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

[Sebastian Andrzej Siewior]

On 2019-01-18 13:17:28 [-0800], 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*.  

I missed that part. I will try to look it up and look see if says
something about optional part.
But ABI means we must keep doing it even if there are no users?

> Inspecting that state is how
> userspace makes sense of MPX or protection keys faults.  We even use
> this in selftests/.

Okay. MPX does not check for FP_XSTATE_MAGIC[12] and simply assumes it
is there. That is why I didn't find it.
So we would break MPX. But then MPX is on its way out, soâ

Sebastian