Re: [PATCH] proc: revalidate dentry returned by proc_pid_follow_link

[Jeff Layton <jlayton@redhat.com>]

On Sun, 08 Nov 2009 02:15:57 -0800
ebiederm@xmission.com (Eric W. Biederman) wrote:

> Jeff Layton <jlayton@redhat.com> writes:
> 
> > On Fri, 6 Nov 2009 20:36:01 +0000
> > Jamie Lokier <jamie@shareable.org> wrote:
> >
> >> Jeff Layton wrote:
> >> > The problem here is that this makes that code shortcut any lookup or
> >> > revalidation of the dentry. In general, this isn't a problem -- in most
> >> > cases the dentry is known to be good. It is a problem however for NFSv4.
> >> > If this symlink is followed on an open operation no actual open call
> >> > occurs and the open state isn't properly established. This causes
> >> > problems when we later try to use this file descriptor for actual
> >> > operations.
> >> 
> >> As NFS uses open() as a kind of fcntl-lock barrier, I can see it's
> >> important to do _something_ on new opens, rather than just cloning
> >> most of the file descriptor.
> >> 
> >
> > I guess you mean the close-to-open cache consistency? If so, this
> > problem doesn't actually break that. The actual nfs_file_open call does
> > occur even when you're opening by following one of these symlinks. I
> > believe the cache consistency code occurs there.
> >
> > The problem here is really nfsv4 specific. There the on-the-wire open
> > call and initialization of state actually happens during d_lookup and
> > d_revalidate. Neither of these happens with these LAST_BIND symlinks so
> > we end up with a filp that has no NFSv4 state attached.
> >
> >> > This patch takes a minimalist approach to fixing this by making the
> >> > /proc/pid follow_link routine revalidate the dentry before returning it.
> >> 
> >> What happens if the file descriptor you are re-opening is for a file
> >> which has been deleted.  Does it still have a revalidatable dentry?
> >> 
> >
> > Well, these LAST_BIND symlinks return a real dget'ed dentry today. If
> > we assume that it always returns a valid dentry (which seems to be the
> > case), then I suppose it's OK to do a d_revalidate against it.
> >
> > It's possible though that that revalidate will either fail though or
> > return that it's no good. In that case, this code just returns ESTALE
> > which should make the path walking code revalidate all the way up the
> > chain. That should (hopefully) make whatever syscall we're servicing
> > return an error.
> 
> 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" ?

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.

-- 
Jeff Layton <jlayton@redhat.com>