linux-kernel - Re: [PATCH] ARM: add get_user() support for 8 byte types

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Message-ID: <20121113110454.GE28341@n2100.arm.linux.org.uk>
Date:	Tue, 13 Nov 2012 11:04:54 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Rob Clark <rob.clark@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, patches@...aro.org,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] ARM: add get_user() support for 8 byte types

On Mon, Nov 12, 2012 at 06:31:50PM -0600, Rob Clark wrote:
> right, that is what I was worried about..  but what about something
> along the lines of:
> 
> 		case 8: {						\
> 			if (sizeof(x) < 8)				\
> 				__get_user_x(__r2, __p, __e, __l, 4);	\
> 			else						\
> 				__get_user_x(__r2, __p, __e, __l, 8);	\
> 			break;						\
> 		}							\
> 
> maybe we need a special variant of __get_user_8() instead to get the
> right 32bits on big vs little endian systems, but I think something
> roughly along these lines could work.

The problem with that is... big endian systems - where the 32-bit word we
want is the second one, so you can't just reduce that down to a 32-bit
access like that.  You also need to add an offset to the pointer in the BE
case (which can be done.)

I'd suggest calling the 4-byte version in there __get_user_xb() and doing
the 4-byte offset for BE inside that (or aliasing it to __get_user_x for
LE systems).
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