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

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Date:	Mon, 22 Feb 2016 15:27:38 -0800
From:	Yu-cheng Yu <yu-cheng.yu@...el.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	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 04/10] x86/xsaves: Introduce a new check that allows
 correct xstates copy from kernel to user directly

On Mon, Feb 22, 2016 at 02:42:54PM -0800, Andy Lutomirski wrote:
> > +static int should_save_registers_directly(void)
> 
> I don't like the name of this function because:
> 
> > +{
> > +       /*
> > +        * In signal handling path, the kernel already checks if
> > +        * FPU instructions have been used before it calls
> > +        * copy_fpstate_to_sigframe(). We check this here again
> > +        * to detect any potential mis-use and saving invalid
> > +        * register values directly to a signal frame.
> > +        */
> > +       WARN_ONCE(!current->thread.fpu.fpstate_active,
> > +                 "direct FPU save with no math use\n");
> 
> ... Here "direct" seems to mean that we're asking whether to directly save ...
> 
> > +
> > +       /*
> > +        * In the case that we are using a compacted kernel
> > +        * xsave area, we can not copy the thread.fpu.state
> > +        * directly to userspace and *must* save it from the
> > +        * registers directly.
> > +        */
> 
> ... and here "directly" means *both* copying directly to userspace and
> saving using xsave directly.
> 
> So can you rename it to something with an obvious meaning like
> "may_memcpy_fpu_regs" or similar?

This function determines whether the kernel should do
copy_fpregs_to_sigframe(). Perhaps we can re-name it to
may_copy_fpregs_to_sigframe()?

--Yu-cheng