RFR: 8252657: JVMTI agent is not unloaded when Agent_OnAttach is failed

[Yasumasa Suenaga]

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.

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!


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
> -----
> 
>> Please see JBS how to reproduce.
>>
>>
>> Thanks,
>>
>> Yasumasa