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Date: Fri, 10 Apr 2015 13:34:12 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Low <jason.low2@hp.com>, Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/uaccess: Implement get_kernel()
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On Fri, Apr 10, 2015 at 01:14:27PM +0200, Ingo Molnar wrote:
> +/*
> + * Simple copy-from-possibly-faulting-kernel-addresses method that
> + * avoids the STAC/CLAC SMAP overhead.
> + *
> + * NOTE: this does not propagate the error code of faulting kernel
> + *	 addresses properly. You can recover it via uaccess_catch()
> + *       if you really need to.
> + */
> +#define get_kernel(dst, src)						\
> +do {									\
> +	typeof(*(src)) __val;						\

Should we make that:

	typeof(*(src)) __val = (dst);

> +									\
> +	switch (sizeof(__val)) {					\
> +	case 1:	__get_kernel_asm_ex(__val, src, "b", "b", "=q"); break;	\
> +	case 2: __get_kernel_asm_ex(__val, src, "w", "w", "=r"); break;	\
> +	case 4: __get_kernel_asm_ex(__val, src, "l", "k", "=r"); break;	\
> +	case 8: __get_kernel_asm_ex(__val, src, "q", " ", "=r"); break;	\
> +	default: __get_kernel_BUILD_ERROR();				\
> +	}								\
> +	(dst) = __val;							\
> +} while (0)

Such that when we fault, the value is unmodified? The way it is we'll
assign whatever was on stack for __val, which seems undesirable, no?