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

[Dave Hansen]

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/.