linux-kernel - Re: [RFC v2 PATCH 1/3] init: add sys-wrapper.h

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Date:	Mon, 30 Aug 2010 14:11:43 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Namhyung Kim <namhyung@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Phillip Lougher <phillip@...gher.demon.co.uk>,
	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 PATCH 1/3] init: add sys-wrapper.h

On Sunday 29 August 2010, Namhyung Kim wrote:
> +
> +/* These macro are called just before/after actual syscalls. */
> +#define KSYS_PREPARE				\
> +	mm_segment_t old_fs = get_fs();		\
> +	set_fs(KERNEL_DS);
> +
> +#define KSYS_RESTORE				\
> +	set_fs(old_fs);

These macros are not that nice, because they depend on context.
I would probably open-code them in each function, or possibly
use a single macro to combine it to something like

#define kern_sys_call(call, ...)	\
({					\
	mm_segment_t old_fs = get_fs();	\
	long result;			\
	set_fs(KERNEL_DS);		\
	result = call(__VA_ARGS__);	\
	set_fs(old_fs);			\
	result;				\
})

static inline int kern_sys_link(const char *oldname, const char *newname)
{
	return kern_sys_call(sys_link, (const char __user __force *)oldname,
			     (const char __user __force *)newname);
}

> +static inline int kern_sys_fchown(unsigned int fd, uid_t user, gid_t group)
> +{
> +	int ret;
> +	KSYS_PREPARE;
> +
> +	ret = sys_fchown(fd, user, group);
> +
> +	KSYS_RESTORE;
> +	return ret;
> +}

When there are no pointer arguments, there is no need to do set_fs
tricks.

	Arnd
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