Preserve Linux xstate in exception contexts

[luciusmagn]

Hi guys,

this is my first time contributing to Clozure, so I am not sure if there is a process I am forgetting.

I am submitting a (maybe a little opinionated) small fix for xstate handling.

On x86-64, the interrupted CPU register is carried by signals in a signal frame, and modern CPUs have AVX/other extended register state stored in the XSAVE area (see https://criu.org/Xsave, https://www.kernel.org/doc/html/latest/arch/x86/xstate.html, https://github.com/torvalds/linux/blob/master/arch/x86/include/uapi/asm/sigcontext.h).

Atm, when relocating a signal context, CCL copies only the old fixed-size FP/SSE portion of the frame. The copy can still advertise an extended XSAVE area that it does not contain, causing AVX and other extended register state to be discarded on signal return.

This should not break anything, if the extended metadata is missing or invalid, we copy the fp/sse portion and clear the marker so linux treats it as legacy.

I believe this is tangentially related to #85 which shows a bad frame crash as well. This should probably be fixed on x86-32 as well, I can do it if yall want.

Thank you for your time

[luciusmagn]

I made a small example here: https://github.com/luciusmagn/xsave-failure

[xrme]

Please write a comment about the source of magic number words and the purpose of the other constants and then this will be ready to merge

A nice change. If you want to make a similar, separate change for 32-bit x86, I'll be happy to accept that.

[luciusmagn]

Please write a comment about the source of magic number words and the purpose of the other constants and then this will be ready to merge

A nice change. If you want to make a similar, separate change for 32-bit x86, I'll be happy to accept that.

Thanks a lot, I have added support for 32-bit x86, please check it out whenever you can :)

[xrme]

Thank you for the patch.