Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows correct xstates copy from kernel to user directly

[Yu-cheng Yu]

On Mon, May 02, 2016 at 10:33:10AM -0700, Dave Hansen wrote:
> On 05/02/2016 10:19 AM, Yu-cheng Yu wrote:
> > On Mon, May 02, 2016 at 09:43:47AM -0700, Dave Hansen wrote:
> >>> If (fpu.fpstate_active == 0), then the task does not use FPU; we don't
> >>> want to save these registers, right?  
> >>
> >> No.  It's possible to have fpstate_active=0 while fpregs_active=1.  Such
> >> a task uses the FPU, but just hasn't done an XSAVE* to save the register
> >> content to the fpstate buffer.
> >>
> >> Note, this is just theoretical, and does not happen in this particular
> >> call path today.
> > 
> > What about...
> > 
> > static int may_copy_fpregs_to_sigframe(void)
> > {
> > 	if (fpregs_active())
> > 		return 1;
> > 
> > 	WARN_ONCE(!current->thread.fpu.fpstate_active,
> > 		  "direct FPU save with no math use\n");
> > 
> > 	if (boot_cpu_has(X86_FEATURE_XSAVES))
> > 		return 1;
> > 
> > 	return 0;
> > } 
> 
> I don't think that changes anything.  We still have a check in there
> that has no purpose.  You've changed the ordering so that the specific
> example that I pointed out no longer triggers it.  But, the underlying
> issue remains.

Before Linux gets into copy_fpstate_to_sigframe(),
current->thread.fpu.fpstate_active must be true.
For eagerfpu, fpregs_active() must also be true.
For lazyfpu, once we try to do FSAVE/FXSAVE/XSAVE,
fpregs_active() will become true as well.

We should have not based on boot_cpu_has(X86_FEATURE_XSAVES)
at all. 

Why don't we make it simple and always copy_fpregs_to_signal_frame()?
Or, only for the lazy case, i.e. !fpregs_active(), we do __copy_to_user().

Anyway, I think we can just replace may_copy_fpregs_to_sigframe() with
!fpregs_active().

Comments?