On 2/09/2020 3:47 pm, Yasumasa Suenaga wrote:
Hi David,
On 2020/09/02 14:00, David Holmes wrote:
Hi Yasumasa,
On 1/09/2020 11:42 pm, Yasumasa Suenaga wrote:
Hi all,
Please review this change:
JBS: https://bugs.openjdk.java.net/browse/JDK-8252657
webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8252657/webrev.00/
I tried to attach JVMTI agent which returns JNI_ERR in Agent_OnUnload
via JVMTI.agent_load dcmd. I expected the library would be unloaded,
but I saw it in VM.dynlibs dcmd even though it was failed to load.
I'm not so sure there is a bug here. The specification is quite lax:
"Instead the VM ignores the error, or takes some implementation
specific action -- for example it might print an error to standard
error, or record the error in a system log. "
I think this spec says about VM behavior only. For example, if
Agent_OnAttach is failed, the action in VM is not specified.
So not unloading the library is not a bug in respect of the
specification. Further by unloading the library do you not now permit
it to be re-loaded and the OnAttach function to be executed
repeatedly? That seems wrong to me. It could also be argued that if
you will re-execute onAttach then you should first have called
OnUnload, which you don't do.
Administrator can retry to attach agent with another options if it was
failed.
Currently we can attach same agent to one JVM as many times as we like.
Of course Agent_OnAttach would be called in each attach operation.
The spec states for startup:
"Exactly one call to a start-up function is made per agent."
and in regards to OnUnload:
" ... and this function is also called if the library is unloaded for
other reasons."
It says nothing about OnUnload requiring a successful OnAttach for it to
be called.
If we can attach an agent multiple times and have OnAttach called
multiple times then that seems a clear violation of the specification.
At best there would have to be an intervening unloading of the agent
with a call to OnUnload.
JVMTI agent might have entry point (e.g. DllMain in Windows) and it
might work something (e.g. multiple launch check). In addition it is
strange agent library exists even if AgentLibrary is discarded when
Agent_OnAttach is failed - nobody would not manage it!
I agree it seems cleaner to unload the library. But I'm not at all clear
what the expected semantics are and exactly what complies with the
specification.
David
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Cheers,
Yasumasa
So while this current behaviour may not meet with your expectations it
is not a bug per-se, and the change in behaviour could have other
implications that are not insignificant.
If the new behaviour is considered desirable then a CSR request should
also be filed for this.
Cheers,
David
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Please see JBS how to reproduce.
Thanks,
Yasumasa
