Fix stack misalignment before calls in get_win32_restore_stack thunk. #2208
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The thunk pushed RBP (−8) and then reserved only 32 bytes of shadow space, leaving RSP misaligned by 8 bytes. This violated the Win64 ABI requirement that RSP be 16-byte aligned at every call instruction. As a result, the calls to _resetstkoflw, mono_tls_get_jit_tls, and mono_restore_context were executed with an unaligned stack. Increase the reservation to 40 bytes (32 shadow + 8 alignment fixup) to restore correct 16-byte alignment after the initial push and ensure all call sites are ABI-compliant.
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Collaborator
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@xiaoxiao921 Is there a submitted bug for this issue? Typically we try not to land changes blind without a reproduction of the bug as well as a case # for tracking. |
Author
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@UnityAlex Hey thanks for getting back to me, I made a bug report through the Editor with the top button Help -> Report a bug Email subject I received: New Incident created: IN-127643 - StackOverflow Managed Exceptions are not always catched properly by the mono native handler. |
Collaborator
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Excellent. Thank you @xiaoxiao921. I'll chase this down with the QA folks. |
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Why
The stack overflow handler fails to execute properly in some cases, it can happen both in game release builds and inside some user editor code / c# scripts.
How
The thunk pushed RBP (−8) and then reserved only 32 bytes of shadow space, leaving RSP misaligned by 8 bytes. This violated the Win64 ABI requirement that RSP be 16-byte aligned at every call instruction. As a result, the calls to _resetstkoflw, mono_tls_get_jit_tls, and mono_restore_context were executed with an unaligned stack.
Increase the reservation to 40 bytes (32 shadow + 8 alignment fixup) to restore correct 16-byte alignment after the initial push and ensure all call sites are ABI-compliant.
I don't know how far back, probably forever since that line of code is probably decade old at this point, I did test this personally on 2019.4.26 and 2021.3.33 but that's it.
In fact, this issue still happens on current mono (see https://github.com/mono/mono/blob/main/mono/mini/exceptions-amd64.c#L94)
I figured i'd do the pr here because i don't use mono outside of an unity environement and it would be nice to get this fixed.