Opening -javaagent classes to named modules

[Alan Bateman]

On 11/05/2021 16:10, Fabian Meumertzheim wrote:
> I am currently working on a JVM fuzzer (
> https://github.com/CodeIntelligenceTesting/jazzer/) and would like to apply
> it also to classes delivered with the JDK, e.g. the image parsers in
> javax.imageio.* in the java.desktop module. The fuzzer uses a Java agent to
> add coverage instrumentation (similar to JaCoCo) to classes at runtime.
>
> As opposed to JaCoCo, for technical reasons the fuzzer needs to inject
> bytecode referencing a public static byte[] field defined on a class of the
> agent. Since -javaagent does not support Java modules, this class will be
> part of the unnamed module and thus can't be "opened" to other modules,
> leading to a "NoClassDefFoundError" when the bytecode is injected into a
> class in a different module, e.g. java.desktop.
>
> Is there any way to have a Java agent open up a class for direct access by
> named modules? I have full control over the JVM args within the fuzzer, but
> haven't found a way to achieve this.
I don't think this a modules issue, it's more likely a visibility issue 
because the boot class loader does not delegate to the application class 
loader. If you are instrumenting classes in java.desktop to call into 
some supporting classes that the agent provides then you need those 
class on the boot class path. 
Instrumentation::appendToBootstrapClassLoaderSearch is the API for that. 
You'll also need to use redefineModule to change java.desktop to read 
the unnamed module of the boot class loader.

-Alan