linux-kernel - Re: [PATCH 02/19] x86, fpu: Wrap get_xsave

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Date:	Fri, 29 May 2015 20:48:29 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	dave.hansen@...ux.intel.com, oleg@...hat.com, bp@...en8.de,
	riel@...hat.com, sbsiddha@...il.com, luto@...capital.net,
	mingo@...hat.com, hpa@...or.com, fenghua.yu@...el.com
Subject: Re: [PATCH 02/19] x86, fpu: Wrap get_xsave_addr() to make it safer


* Dave Hansen <dave@...1.net> wrote:

> On 05/28/2015 01:41 AM, Ingo Molnar wrote:
> >> > +	union fpregs_state *xstate;
> >> > +
> >> > +	if (!current->thread.fpu.fpstate_active)
> >> > +		return NULL;
> >> > +	/*
> >> > +	 * fpu__save() takes the CPU's xstate registers
> >> > +	 * and saves them off to the 'fpu memory buffer.
> >> > +	 */
> >> > +	fpu__save(&current->thread.fpu);
> >> > +	xstate = &current->thread.fpu.state;
> >> > +
> >> > +	return get_xsave_addr(&xstate->xsave, xsave_state);
> > Small nit, this would become a lot shorter if you introduced a helper local 
> > variable:
> > 
> > 	struct fpu *fpu = &current->thread.fpu;
> > 
> > But more importantly, for a generic get_xsave_field_ptr() API, fpu__save() is 
> > not enough: fpu__save() will only save FPU registers into memory if necessary 
> > (i.e. if the FPU is already in use), and if you call it on a task with no FPU 
> > state then it will still have an !fpu->fpstate_active FPU state after the 
> > call, with random, invalid data in the xsave area.
> 
> But why does this matter?  We just did a !fpu.fpstate_active check, so we can't 
> have a !fpu.fpstate_active before or after the call.

Ah yes, you are right, I missed this:

> >> > +	if (!current->thread.fpu.fpstate_active)
> >> > +		return NULL;

because the usual pattern is:

		if (!fpu->fpstate_active)
			return NULL;

:-)

So your variant is fine too.

Thanks,

	Ingo
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