RFR: 8318737: Fallback linker passes bad JNI handle [v3]

[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...

Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:

  move fallback linker initialization outside of try block

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16349/files
  - new: https://git.openjdk.org/jdk/pull/16349/files/dd74cdb9..14ea99a2

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16349&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16349&range=01-02

  Stats: 10 lines in 1 file changed: 5 ins; 5 del; 0 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