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

From: Dave Hansen
Date: Mon May 02 2016 - 12:44:32 EST

On 05/02/2016 09:34 AM, Yu-cheng Yu wrote:
> On Mon, May 02, 2016 at 09:06:41AM -0700, Dave Hansen wrote:
>> On 05/02/2016 08:57 AM, Yu-cheng Yu wrote:
>>> On Fri, Apr 29, 2016 at 05:36:48PM -0700, Dave Hansen wrote:
>>>> If may_copy_fpregs_to_sigframe() were called from a slightly different
>>>> context, or if we change the call-site, what breaks?
>>>> In other words. if we can still "may_copy_fpregs_to_sigframe()" no
>>>> matter the state of fpu.fpstate_active, then I don't think we should be
>>>> checking it in may_copy_fpregs_to_sigframe().
>>> Do you mean, don't check fpu.fpstate_active here?
>> Not really. I'm asking *why* the check is there.
> 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.