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

Yasumasa Suenaga ysuenaga at openjdk.java.net
Fri Dec 18 05:11:56 UTC 2020


On Tue, 1 Dec 2020 01:43:15 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>>> * Q1: Is it necessary to call the Agent_OnUnload()?
>> 
>> [JVMTI spec of Agent_OnUnload()](https://docs.oracle.com/en/java/javase/15/docs/specs/jvmti.html#onunload) says this function will be called when the agent library will be unloaded by platform specific mechanism. OTOH it also says `Agent_OnUnload()` will be called both at VM termination and **by other reasons**.
>> The spec don't say for the case if `Agent_OnAttach()` would be failed. IMHO `Agent_OnUnload()` should be called because this PR would unload library if `Agent_OnAttach()` failed.
>> 
>>> * Q2: Would it be a JVMTI spec violation to call the Agent_OnAttach() multiple times? (It seems to be the case to me.)
>> 
>> `Agent_OnAttach()` should be called only once per attach request, but VM should accept multiple attach request for same agent library.
>> 
>> For example, we can add multiple `-agentlib` and `-agentpath` request as below. JVMTI agent might change behavior due to arguments or configuration file.
>> 
>> -agentlib:test=profile=A -agentlib:test=profile=B -agentpath:/path/to/libtest=profile=C
>> 
>> Agent developers should have responsibility for the behavior when more than one agent is loaded at a time.
>> 
>>> * Q3: What has to be done for statically linked agent?
>> 
>> JVMTI spec says "unless it is statically linked into the executable", so I think we can ignore about Agent_OnUnload_L() in this PR.
>> 
>>> * Q4: Should the agent be correctly loadable in the first place? What were the reasons its loading to fail?
>> 
>> Agent (`Agent_OnAttach()`) might fail due to error in agent logic. For example, some agents load configuration file at initialization. If the user gives wrong value, it will fail.
>> 
>>> Yes, at least, a CSR is needed for this.
>> 
>> I will file CSR for this PR after this discussion.
>
>> > * Q3: What has to be done for statically linked agent?
>> 
>> JVMTI spec says "unless it is statically linked into the executable", so I think we can ignore about Agent_OnUnload_L() in this PR.
> 
> I don't think that makes sense. If you call it for dynamically linked then you need to call it for statically linked also.

@dholmes-ora Thank you for explanation! I updated [CSR](https://bugs.openjdk.java.net/browse/JDK-8256918). Could you review it?

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PR: https://git.openjdk.java.net/jdk/pull/19


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