JVMTI retransformation and addition of private methods

Wed Feb 21 17:55:19 UTC 2018

Dan,

Thank you for all the background digging. This is really helpful.

Serguei - do you know what tests exist for this behavior?

The way I read the source code - we currently allow ADD and DELETE for
PRIVATE OR STATIC OR FINAL methods. Did I read that correctly?

With the current implementation, I am not sure if deletion works for private methods - do we
have a test for that? Or could you add one as part of this exercise?

Today we create a vtable entry for private methods (my misunderstanding ~ 2006ish). After discussions
with David I no longer believe we need those.
Today, klassVtable::adjust_method_entries has an assertion
  assert(!old_method->is_deleted(), “vtable methods may not be deleted”)

I may have read the code incorrectly - but I would expect to hit this assertion if you had a private
method you were deleting that was not final and not static.

option 1) we could explicitly tighten the restrictions to match what we have implemented
option 2) we could make this work by changing klassVtable.cpp::update_inherited_vtable
  handling of private fields to be done the way it handles final fields.
option 3) I read the code incorrectly?

thanks,
Karen

> On Feb 21, 2018, at 10:40 AM, Daniel D. Daugherty <daniel.daugherty at oracle.com> wrote:
> 
> On 2/21/18 2:45 AM, serguei.spitsyn at oracle.com <mailto:serguei.spitsyn at oracle.com> wrote:
>> On 2/20/18 23:01, David Holmes wrote: 
>>> On 21/02/2018 4:50 PM, serguei.spitsyn at oracle.com <mailto:serguei.spitsyn at oracle.com> wrote: 
>>>> Hi Karen and David, 
>>>> 
>>>> 
>>>> On 2/20/18 19:52, David Holmes wrote: 
>>>>> Hi Karen, 
>>>>> 
>>>>> On 21/02/2018 1:54 AM, Karen Kinnear wrote: 
>>>>>> Folks, 
>>>>>> 
>>>>>> As part of the Valhalla EG discussions for JVMTI changes for nestmates (thank you Serguei and David), 
>>>>>> IBM brought up a request that we update the JVMTI documentation to reflect that we allow addition 
>>>>>> of private methods. 
>>>>>> 
>>>>>> Is there a reason we do not document this? I’m inviting those who were involved at the time - please include 
>>>>>> others if needed. 
>>>> 
>>>> I support documenting this in the JVMTI spec and had a plan to fix it in 11. 
>>>> However, it is not clear to me yet if we have a consensus on it. 
>>> 
>>> I would like to see a detailed analysis of the implications of allowing this. I _think_ it is safe but ... 
>> 
>> Valid concern. 
>> Also, I'd love to collect more details on the initial motivation to relax the JVMTI spec. 
>> Most likely we had no CCC/CSR filed on this change. 
>> 
>> 
>>>>> This issue is tracked by: 
>>>>> 
>>>>> https://bugs.openjdk.java.net/browse/JDK-8192936 <https://bugs.openjdk.java.net/browse/JDK-8192936> 
>>>>> 
>>>>> "RI does not follow the JVMTI RedefineClasses spec that is too strict in the definition" 
>>>> 
>>>> Yes, this is the one. 
>>>> Thank you, David, for posting the link. 
>>>> 
>>>> 
>>>>> As I wrote there ... It is not at all clear how JDK-6404550 morphed into "Permit the adding or deleting of private final/static methods with redefine" - nor why those changes failed to make any change to the spec itself. It is also unclear whether the add/delete is restricted to final/static methods or any private method? I can see that the intent was to only allow something that would not perturb the vtable for existing instances. 
>>>> 
>>>> I agree, there is a confusion somewhere. 
>>>> Is it possible, the JDK-6404550 in JIRA is a different bug than the one in the Bugtraq system? 
>>>> 
>>>> The JDK-6404550 in JIRA has a different synopsis: 
>>>> https://bugs.openjdk.java.net/browse/JDK-6404550 <https://bugs.openjdk.java.net/browse/JDK-6404550> 
>>>>       Cannot implement late attach in NetBeans Profiler due to missing functionality in JVMTI 
>>> 
>>> Digging deeper ... to fix the problem described in that bug they augmented JVM TI to allow private method redefinition as an alternate to the "native rebinding" technique that had been used previously. See the final comment in: 
>>> 
>>> https://bugs.openjdk.java.net/browse/JDK-6341303 <https://bugs.openjdk.java.net/browse/JDK-6341303> 
>>> 
>>> "JVMTI Spec: Need a way how to rebind Object.wait and Thread.sleep with late attach" 
>>> 
>>> which was closed as a duplicate. 
>> 
>> Thank you for the point. 
>> This explains it. 
>> It seems, the bug synopsis was changed at some moment. 
> 
> The synopsis for 6404550 has never changed. Here's the subject line when
> it was created on 2006.03.27:
> 
> > CR 6404550 *HOT* Created P1 hotspot/jvmti Cannot implement late attach in NetBeans Profiler due to missing functionality in JVMTI
> 
> I think the confusion arises over comments like this in 6341303:
> 
>>  Robert Field <https://bugs.openjdk.java.net/secure/ViewProfile.jspa?name=rfield> added a comment - 2006-05-04 11:54
>> BT2:EVALUATION
>> 
>> This can now be accomplished with Java programming language instrumentation, via:
>> 
>>      6404550: missing late attach (JVM TI redefine) functionality
>>            Permit the adding or deleting of private final/static methods with redefine
>> 
>> Closing this bug as a duplicate.
> 
> That's just Robert's style for an sccs delta comment:
> 
> D 1.65.2.3 06/04/25 23:36:35 rfield 140 139     00023/00013/03263
> MRs:
> COMMENTS:
> 6404550: missing late attach (JVM TI redefine) functionality
>             Add/delete private methods, continued: changes per review
> 
> Back in the ancient past we tried to include some brief
> info about the change in the delta comment. This was one of many
> deltas associated with 6404550.
> 
> Please see the attached email that I sent on 2012.12.17 about the
> history behind this issue... (sent to Karen, Mikael V, and Serguei)
> 
> It seems I forwarded that same email to Coleen, Markus G and Serguei
> back on 2014.05.20. Since Markus is on that thread, it must have had
> something to do with research about JFR...
> 
> I need to do a detailed read thru my e-mail archive for 6404550 to
> see if I can spot some clues about why we didn't do a spec update.
> 
> Dan
> 
> 
>> 
>> Thanks, 
>> Serguei 
>> 
>>> 
>>> David 
>>> ----- 
>>> 
>>>> 
>>>> Thanks, 
>>>> Serguei 
>>>> 
>>>> 
>>>>> -- 
>>>>> David 
>>>>> 
>>>>> 
>>>>>> thanks, 
>>>>>> Karen 
>>>>>> 
>>>> 
>> 
>> 
> 
> <Attached Message.eml>

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