RFR: 8318737: Fallback linker passes bad JNI handle

[Aleksey Shipilev]

On Wed, 25 Oct 2023 10:05:22 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> The old code would throw `ShouldNotReachHere()` if we did not recognize the handle type:
>> https://github.com/openjdk/jdk/blob/d2d1592dd94e897fae6fc4098e43b4fffb6d6750/src/hotspot/share/runtime/jniHandles.cpp#L207
>> 
>> I think the new code should still keep it, like so:
>> 
>> 
>>  } else if (is_local_handle(thread, handle) || is_frame_handle(thread, handle)) {
>>    ...
>>  } else {
>>    ShouldNotReachHere();
>>  }
>> 
>> 
>> That way, if we ever have another handle type, we would hit the `else` branch instead of silently returning `JNIInvalidRefType`. That is, our condition chain would still be exhaustive, catching unexpected values explicitly.
>
> But that just re-introduces the bug? I don't see how we can have both `ShouldNotReachHere()` in the 'else branch' and return `JNIInvalidRefType` for unrecognized handles at the same time.
> 
> Not that this function is used to check potentially stale/invalid/corrupt JNI handles, so the input handle can be complete garbage.

Ah, d'uh! Not enough coffee today, sorry. Yes, you are right.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16349#discussion_r1371490180