linux-kernel - Re: [PATCH] proc: revalidate dentry returned by proc_pid_follow

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Message-ID: <m1hbt49xxs.fsf@fess.ebiederm.org>
Date:	Sun, 08 Nov 2009 19:30:55 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Jeff Layton <jlayton@...hat.com>
Cc:	Jamie Lokier <jamie@...reable.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	adobriyan@...il.com, viro@...IV.linux.org.uk
Subject: Re: [PATCH] proc: revalidate dentry returned by proc_pid_follow_link

Jeff Layton <jlayton@...hat.com> writes:

>> Hmm.  Looking at the code I get the impression that a file bind mount
>> will have exactly the same problem.
>> 
>> Can you confirm.
>> 
>> If file bind mounts also have this problem a bugfix to to just
>> proc seems questionable.
>> 
>
> I'm not sure I understand what you mean by "file bind mount". Is that
> something like mounting with "-o loop" ?

# cd /tmp
# echo foo > foo
# echo bar > bar
# mount --bind foo bar
# cat bar
foo
#

> I'm not at all opposed to fixing this in a more broad fashion, but as
> best I can tell, the only place that LAST_BIND is used is in procfs.

proc does appear to be the only user of LAST_BIND.  With a file bind
mount we can get to the same ok: label without a revalidate.  The
difference is that we came from __follow_mount instead of follow_link.

At least that is how I read the code.

Eric
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