RFR: 8303624: The java.lang.Thread.FieldHolder can be null for JNI attaching threads

[David Holmes]

On Mon, 6 Mar 2023 21:52:47 GMT, David Holmes <dholmes at openjdk.org> wrote:

> To support virtual threads a number of fields were moved out of `java.lang.Thread` into a separate `FieldHolder` object. The VM was updated to then access certain thread fields via the `FieldHolder`. 
> 
> The code for attaching a thread to the VM, specifically `allocate_threadObj` didn't allow for the `Thread` constructor throwing an exception, and so failing to allocate the `FieldHolder` before attempting to access a field through the `FieldHolder`. This resulted in assertion failures in `javaClasses.cpp` (or crashes in a product build). That code is fixed to ensure we cease processing if the constructor throws an exception.
> 
> In addition, we need to recognise that whilst a native thread is attaching via JNI, it is partially initialized (to varying degrees) but also visible through JVMTI. Though the window is small JVMTI could get hold of an attaching thread and then invoke methods that would try to access uninitialized state. When this was state of `Thread` instance there was no problem as the object existed in its zero-initialized form (it had been allocated directly but no constructor run). However, anythng accessed via the `FieldHolder` is now a problem as the `FieldHolder` may not exist. So we modify all of the `FieldHolder` get/set methods to account for it being null: setters will do nothing, while getters return the default zero value for the field.
> 
> Testing: tiers 1-3 as a sanity test
> 
> There's no way to write a regression test for this.
> 
> Thanks.

I re-examined the setter methods and none of them can be called on a JNI attaching thread whilst it is partially constructed. Such a thread could have regular Java methods called on it, but they would throw NPE when trying to access the FieldHolder. So it suffices to keep the assertion that holder!=nullptr in the setter methods.

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PR: https://git.openjdk.org/jdk/pull/12892