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Hi Karen,
Thank you for sorting this out!
On 2/21/18 09:55, Karen Kinnear wrote:
Dan,
Thank you for all the background digging. This is
really helpful.
Serguei - do you know what tests exist for this
behavior?
Dan already replied (thanks, Dan!)
There are two tests in the open/test/jdk/java/lang/instrument:
RedefineMethodAddInvoke*
RedefineMethodDelInvoke*
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?
The above does not look correct to me.
We have the same check for both ADD and DELETE method:
if ((old_flags & JVM_ACC_PRIVATE) == 0
// hack: private should be treated as final, but alas
|| (old_flags & (JVM_ACC_FINAL|JVM_ACC_STATIC)) == 0
) {
// deleted methods must be private
return JVMTI_ERROR_UNSUPPORTED_REDEFINITION_METHOD_DELETED;
}
I read it as we allow ADD and DELETE for PRIVATE && (STATIC
|| FINAL) methods.
(Rephrase: We allow PRIVATE FINAL or PRIVATE STATIC methods.)
As private should always be treated final then we can simplify the
above to:
We allow and and delete PRIVATE INSTANCE or PRIVATE STATIC methods
which is equal to just "PRIVATE methods".
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?
Yes, we have one j.l.instrument test: RedefineMethodDelInvoke.sh.
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?
If we create a vtable entry for private methods then we should hit
the assert above.
If we no longer need this then we have no problem.
Thanks,
Serguei
thanks,
Karen
On 2/21/18 2:45 AM, serguei.spit...@oracle.com
wrote:
On 2/20/18 23:01, David Holmes wrote:
On 21/02/2018 4:50
PM, serguei.spit...@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
"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
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
"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:
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
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