Linked code made not entrant

[TomerYogev]

I tried using nalim to link native code to a native Java function. It sometimes works but in certain flows the JIT decides to make it not entrant, causing an UnsatisfiedLinkError. Any ideas?
(I'm running on JDK11)

[apangin]

Do you have an example that demonstrates the issue?

[TomerYogev]

Managed to boil it down to this: I'm attaching a Java agent with an agentmain that does:

1. Link native code to function.
2. instrumentation.retransformClasses(String.class);
3. Call the linked function.

This throws an UnsatisfiedLinkError for me. It doesn't seem to matter which class I retransform, although it doesn't happen if I do it from premain! It doesn't reproduce on JDK19+.

[apangin]

Thank you. This seems to be related to the flushing of the enitre CodeCache on the first class redefinition.
This has been fixed in JDK-8324241.

If the assumption is correct, then claiming Can-Redefine-Classes capability in nalim java agent should solve the issue.

[TomerYogev]

Very interesting! Regarding Can-Redefine-Classes, do you mean defining it for the agent that is loaded when the process starts? Is there a way to fix this for when the process starts without it and I need to attach it dynamically? Other than manually retransforming some class before linking the native code?

[apangin]

do you mean defining it for the agent that is loaded when the process starts?

Yes.

Is there a way to fix this for when the process starts without it and I need to attach it dynamically? Other than manually retransforming some class before linking the native code?

The agent can do this automatically on the first attach. I'm not aware of other workarounds right now; this requires deeper research.