Re: [PATCH 3/4] x86/fpu: use restore_init_xstate() instead of math_state_restore() on kthread exec

[Oleg Nesterov <oleg@redhat.com>]

On 03/13, Borislav Petkov wrote:
>
> On Wed, Mar 11, 2015 at 06:34:49PM +0100, Oleg Nesterov wrote:
> > Change flush_thread() to do user_fpu_begin() + restore_init_xstate()
> > and avoid math_state_restore().
> >
> > Note: "TODO: cleanup this horror" is still valid. We do not need
> > init_fpu() at all, we only need fpu_alloc() + memset(0). But this needs
> > other changes, in particular user_fpu_begin() should set used_math().
> >
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > ---
> >  arch/x86/kernel/process.c |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> >
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index dd9a069..c396de2 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -142,7 +142,8 @@ void flush_thread(void)
> >  		/* kthread execs. TODO: cleanup this horror. */
> >  		if (WARN_ON(init_fpu(current)))
> >  			force_sig(SIGKILL, current);
> > -		math_state_restore();
> > +		user_fpu_begin();
> > +		restore_init_xstate();
>
> Ok, question: so math_state_restore() does kernel_fpu_disable() before
> doing those, why is it ok for flush_thread() to not do it?

You mean, why restore_init_xstate() is safe?

Because in math_state_restore() case kernel_fpu_begin()->__save_init_fpu()
will overwrite (corrupt) the same fpu->state buffer we need to restore.
Without kernel_fpu_disable().

restore_init_xstate() obviously differs because it reads init_xstate_buf,
we do not care at all if kernel_fpu_begin() in between overwrites ->state.

And note! this is the yet another proof that init_fpu()->fpu_finit() is
pointless. This (and almost all) users need fpu_alloc() only.

Oleg.