[release/6.0] Fix exception propagation over HW exception frame on macOS arm64 #64262
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There is a problem unwinding over the PAL_DispatchExceptionWrapper to the actual hardware exception location. The unwinder is unable to get distinct LR and PC in that frame and sets both of them to the same value. This is caused by the fact that the PAL_DispatchExceptionWrapper is just an injected fake frame and there was no real call. Calls always return with LR and PC set to the same value. The fix unifies the hardware exception frame unwinding with Linux where we had problems unwinding over signal handler trampoline, so PAL_VirtualUnwind skips the trampoline and now also the PAL_DispatchExceptionWrapper frame by copying the context of the exception as the unwound context.
The context that is restored in the PAL_DispatchException needs to be the one from the exception, not the original saved one. That ensures that the registers updated by the GC in GC stress C in the context are properly restored after the execution is resumed.
The PAL_SEHException had the records allocated on stack, so the direct context restoration after the EH for GC stress C completed leaked those.
janvorli
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jkotas
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Jan 25, 2022
jeffschwMSFT
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Jan 25, 2022
leecow
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ghost
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Backport of #63596 to release/6.0
/cc @janvorli
There is a bug in hardware exception handling on macOS arm64 in setting up
the fake stack frame for the PAL_DispatchExceptionWrapper. Unwinding
while propagating an exception through the frame that caused the hardware
exception restores incorrect value of the LR register.
Customer Impact
Attempt to throw a new exception or rethrow from a catch handler of a NullReferenceException that stemmed from a virtual call on a
nullreference results in a SIGSEGV crash of the process on macOS arm64.Testing
coreclr and libraries tests, targeted regression test.
Risk
Low. With the change, the unwind gets an exact copy of the frame of the hardware exception that was reported by the OS. So it is correct by definition.