From: Arnd Bergmann <arnd@arndb.de>
To: "Guan Xuetao" <gxt@mprc.pku.edu.cn>
Subject: Re: [PATCH 05/12] unicore32 additional architecture files: low-level lib: uaccess
Date: Thu, 17 Feb 2011 18:45:00 +0100
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Cc: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
        "'Greg KH'" <greg@kroah.com>
References: <014b01cbcdad$f110afb0$d3320f10$@mprc.pku.edu.cn>
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On Wednesday 16 February 2011, Guan Xuetao wrote:
> +
> +#define __kernel_ok		(segment_eq(get_fs(), KERNEL_DS))
> +#define __user_ok(addr, size)	(((size) <= TASK_SIZE)			\
> +				&& ((addr) <= TASK_SIZE - (size)))
> +#define __access_ok(addr, size)	(__kernel_ok || __user_ok((addr), (size)))

A nice trick to simplify this is to introduce a per-process variable for comparing
the pointer, set_fs() then sets this variable to either TASK_SIZE or
UINT_MAX.

This saves one conditional branch for each call to __access_ok.

	Arnd
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