Bug 1456494 - Initialize Zone::helperThreadUse_ first to avoid accessing uninitialized memory in debug builds. r=jonco

--HG--
extra : rebase_source : 9abb7cecd084e7f8c4bbe7ce1ff3e34c10e024ce

js/src/gc/Zone.cpp

@@ -26,6 +26,10 @@ Zone * const Zone::NotOnList = reinterpret_cast<Zone*>(1);
 
 JS::Zone::Zone(JSRuntime* rt)
   : JS::shadow::Zone(rt, &rt->gc.marker),
+    // Note: don't use |this| before initializing helperThreadUse_!
+    // ProtectedData checks in CheckZone::check may read this field.
+    helperThreadUse_(HelperThreadUse::None),
+    helperThreadOwnerContext_(nullptr),
     debuggers(this, nullptr),
     uniqueIds_(this),
     suppressAllocationMetadataBuilder(this, false),
@@ -54,8 +58,6 @@ JS::Zone::Zone(JSRuntime* rt)
     nurseryShapes_(this),
     data(this, nullptr),
     isSystem(this, false),
-    helperThreadOwnerContext_(nullptr),
-    helperThreadUse(HelperThreadUse::None),
 #ifdef DEBUG
     gcLastSweepGroupIndex(this, 0),
 #endif
@@ -78,7 +80,7 @@ JS::Zone::Zone(JSRuntime* rt)
 
 Zone::~Zone()
 {
-    MOZ_ASSERT(helperThreadUse == HelperThreadUse::None);
+    MOZ_ASSERT(helperThreadUse_ == HelperThreadUse::None);
 
     JSRuntime* rt = runtimeFromAnyThread();
     if (this == rt->gc.systemZone)

js/src/gc/Zone.h

@@ -151,6 +151,44 @@ struct Zone : public JS::shadow::Zone,
     MOZ_MUST_USE bool init(bool isSystem);
     void destroy(js::FreeOp *fop);
 
+  private:
+    enum class HelperThreadUse : uint32_t {
+        None,
+        Pending,
+        Active
+    };
+    mozilla::Atomic<HelperThreadUse> helperThreadUse_;
+
+    // The helper thread context with exclusive access to this zone, if
+    // usedByHelperThread(), or nullptr when on the main thread.
+    js::UnprotectedData<JSContext*> helperThreadOwnerContext_;
+
+  public:
+    bool ownedByCurrentHelperThread();
+    void setHelperThreadOwnerContext(JSContext* cx);
+
+    // Whether this zone was created for use by a helper thread.
+    bool createdForHelperThread() const {
+        return helperThreadUse_ != HelperThreadUse::None;
+    }
+    // Whether this zone is currently in use by a helper thread.
+    bool usedByHelperThread() {
+        MOZ_ASSERT_IF(isAtomsZone(), helperThreadUse_ == HelperThreadUse::None);
+        return helperThreadUse_ == HelperThreadUse::Active;
+    }
+    void setCreatedForHelperThread() {
+        MOZ_ASSERT(helperThreadUse_ == HelperThreadUse::None);
+        helperThreadUse_ = HelperThreadUse::Pending;
+    }
+    void setUsedByHelperThread() {
+        MOZ_ASSERT(helperThreadUse_ == HelperThreadUse::Pending);
+        helperThreadUse_ = HelperThreadUse::Active;
+    }
+    void clearUsedByHelperThread() {
+        MOZ_ASSERT(helperThreadUse_ != HelperThreadUse::None);
+        helperThreadUse_ = HelperThreadUse::None;
+    }
+
     void findOutgoingEdges(js::gc::ZoneComponentFinder& finder);
 
     void discardJitCode(js::FreeOp* fop, bool discardBaselineCode = true);
@@ -513,48 +551,6 @@ struct Zone : public JS::shadow::Zone,
 
     js::ZoneData<bool> isSystem;
 
-  private:
-    // The helper thread context with exclusive access to this zone, if
-    // usedByHelperThread(), or nullptr when on the main thread.
-    js::UnprotectedData<JSContext*> helperThreadOwnerContext_;
-
-  public:
-    bool ownedByCurrentHelperThread();
-    void setHelperThreadOwnerContext(JSContext* cx);
-
-  private:
-    enum class HelperThreadUse : uint32_t
-    {
-        None,
-        Pending,
-        Active
-    };
-
-    mozilla::Atomic<HelperThreadUse> helperThreadUse;
-
-  public:
-    // Whether this zone was created for use by a helper thread.
-    bool createdForHelperThread() const {
-        return helperThreadUse != HelperThreadUse::None;
-    }
-    // Whether this zone is currently in use by a helper thread.
-    bool usedByHelperThread() {
-        MOZ_ASSERT_IF(isAtomsZone(), helperThreadUse == HelperThreadUse::None);
-        return helperThreadUse == HelperThreadUse::Active;
-    }
-    void setCreatedForHelperThread() {
-        MOZ_ASSERT(helperThreadUse == HelperThreadUse::None);
-        helperThreadUse = HelperThreadUse::Pending;
-    }
-    void setUsedByHelperThread() {
-        MOZ_ASSERT(helperThreadUse == HelperThreadUse::Pending);
-        helperThreadUse = HelperThreadUse::Active;
-    }
-    void clearUsedByHelperThread() {
-        MOZ_ASSERT(helperThreadUse != HelperThreadUse::None);
-        helperThreadUse = HelperThreadUse::None;
-    }
-
 #ifdef DEBUG
     js::ZoneData<unsigned> gcLastSweepGroupIndex;
 #endif