From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752779AbZKIBMf (ORCPT ); Sun, 8 Nov 2009 20:12:35 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751459AbZKIBMe (ORCPT ); Sun, 8 Nov 2009 20:12:34 -0500 Received: from mx1.redhat.com ([209.132.183.28]:29407 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750851AbZKIBMe (ORCPT ); Sun, 8 Nov 2009 20:12:34 -0500 Date: Sun, 8 Nov 2009 20:12:37 -0500 From: Jeff Layton To: ebiederm@xmission.com (Eric W. Biederman) Cc: Jamie Lokier , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org, adobriyan@gmail.com, viro@ZenIV.linux.org.uk Subject: Re: [PATCH] proc: revalidate dentry returned by proc_pid_follow_link Message-ID: <20091108201237.79af5cac@tupile.poochiereds.net> In-Reply-To: References: <1257513594-31071-1-git-send-email-jlayton@redhat.com> <20091106203601.GD27751@shareable.org> <20091106160612.4cd05e76@tlielax.poochiereds.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 08 Nov 2009 02:15:57 -0800 ebiederm@xmission.com (Eric W. Biederman) wrote: > Jeff Layton writes: > > > On Fri, 6 Nov 2009 20:36:01 +0000 > > Jamie Lokier 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