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

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Date:	Mon, 18 May 2015 21:38:09 +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 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.

> +
> +/*
> + * 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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