Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry

[Hugh Dickins <hughd@google.com>]

On Mon, 18 Jul 2011, Linus Torvalds wrote:
> On Sun, Jul 17, 2011 at 11:31 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > Now, I do agree that maybe that case simply should check the dentry
> > sequence count. I wish all cases did. Hugh patch did that. But the
> > reason I dislike Hugh's patch is that when I say "I wish they all
> > did", I mean that I dislike the special casing. And Hugh's patch just
> > adds *more* special casing for that NULL entry - I'd wish we just
> > always did it regardless of whether it was NULL or not.
> 
> Btw, looking at that, I think Hugh's patch is wrong. It does
> 
>   if (!read_seqcount_retry(&dentry->d_seq, nd->seq))
> 
> but that's after we've done the __follow_mount_rcu() that may actually
> have changed "nd->seq" to the mount-point inode (and has changed
> path->dentry to match it).

Yes, my patch is wrong there.  I started out with
	if (!read_seqcount_retry(&dentry->d_seq, seq);
but seeing __follow_mount_rcu() updates inode and nd->seq, I changed to
	if (!read_seqcount_retry(&dentry->d_seq, nd->seq);
missing the obvious, that it's changing path->dentry when it updates nd->seq.

> 
> Now, it only does it if inode is NULL, so I guess it doesn't matter,
> but it's the kind of inconsistency that I think is really dangerous,
> because it basically compares incompatible sequence numbers.

Yes, it's simply wrong.

> 
> Also, looking at that whole mount-point traversal sequence, it looks
> like __follow_mount_rcu() will happily totally ignore the old sequence
> number when it replaces it with the mount-point sequence number. So it
> looks to me that we have a case where we miss the sequence number
> check that can happen with a positive dentry too!

Al has commented on that.  I'd feel more confident with a patch like
yours, and corrected final check in mine (if we used mine at all)
	if (!read_seqcount_retry(&path->dentry->d_seq, nd->seq);
but ignore me, I'm easily confused by mounts ;)

Hugh