RFR (S) 8229900: RedefineDeleteJmethod.java fails with -Xcheck:jni

David Holmes david.holmes at oracle.com
Thu Oct 3 07:04:51 UTC 2019

Hi Coleen,

On 2/10/2019 11:36 pm, coleen.phillimore at oracle.com wrote:
> On 10/2/19 12:53 AM, David Holmes wrote:
>> Hi Coleen,
>> Sorry rather long-winded reply ...
>> On 1/10/2019 11:52 pm, coleen.phillimore at oracle.com wrote:
>>> Summary: Remove RedefineClasses adjustment and test, but improve 
>>> checking for method/class matching.
>>> Tested with tier1 with -Xcheck:jni locally, and tier1 on Oracle 
>>> platforms.
>>> open webrev at 
>>> http://cr.openjdk.java.net/~coleenp/2019/8229900.01/webrev
>>> bug link https://bugs.openjdk.java.net/browse/JDK-8229900
>> I was troubled by this change because it made me think more deeply 
>> about what should reasonably happen with respect to jmethodIDs and 
>> class redefinition.
>> I had a (simplistic?) mental model that a jmethodID is a "pointer" to 
>> a specific method in a specific class (the defining class of the 
>> method). As such a jmethodID should only be used when operating on 
>> that class (or subclass thereof) or an instance of that class (or 
>> subclass thereof). With that mental model it makes sense for 
>> Xcheck:jni to validate the defining class is the target class, or a 
>> superclass thereof. But it doesn't then make sense for class 
>> redefinition to update jmethodIDs as the redefined class is not the 
>> original class! If a class is redefined then it should, in my mental 
>> model, invalidate all existing jmethodIDs that pertained to the 
>> original class.
> Your simplistic model is correct.  The jmethodID is a pointer to a 
> specific method in a class.  In the call, we check that it applies to a 
> class or subclass.
> The current behavior is that if the method is "obsolete", meaning that 
> the bytecodes are new, the methodID is not replaced:
>        // obsolete methods need a unique idnum so they become new 
> entries in
>        // the jmethodID cache in InstanceKlass
>        assert(old_method->method_idnum() == new_method->method_idnum(), 
> "must match");
>        u2 num = InstanceKlass::cast(_the_class)->next_method_idnum();
>        if (num != ConstMethod::UNSET_IDNUM) {
>          old_method->set_method_idnum(num);
>        }
> If the redefined method is the same *bytecodes* as the old method 
> "emcp", the methodID *is* replaced.   This is good.

Okay I don't have a good enough understanding for how this is all 
supposed to work to continue discussing this aspect. Seems we don't 
actually invalidate jmethodIDs they either keep referring to the old 
method implementation, or they are updated to the new one if it is 
equivalent to the old one.

>> However, that's not very user friendly in the face of redefinition of 
>> a superclass as code that only works with the subclass may reasonably 
>> expect jmethodIDs to remain valid even if they refer to an inherited 
>> method that has been redefined. So we update them to refer to the 
>> redefined method implementation.
> Not sure I follow this.  We only update them for emcp methods, but I 
> don't see why calling through a subclass is a distinction.

Let's say I am working with a subclass and obtained jmethodIDs that 
refer to methods in the superclass. If the superclass is redefined 
outside the knowledge of the code using the subclass and jmethodIDs, 
then suddenly making those jmethodIDs be invalid would come as a bit of 
a surprise to that code. But I see now that they never become invalid so 
the point is moot.

>> So in that regards the update to jniCheck::validate_call_class seems 
>> correct. Though I wonder if we also need to check that obj->class is a 
>> subtype of clazz? As far as I can see we never actually validate that! 
>> We use the jmethodID to find the method and we then find the vtable 
>> index wrt. the method->holder class, and then we use that vtable index 
>> to lookup a method in the receiver object's class - which could lead 
>> to a random method being selected in a different class!
> We could add that check in a new RFE.
>> Continuing on, if we do expect jmethodIDs to get updated upon class 
>> redefinition then it makes sense to me to keep the logic that handles 
>> deleted methods, by redirecting them to a method that throws NSME. The 
>> fact that method is in Unsafe is unfortunate but it is what we do 
>> elsewhere in the VM. I'm assuming the problem here is that the 
>> augmented jniCheck::validate_call_class will fail in such cases? That 
>> is a problem, but I think I'd rather see it special-cased than change 
>> the existing behaviour:
>>   if (obj != NULL) {
>>     jniCheck::validate_object(thr, obj);
>>   }
>> + if (m == Universe::throw_no_such_method_error())
>> +   return;  // skip class checks in this case
>> then the test could also remain.
>> Also note that while we generally require JNI programmers to ensure 
>> everything is called correctly, jmethodIDs are also used by JVM TI and 
>> we tend to want JVM TI to have well defined semantics. I'm unclear now 
>> what happens if we invoke a deleted method through JVM TI ?
> JVMTI semantics should follow JNI semantics.   If JVMTI uses jmethodIDs 
> and invoke a deleted method, it should get undefined behavior.   Not 
> sure what you mean here either.

I hold JVM TI to a higher standard than JNI when it comes to dealing 
with these kinds of errors. But I was under the mistaken impression that 
JVM TI provided an API to invoke methods via jmethodIDs, but seems I was 
wrong and that it is only available in JNI.

I still dislike the idea that there is "undefined behaviour" here, and I 
would expect any jmethodID that referred to a now deleted method to 
actually still refer to it as it is the "old version". But that's not 
related to this change so ...

I don't know the exact context under which the NSME handling was put in, 
so can't really comment on its removal. Throwing the NSME doesn't seem 
unreasonable to me but that's besides the point.


> When looking at this change and trying to decide whether to keep this 
> replacement with the special cases in jniCheck.cpp above or remove it, 
> these things led me to decide to remove it.
> 1. This is JNI and we don't generally protect people from undefined 
> behavior.   In fact, if you're using JNI, I think a crash is a better 
> debugging tool than a Java NSME exception, which will propagate out of 
> the JNI context.  That would be my preference if I were a developer.
> 2. Adding/deleting methods in redefinition has been removed, except with 
> a command line switch to enable old behavior, so adding something 
> helpful for something deprecated seemed silly.
> 3. This "helpful" NSME for jmethodID was a recent addition for a 
> different problem.  Nobody relies on this.
> 4. It looks like the "helpful" NSME replacement unintentionally affected 
> more than deleted methods, so old code might get different results (see 
> 1 above).
> 5. We have so many special cases in the jvm for so many different 
> things, we don't need another.
> Thanks,
> Coleen
>> Thanks,
>> David
>> ----
>>> Thanks,
>>> Coleen

More information about the hotspot-runtime-dev mailing list