RFR: 8287325: AArch64: fix virtual threads with -XX:UseBranchProtection=pac-ret

[Ron Pressler]

On Tue, 7 Jun 2022 16:42:14 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

> The continuation free/thaw mechanism relies on being able to move thread stacks around in memory.  However when PAC is enabled on supported AArch64 CPUs, the saved LR on the stack contains a "pointer authentication code" signed with the stack pointer at the time the frame was created.  When a stack frame is relocated we need to re-sign the LR with the new stack pointer to ensure it authenticates successfully when the method returns.
> 
> Introduced `ContinuationHelper::return_pc_at()` to avoid directly reading the saved PC from the stack in shared code.  On AArch64 with PAC it enabled it strips the PAC from the address after reading it, on all other platforms it just loads the PC from the stack as before.

I've read some about the ARM PAC feature, and it seems that pointers are signed after being combined with any arbitrary value stored in some register. The current implementation uses a frame pointer value in the fp register as the other register, which is not only problematic for virtual threads, but with some future planned repurposing of that register. Perhaps some other secret value, that is amenable to copying stacks, could be used to achieve the same effect (although not in JDK 19, obviously).

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PR: https://git.openjdk.java.net/jdk/pull/9067