linux-kernel - Re: [PATCH] vfs: remove unneeded permission check from path

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Date:	Wed, 12 Dec 2012 18:40:49 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jeff Layton <jlayton@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	dwysocha@...hat.com
Subject: Re: [PATCH] vfs: remove unneeded permission check from path_init

On Tue, Dec 11, 2012 at 08:56:16AM -0500, Jeff Layton wrote:
> When path_init is called with a valid dfd, that code checks permissions
> on the open directory fd and returns an error if the check fails. This
> permission check is redundant, however.
> 
> Both callers of path_init immediately call link_path_walk afterward. The
> first thing that link_path_walk does is to check for exec permissions
> at the starting point of the path walk.
> 
> In most cases, these checks are very quick, but when the dfd is for a
> file on a NFS mount with the actimeo=0, each permission check goes
> out onto the wire. The result is 2 identical ACCESS calls.
> 
> Given that these codepaths are fairly "hot", I think it makes sense to
> eliminate the permission check in path_init and simply assume that the
> caller will eventually check the permissions before proceeding.

Applied, with one modification to commit message - the second paragraph
replaced with

Both callers of path_init immediately call link_path_walk afterward. The
first thing that link_path_walk does for pathnames that do not consist
only of slashes is to check for exec permissions at the starting point of
the path walk.  And this check in path_init() is on the path taken only
when *name != '/' && *name != '\0'.
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