[PATCH 5/6] arm64/signal: Avoid corruption of SME state when entering signal handler

[Mark Brown]

On Tue, Dec 03, 2024 at 03:33:18PM +0000, Dave Martin wrote:
> On Tue, Dec 03, 2024 at 12:45:57PM +0000, Mark Brown wrote:

> > +	get_cpu_fpsimd_context();

> > +		if (current->thread.svcr & SVCR_SM_MASK) {
> > +			memset(&current->thread.uw.fpsimd_state.vregs, 0,
> > +			       sizeof(current->thread.uw.fpsimd_state.vregs));

> Do we need to hold the CPU fpsimd context across this memset?

> IIRC, TIF_FOREIGN_FPSTATE can be spontaneously cleared along with
> dumping of the regs into thread_struct (from current's PoV), but never
> spontaneously set again.  So ... -> [*]

Yes, we could drop the lock here.  OTOH this is very simple and easy to
understand.

> > +		/* Ensure any copies on other CPUs aren't reused */
> > +		fpsimd_flush_task_state(current);

> (This is very similar to fpsimd_flush_thread(); can they be unified?)

I have a half finished series to replace the whole setup around
accessing the state with get/put operations for working on the state
which should remove all these functions.  The pile of similarly and
confusingly named operations we have for working on the state is one of
the major sources of issues with this code, even when actively working
on the code it's hard to remember exactly which operation does what
never mind the rules for which is needed.
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