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Date:	Mon, 18 Jun 2012 17:19:30 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Ted Ts'o <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>,
	linux-fsdevel@...r.kernel.org,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	viro@...IV.linux.org.uk, sami.liedes@....fi
Subject: Re: [PATCH] vfs: avoid hang caused by attempting to rmdir an
 invalid file system

On Wed, May 30, 2012 at 10:12:57PM +0200, Jan Kara wrote:
> On Wed 30-05-12 13:37:09, J. Bruce Fields wrote:
> > On Tue, May 29, 2012 at 10:08:56PM +0200, Jan Kara wrote:
> > > On Tue 29-05-12 21:50:19, Jan Kara wrote:
> > > > On Mon 28-05-12 17:05:11, Ted Tso wrote:
> > > > > On Mon, May 28, 2012 at 02:29:05PM -0600, Andreas Dilger wrote:
> > > > > > This patch is good from the POV of covering all filesystems, and
> > > > > > avoiding the deadlock at the dcache level.  It would be possible to
> > > > > > detect this problem in the filesystem itself during lookup, before
> > > > > > the bad link got into the dcache itself.  Something like:
> > > > > 
> > > > > I like that as a solution for detecting the problem in ext4.  As you
> > > > > say, it's still an issue for other file systems, and so the patch I
> > > > > proposed is still probably a good idea for the VFS.  But this way ext4
> > > > > (and ext3 when Jan backports it) will be able to detect the problem
> > > > > and mark the file system as being corrupted.
> > > >   Actually, I think there's even better way. d_splice_alias() can rather
> > > > easily detect the problem and report it to filesystem. The advantage is
> > > > that the check in d_splice_alias() can catch any "hardlinks" to
> > > > directories, not just self loops. The patch is attached, I also have
> > > > corresponding handling written for ext? filesystems but that's trivial.
> > > > I'll post the whole series to Al to have a look.
> > >   And now with the attachment. Sorry.
> > 
> > Well, my understanding of d_splice_alias is that it should just return
> > the existing dentry instead of failing.  (It does that now for
> > DISCONNECTED dentries, but I don't understand why they're special.)
> > So that's what:
> > 
> > http://git.kernel.org/?p=linux/kernel/git/viro/vfs.git;a=commit;h=9d345b3217b384813680901d42eae3fb380b9f77
> > 
> > does.
>   Thanks for the pointer. In the case I tried to solve, returning the
> existing dentry will solve the deadlocks, just user won't be warned that
> the filesystem is corrupted. Since you seem to describe a valid case where
> we can spot other !DISCONNECTED dentry of a directory, I guess we have no
> other choice than using your approach.

But my patch got reverted, on suspicion that it was either wrong or
covering up some other problem:

	http://marc.info/?l=linux-fsdevel&m=133917767003505&w=2

... which an approach like yours might help at least find?  So maybe
it's worth another try.

--b.

> 
> We could do some sanity checks in ->lookup method (like Andreas suggested)
> but they are not that powerful as a check in d_splice_alias() can be. But
> what can one do...
> 
> 								Honza
> 
> 
> > > >From 0715b656ac88ce1bb62800b14d99ef2e25c26d28 Mon Sep 17 00:00:00 2001
> > > From: Jan Kara <jack@...e.cz>
> > > Date: Tue, 29 May 2012 21:19:01 +0200
> > > Subject: [PATCH 1/4] vfs: Avoid creation of directory loops for corrupted filesystems
> > > 
> > > When a directory hierarchy is corrupted (e. g. due to a bit flip on the media),
> > > it can happen that it contains loops of directories. That creates possibilities
> > > for deadlock when locking directories.
> > > 
> > > Fix the problem by checking in d_splice_alias() that when we splice a
> > > directory, it does not have any other connected alias.
> > > 
> > > Reported-by: Sami Liedes <sami.liedes@....fi>
> > > Signed-off-by: Jan Kara <jack@...e.cz>
> > > ---
> > >  fs/dcache.c |    4 ++++
> > >  1 files changed, 4 insertions(+), 0 deletions(-)
> > > 
> > > diff --git a/fs/dcache.c b/fs/dcache.c
> > > index 4435d8b..ca31a1e 100644
> > > --- a/fs/dcache.c
> > > +++ b/fs/dcache.c
> > > @@ -1658,6 +1658,10 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
> > >  			d_move(new, dentry);
> > >  			iput(inode);
> > >  		} else {
> > > +			if (unlikely(!list_empty(&inode->i_dentry))) {
> > > +				spin_unlock(&inode->i_lock);
> > > +				return ERR_PTR(-EIO);
> > > +			}
> > >  			/* already taking inode->i_lock, so d_add() by hand */
> > >  			__d_instantiate(dentry, inode);
> > >  			spin_unlock(&inode->i_lock);
> > > -- 
> > > 1.7.1
> > > 
> > 
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
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