Re: [PATCH 05/22] x86/fpu: Remove fpu->initialized usage in copy_fpstate_to_sigframe()

[Oleg Nesterov]

On 01/18, Dave Hansen wrote:
>
> On 1/18/19 1:14 PM, Sebastian Andrzej Siewior wrote:
> > The kernel saves task's FPU registers on user's signal stack before
> > entering the signal handler. Can we avoid that and have in-kernel memory
> > for that? Does someone rely on the FPU registers from the task in the
> > signal handler?
>
> This is part of our ABI for *sure*.  Inspecting that state is how
> userspace makes sense of MPX or protection keys faults.  We even use
> this in selftests/.

Yes.

And in any case I do not understand the idea to use the second in-kernel struct fpu.
A signal handler can be interrupted by another signal, this will need to save/restore
the FPU state again.

Oleg.