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

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Date:	Mon, 18 May 2015 21:42:27 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dave Hansen <dave@...1.net>
cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	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



On Mon, 18 May 2015, Thomas Gleixner wrote:

> On Fri, 8 May 2015, Dave Hansen wrote:
> > The MPX code appears to be saving off the FPU in an unsafe
> > way.   It does not disable preemption or ensure that the
> > FPU state has been allocated.
> > 
> > This patch introduces a new helper which will do both of
> > those things internally.
> 
> This changelog does not really match the implementation. Unless I'm
> missing something I can't find anything preemption related.

Gah. Hit send before finishing the mail.

It's unlazy_fpu (which I agree is a horrible name) which does the
right thing.

> > +
> > +/*
> > + * This wraps up the common operations that need to occur when retrieving
> > + * data from xsave state.  It first ensures that the current task was
> > + * using the FPU and retrieves the data in to a buffer.  It then calculates
> > + * the offset of the requested field in the buffer.
> > + *
> > + * This function is safe to call whether the FPU is in use or not.
> > + *
> > + * Note that this only works on the current task.
> > + *
> > + * Inputs:
> > + *	@xsave_field: state which is defined in xsave.h (e.g. XSTATE_FP,
> > + *	XSTATE_SSE, etc...)
> > + * Output:
> > + *	address of the state in the xsave area.
> 
>   or NULL in case of .....
> 
> > + */
> > +void *get_xsave_field(int xsave_field)
> > +{
> > +	union thread_xstate *xstate;
> > +
> > +	if (!tsk_used_math(current))
> > +		return NULL;
> > +	/*
> > +	 * unlazy_fpu() is poorly named and will actually
> > +	 * save the xstate off in to the memory buffer.
> > +	 */
> > +	unlazy_fpu(current);
> > +	xstate = current->thread.fpu.state;
> > +
> > +	return get_xsave_addr(&xstate->xsave, xsave_field);
> > +}
> 
> Thanks,
> 
> 	tglx
> 
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