RFR: 8318737: Fallback linker passes bad JNI handle

[Jorn Vernee]

The result of `FindClass` is a local JNI handle (in `find_class_from_class_loader`, called from `jni_FindClass` [1]). As such, we need to wrap the return value of `FindClass` in a global reference when storing it inside fallbackLinker.c.

While investigating this, I also noticed an existing bug in `JNIHandles::handle_type`. This method is used from the implementation of `GetObjectRefType` ([2]), and from the implementation of `-Xcheck:jni` code. The former specifies that `JNIInvalidRefType` is a valid return value, and the latter compares the result against `JNIInvalidRefType`. However, if the handle is not any valid type, the implementation bottoms out in a `ShouldNotReachHere()`, meaning `JNIHandles::handle_type` can never return `JNIInvalidRefType`. I've fixed this by letting the enclosing if/else chain fall through to just returning the default result, which is `JNIInvalidRefType`. In that case, I observe the expected stack trace when running with `-Xcheck:jni`. For example:


FATAL ERROR in native method: Bad global or local ref passed to JNI
        at jdk.internal.foreign.abi.fallback.LibFallback.doDowncall(java.base at 22-internal/Native Method)
        at jdk.internal.foreign.abi.fallback.LibFallback.doDowncall(java.base at 22-internal/LibFallback.java:94)
        at jdk.internal.foreign.abi.fallback.FallbackLinker.doDowncall(java.base at 22-internal/FallbackLinker.java:197)
        at java.lang.invoke.LambdaForm$DMH/0x000001b585008000.invokeStaticInit(java.base at 22-internal/LambdaForm$DMH)
        at java.lang.invoke.LambdaForm$MH/0x000001b585029400.invoke(java.base at 22-internal/LambdaForm$MH)
        at java.lang.invoke.LambdaForm$MH/0x000001b58502d000.invokeExact_MT(java.base at 22-internal/LambdaForm$MH)
        at TestUpcallDeopt.payload(TestUpcallDeopt.java:93)
        at TestUpcallDeopt.main(TestUpcallDeopt.java:84)
        at java.lang.invoke.LambdaForm$DMH/0x000001b585006800.invokeStatic(java.base at 22-internal/LambdaForm$DMH)
        at java.lang.invoke.LambdaForm$MH/0x000001b58502a800.invoke(java.base at 22-internal/LambdaForm$MH)
        at java.lang.invoke.Invokers$Holder.invokeExact_MT(java.base at 22-internal/Invokers$Holder)
        at jdk.internal.reflect.DirectMethodHandleAccessor.invokeImpl(java.base at 22-internal/DirectMethodHandleAccessor.java:154)
        at jdk.internal.reflect.DirectMethodHandleAccessor.invoke(java.base at 22-internal/DirectMethodHandleAccessor.java:103)
        at java.lang.reflect.Method.invoke(java.base at 22-internal/Method.java:580)
        at com.sun.javatest.regtest.agent.MainWrapper$MainTask.run(MainWrapper.java:138)
        at java.lang.Thread.runWith(java.base at 22-internal/Thread.java:1583)
        at java.lang.Thread.run(java.base at 22-internal/Thread.java:1570)


(while with the current code, we crash and generate an hs_err file)

Testing: tier 1-5, local run of `jdk_foreign` with fallback linker and `-Xcheck:jni`

[1]: https://github.com/openjdk/jdk/blob/6f352740cb5e7c47d226fd4039cfb977c0622488/src/hotspot/share/prims/jvm.cpp#L3548
[2]: https://docs.oracle.com/en/java/javase/21/docs/specs/jni/functions.html#getobjectreftype

-------------

Commit messages:
 - bail out when FindClass returns NULL
 - add -Xcheck:jni to tests
 - use global ref for LibFallback_class

Changes: https://git.openjdk.org/jdk/pull/16349/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16349&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318737
  Stats: 15 lines in 6 files changed: 4 ins; 4 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/16349.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16349/head:pull/16349

PR: https://git.openjdk.org/jdk/pull/16349