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

From Dave Hansen <dave.hansen@linux.intel.com>
Newsgroups linux.kernel
Subject Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows correct xstates copy from kernel to user directly
Date 2016-05-02 18:50 +0200
Message-ID <rupcJ-6XH-3@gated-at.bofh.it> (permalink)
References (3 earlier) <rtpou-5CD-15@gated-at.bofh.it> <rtr6V-76c-1@gated-at.bofh.it> <ruoA2-6AD-25@gated-at.bofh.it> <ruoA2-6AD-23@gated-at.bofh.it> <rup35-6RX-21@gated-at.bofh.it>
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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.

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Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Dave Hansen <dave.hansen@linux.intel.com> - 2016-04-29 22:10 +0200
  Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-04-30 00:50 +0200
    Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Dave Hansen <dave.hansen@linux.intel.com> - 2016-04-30 02:40 +0200
      Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Dave Hansen <dave.hansen@linux.intel.com> - 2016-05-02 18:10 +0200
        Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-05-02 18:40 +0200
          Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Dave Hansen <dave.hansen@linux.intel.com> - 2016-05-02 18:50 +0200
            Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-05-02 19:30 +0200
              Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Dave Hansen <dave.hansen@linux.intel.com> - 2016-05-02 19:40 +0200
                Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-05-02 23:30 +0200
                Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-05-02 23:30 +0200
                Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Dave Hansen <dave.hansen@linux.intel.com> - 2016-05-02 23:40 +0200
                Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-05-03 00:30 +0200
                Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Dave Hansen <dave.hansen@linux.intel.com> - 2016-05-03 00:40 +0200
      Re: [PATCH v4 04/10] x86/xsaves: Introduce a new check that allows  correct xstates copy from kernel to user directly Yu-cheng Yu <yu-cheng.yu@intel.com> - 2016-05-02 18:10 +0200

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