linux-kernel - Re: [PATCH] Fix __d_path for lazy unmounts

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Date:	Fri, 26 Feb 2010 13:07:21 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	john.johansen@...onical.com
CC:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	john.johansen@...onical.com
Subject: Re: [PATCH] Fix __d_path for lazy unmounts

On Sat, 20 Feb 2010, john.johansen@...onical.co wrote:
> From: John Johansen <john.johansen@...onical.com>
> 
> When __d_path() hits a lazily unmounted mount point, it tries to prepend
> the name of the lazily unmounted dentry to the path name.  It gets this wrong,
> and also overwrites the slash that separates the name from the following
> pathname component. This patch fixes that; if a process was in directory
> /foo/bar and /foo got lazily unmounted, the old result was ``foobar'' (note the
> missing slash), while the new result with this patch is ``/foo/bar''.

Example:

# mkdir -p /tmp/foo/bar
# mkdir /tmp/mnt
# mount --bind /tmp/foo /tmp/mnt
# cd /tmp/mnt/bar
# /bin/pwd
/tmp/mnt/bar
# umount -l /tmp/mnt
# /bin/pwd
foobar

After the patch it will be /foo/bar.

Why is the path starting with "/foo"?  Does that make any sense?

Last time this was discussed the proposals which are halfway sane
were:

 a) "(unreachable)/bar" or something along those lines
 b) ENOENT

And with either one care needs to be taken to limit this change to
interfaces (both internal and userspace) where it's not likely to
cause breakage.

Thanks,
Miklos
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