<AWT Dev> [15] Review request for 8242498: Invalid "sun.awt.TimedWindowEvent" object leads to JVM crash

Anton Litvinov anton.litvinov at oracle.com
Wed Apr 15 19:26:48 UTC 2020

Hi Sergey,

This bug happens, when original "opposite" is HWND of the peer object of 
the "java.awt.Component", like "java.awt.Label", "java.awt.TextArea" 
etc. which are strictly not instances of "java.awt.Window" class. These 
AWT components cannot be displayed on a screen without being located in 
some instance of "java.awt.Window". Therefore getting parent HWND of the 
peer's HWND of these AWT components works and resolves the bug. This 
approach resolves the bug also because there is a reproducer of this bug 
which you saw yourself and which proves that the fix resolves the crash. 
This is the reply to your question "why it will work fine at step(2)?".

When JDK is used in a normal way and not as it is used in the 
reproducer, "opposite" is always HWND of a peer of "java.awt.Window" 
instance, thus for such cases there is no need to search for some parent 
HWND of the "opposite" HWND. I do not want to break behavior of JDK for 
these normal scenarios, by getting some parent HWND of the valid 
"opposite" HWND. I do not want to rely on expectation that for valid 
"opposite" HWND the method "AwtComponent::GetTopLevelParentForWindow" 
must return the same "opposite" HWND, I do not want to rely on this 
expectation and run the risk of using some incorrect HWND as opposite, 
when there was already valid "opposite" HWND in hands, and by this to 
break existing JDK behavior related to focus handling, because this 
edited "AwtWindow::SendWindowEvent" method is really involved in focus 

Thank you,

On 15/04/2020 07:56, Sergey Bylokhov wrote:
> On 4/14/20 11:12 am, Anton Litvinov wrote:
>> I think that it is better not to change current well established old 
>> behavior in the method "AwtWindow::SendWindowEvent(jint, HWND, jint, 
>> jint)" - what is first to get "jobject" from the original "opposite" 
>> HWND and try to use it for construction of "TimedWindowEvent". I was 
>> trying to make a fix which will just extend the code and will address 
>> just this very unreal and very narrow scenario without running the 
>> risk of affecting or breaking wide range of other normal scenarios 
>> under which this code was working well from the moment of its 
>> creation. If we assign parent HWND to the "opposite" from the very 
>> beginning, then we will change the currently existing behavior in 
>> favor of addressing this almost unreal bug and may for some cases, 
>> which we do not know yet, start using parent HWND instead of a valid 
>> original "opposite" HWND whose target will be instance of 
>> "java.awt.Window".
>> In my opinion it is better not to omit first attempt to use original 
>> "opposite" with related to it "jobject" instance.
> If it is not safe to replace the opposite by the 
> GetTopLevelParentForWindow(1) in the first place then
> why it is safe to use it in the fix later(1)? What could go wrong at 
> step (1) and why it will work fine
> at step(2)?
>> Thank you,
>> Anton
>> On 14/04/2020 16:41, Sergey Bylokhov wrote:
>>> Hi, Anton.
>>> Probably it is possible to simplify the code a little bit:
>>> Can we just replace initial "opposite" by the top level HWND?
>>> + opposite = AwtComponent::GetTopLevelParentForWindow(opposite);
>>>   AwtComponent *awtOpposite = AwtComponent::GetComponent(opposite);
>>> and add only one check
>>>   jOpposite = awtOpposite->GetTarget(env);
>>> + if ((jOpposite != NULL) &&
>>> +   !env->IsInstanceOf(jOpposite, windowCls)) {
>>> +       env->DeleteLocalRef(jOpposite);
>>> +       jOpposite = NULL;
>>> +  }
>>> On 4/10/20 1:32 pm, Anton Litvinov wrote:
>>>> Hello,
>>>> Could you please review the following fix for the bug.
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8242498
>>>> Webrev: http://cr.openjdk.java.net/~alitvinov/8242498/jdk15/webrev.00
>>>> The bug is the JVM crash, which occurs because a not existing 
>>>> method is called on a Java object which is not an instance of the 
>>>> expected Java class that has such a method. Such discrepancy of the 
>>>> expected type and the type in runtime is possible, because the Java 
>>>> object, whose field value is set to the instance of the not 
>>>> expected Java class, is instantiated by AWT native code through JNI 
>>>> invocation. Since JNI does not validate arguments passed to Java 
>>>> class constructor and since AWT native code does not validate 
>>>> arguments prior to invoking Java class constructor through JNI, 
>>>> such invalid object is created.
>>>> The fact that in the method 
>>>> "java.awt.DefaultKeyboardFocusManager.dispatchEvent(AWTEvent)" in 
>>>> the case "WindowEvent.WINDOW_LOST_FOCUS" of switch operator the 
>>>> variable defined by the expression "Window oppositeWindow = 
>>>> we.getOppositeWindow();" in runtime is instance of 
>>>> "java.awt.Component" class instead of "java.awt.Window" class. The 
>>>> crash occurs during attempt to call the method 
>>>> "java.awt.Window.getTemporaryLostComponent()" on the object 
>>>> "oppositeWindow" which in runtime is "Component" instead of the 
>>>> expected "Window" object, and since the method 
>>>> "getTemporaryLostComponent()" does not exist in 
>>>> "java.awt.Component" class JVM cannot find this method and 
>>>> initiates the crash.
>>>> Transfer of the object of the incompatible type 
>>>> "java.awt.Component" instead of an object of "java.awt.Window" type 
>>>> as "opposite" argument to the constructor "TimedWindowEvent(Window 
>>>> source, int id, Window opposite, int oldState, int newState, long 
>>>> time)" of the class "sun.awt.TimedWindowEvent" through JNI 
>>>> invocation. This JNI invocation occurs in the C++ class method 
>>>> "AwtWindow::SendWindowEvent(jint, HWND, jint, jint)" in the file 
>>>> "src/java.desktop/windows/native/libawt/windows/awt_Window.cpp". 
>>>> The exact expression creating the instance of Java class 
>>>> "TimedWindowEvent" with the invalid value of the field "opposite" 
>>>> is following:
>>>> jobject event = env->NewObject(wClassEvent, wEventInitMID, target, id,
>>>>          jOpposite, oldState, newState, 
>>>> ::JVM_CurrentTimeMillis(NULL, 0));
>>>> THE FIX:
>>>> The fix changes "AwtWindow::SendWindowEvent(jint, HWND, jint, 
>>>> jint)" method in the file "awt_Window.cpp" to introduce the code 
>>>> which verifies that the Java object "jOpposite" is really instance 
>>>> of the class "java.awt.Window", and if it is not then the fix tries 
>>>> to get Java object corresponding to parent window of the original 
>>>> "opposite" HWND. And if this parent window object also is not 
>>>> instance of "java.awt.Window" class, then NULL value is passed to 
>>>> the constructor of "TimedWindowEvent" class.
>>>> Thank you,
>>>> Anton

More information about the awt-dev mailing list