[PATCH v2] arm64/signal: Avoid corruption of SME state when entering signal handler

[Mark Brown]

On Tue, Nov 05, 2024 at 03:18:33PM +0000, Mark Rutland wrote:

> I don't think that the foreign / non-foreign cases are equivalent. In
> the foreign case we clear the entire fpsimd_state structure, i.e. all
> of:

You're right, they're not - thanks for spotting this.

> AFAICT either:

> (a) Our intended ABI is that signal handlers are entered as-if an SMSTOP
>     is executed to exit streaming mode and disable ZA storage.
> 
>     In this case we'll need a more elaborate sequence here to simulate
>     that effect.

That's the intention, so we do need to just clear the vregs instead of
the whole user_fpsimd_state and add clearing of FPMR.

> ... the description of FPMR (which is not in the latest ARM ARM) says:

> | On entry to or exit from Streaming SVE mode, FPMR is set to 0.

> ... so we'd need code to clobber that.

Right, that was missed with the addition of FPMR support.  We'll have
the same thing in ptrace streaming mode enter/exits, FPCR and FPSR
should be better there as in most cases register state is provided when
changing mode.

> Our documentation in Documentation/arch/arm64/sme.rst says:

> | Signal handlers are invoked with streaming mode and ZA disabled.

> ... and doesn't mention FPCR/FPMR/FPSR, so we could go either way,
> though I suspect we intended case (a) ?

Yes.  The the intended goal is literally just that, but if we accomplish
it by issuing a SMSTOP in the live registers case (which is the only
reasonable implementation) then we should obviously behave the same in
the live memory case.  I'll add a patch which makes this explicit in the
documentation.
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