Inappropriate use of the implicit session in allocating the upcall stub in SafeFunctionAccessTest.java

[Cheng Jin]

Hi there,

I notice there might be an issue with the allocation of upcall stub under an implicit session at test/jdk/java/foreign/SafeFunctionAccessTest.java in JDK19+ as follows:

e.g.

    @Test
    public void testClosedStructCallback() throws Throwable {
        MethodHandle handle = Linker.nativeLinker().downcallHandle(
                findNativeOrThrow("addr_func_cb"),
                FunctionDescriptor.ofVoid(C_POINTER, C_POINTER));

        try (MemorySession session = MemorySession.openConfined()) {
           MemorySegment segment = MemorySegment.allocateNative(POINT, session);
            handle.invoke(segment, sessionChecker(session)); <----------------- the upcall stub is allocated in sessionChecker()
        }
    }

    MemorySegment sessionChecker(MemorySession session) {
        try {
            MethodHandle handle = MethodHandles.lookup().findStatic(SafeFunctionAccessTest.class, "checkSession",
                    MethodType.methodType(void.class, MemorySession.class));
            handle = handle.bindTo(session);
            return Linker.nativeLinker().upcallStub(handle, FunctionDescriptor.ofVoid(), MemorySession.openImplicit()); <-------the upcall stub is allocated with an implicit session
        } catch (Throwable ex) {
            throw new AssertionError(ex);
        }
}

It is correct to allocate the upcall stub with a session different from the current session in tests given these tests intend to close the current session in upcall to verify the behavior. But it is problematic to exploit an implicit session backed by GC, in which case the memory of the upcall stub is more likely to be forced to release by GC (when implicitly closing the session to free memory) especially on the memory-restricted machines, which leads to unexpected behavior in upcall (e.g. crash which was captured recently). To work around this case, I'd suggest to replace it with a global session or others so as to keep the upcall stub alive during the upcall.

Best Regards
Cheng Jin
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