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Subject: Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows
 correct xstates copy from kernel to user directly
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <cover.1457038929.git.yu-cheng.yu@intel.com>
 <b4b6fd6f2b928474748abb3d842aef025cbd10da.1457038929.git.yu-cheng.yu@intel.com>
 <5723BF63.2000100@linux.intel.com> <20160429224338.GA15757@test-lenovo>
 <5723FE20.7000906@linux.intel.com> <20160502155734.GA21577@test-lenovo>
 <57277B11.9080802@linux.intel.com> <20160502163410.GA21734@test-lenovo>
 <572783C3.8040907@linux.intel.com> <20160502171936.GA22224@test-lenovo>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <57278F56.6050600@linux.intel.com>
Date: Mon, 2 May 2016 10:33:10 -0700
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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.