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

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Message-ID: <20160502163410.GA21734@test-lenovo>
Date:	Mon, 2 May 2016 09:34:10 -0700
From:	Yu-cheng Yu <yu-cheng.yu@...el.com>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows
 correct xstates copy from kernel to user directly

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?