_FOR_SIMPLE_TYPE(capitalName, lowerName, properName, instanceType, jsName, prototypeBase) \
     WriteBarrier<capitalName ## Prototype> m_ ## lowerName ## Prototype; \
@@ -670,8 +672,13 @@
         switch (sharingMode) {
         case ArrayBufferSharingMode::Default:
             return m_arrayBufferPrototype.get();
+#if ENABLE(SHARED_ARRAY_BUFFER)
         case ArrayBufferSharingMode::Shared:
             return m_sharedArrayBufferPrototype.get();
+#else
+        default:
+            return m_arrayBufferPrototype.get();
+#endif
         }
     }
     Structure* arrayBufferStructure(ArrayBufferSharingMode sharingMode) const
@@ -679,8 +686,13 @@
         switch (sharingMode) {
         case ArrayBufferSharingMode::Default:
             return m_arrayBufferStructure.get();
+#if ENABLE(SHARED_ARRAY_BUFFER)
         case ArrayBufferSharingMode::Shared:
             return m_sharedArrayBufferStructure.get();
+#else
+        default:
+            return m_arrayBufferStructure.get();
+#endif
         }
         RELEASE_ASSERT_NOT_REACHED();
         return nullptr;
--- webkitgtk-2.18.4/Source/JavaScriptCore/runtime/JSGlobalObject.cpp.orig	2017-12-19 02:23:07.000000000 -0500
+++ webkitgtk-2.18.4/Source/JavaScriptCore/runtime/JSGlobalObject.cpp	2018-01-06 19:27:16.628574304 -0500
@@ -574,8 +574,10 @@
     
     m_arrayBufferPrototype.set(vm, this, JSArrayBufferPrototype::create(vm, this, JSArrayBufferPrototype::createStructure(vm, this, m_objectPrototype.get()), ArrayBufferSharingMode::Default));
     m_arrayBufferStructure.set(vm, this, JSArrayBuffer::createStructure(vm, this, m_arrayBufferPrototype.get()));
+#if ENABLE(SHARED_ARRAY_BUFFER)
     m_sharedArrayBufferPrototype.set(vm, this, JSArrayBufferPrototype::create(vm, this, JSArrayBufferPrototype::createStructure(vm, this, m_objectPrototype.get()), ArrayBufferSharingMode::Shared));
     m_sharedArrayBufferStructure.set(vm, this, JSArrayBuffer::createStructure(vm, this, m_sharedArrayBufferPrototype.get()));
+#endif
 
     m_iteratorPrototype.set(vm, this, IteratorPrototype::create(vm, this, IteratorPrototype::createStructure(vm, this, m_objectPrototype.get())));
     m_generatorPrototype.set(vm, this, GeneratorPrototype::create(vm, this, GeneratorPrototype::createStructure(vm, this, m_iteratorPrototype.get())));
@@ -620,10 +622,11 @@
     
     JSArrayBufferConstructor* arrayBufferConstructor = JSArrayBufferConstructor::create(vm, JSArrayBufferConstructor::createStructure(vm, this, m_functionPrototype.get()), m_arrayBufferPrototype.get(), m_speciesGetterSetter.get(), ArrayBufferSharingMode::Default);
     m_arrayBufferPrototype->putDirectWithoutTransition(vm, vm.propertyNames->constructor, arrayBufferConstructor, DontEnum);
+#if ENABLE(SHARED_ARRAY_BUFFER)
     JSArrayBufferConstructor* sharedArrayBufferConstructor = nullptr;
     sharedArrayBufferConstructor = JSArrayBufferConstructor::create(vm, JSArrayBufferConstructor::createStructure(vm, this, m_functionPrototype.get()), m_sharedArrayBufferPrototype.get(), m_speciesGetterSetter.get(), ArrayBufferSharingMode::Shared);
     m_sharedArrayBufferPrototype->putDirectWithoutTransition(vm, vm.propertyNames->constructor, sharedArrayBufferConstructor, DontEnum);
-    
+#endif
 #define CREATE_CONSTRUCTOR_FOR_SIMPLE_TYPE(capitalName, lowerName, properName, instanceType, jsName, prototypeBase) \
 capitalName ## Constructor* lowerName ## Constructor = capitalName ## Constructor::create(vm, capitalName ## Constructor::createStructure(vm, this, m_functionPrototype.get()), m_ ## lowerName ## Prototype.get(), m_speciesGetterSetter.get()); \
 m_ ## lowerName ## Prototype->putDirectWithoutTransition(vm, vm.propertyNames->constructor, lowerName ## Constructor, DontEnum); \
@@ -686,7 +689,9 @@
     putDirectWithoutTransition(vm, vm.propertyNames->builtinNames().ArrayPrivateName(), arrayConstructor, DontEnum | DontDelete | ReadOnly);
 
     putDirectWithoutTransition(vm, vm.propertyNames->ArrayBuffer, arrayBufferConstructor, DontEnum);
+#if ENABLE(SHARED_ARRAY_BUFFER)
     putDirectWithoutTransition(vm, vm.propertyNames->SharedArrayBuffer, sharedArrayBufferConstructor, DontEnum);
+#endif
 
 #define PUT_CONSTRUCTOR_FOR_SIMPLE_TYPE(capitalName, lowerName, properName, instanceType, jsName, prototypeBase) \
 putDirectWithoutTransition(vm, vm.propertyNames-> jsName, lowerName ## Constructor, DontEnum); \
@@ -1288,8 +1293,10 @@
     
     visitor.append(thisObject->m_arrayBufferPrototype);
     visitor.append(thisObject->m_arrayBufferStructure);
+#if ENABLE(SHARED_ARRAY_BUFFER)
     visitor.append(thisObject->m_sharedArrayBufferPrototype);
     visitor.append(thisObject->m_sharedArrayBufferStructure);
+#endif
 
 #define VISIT_SIMPLE_TYPE(CapitalName, lowerName, properName, instanceType, jsName, prototypeBase) \
     visitor.append(thisObject->m_ ## lowerName ## Prototype); \
--- webkitgtk-2.18.4/Source/WTF/wtf/Platform.h.orig	2017-10-16 08:18:56.000000000 -0400
+++ webkitgtk-2.18.4/Source/WTF/wtf/Platform.h	2018-01-06 19:29:52.897349199 -0500
@@ -1190,6 +1190,9 @@
 #define HAVE_NS_ACTIVITY 1
 #endif
 
+/* Disable SharedArrayBuffers until Spectre security concerns are mitigated. */
+#define ENABLE_SHARED_ARRAY_BUFFER 0
+
 #if (OS(DARWIN) && USE(CG)) || (USE(FREETYPE) && !PLATFORM(GTK)) || (PLATFORM(WIN) && (USE(CG) || USE(CAIRO)))
 #undef ENABLE_OPENTYPE_MATH
 #define ENABLE_OPENTYPE_MATH 1