Message-ID: <20090211195352.GA23974@fieldses.org>
Date:	Wed, 11 Feb 2009 14:53:52 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Trond Myklebust <Trond.Myklebust@...app.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk
Subject: Re: [RFC PATCH 0/2] Add VFS support for looking up paths on remote
	servers using a temporary mount namespace

On Tue, Feb 10, 2009 at 05:48:55PM -0500, Trond Myklebust wrote:
> On Tue, 2009-02-10 at 16:48 -0500, J. Bruce Fields wrote:
> > On Tue, Feb 10, 2009 at 01:31:48PM -0500, Trond Myklebust wrote:
> > > On Tue, 2009-02-10 at 10:58 -0500, J. Bruce Fields wrote:
> > > > On Mon, Feb 09, 2009 at 01:45:34PM -0500, Trond Myklebust wrote:
> > > > > The following two patches attempt to improve NFSv4's ability to look up
> > > > > the mount path on a remote server.
> > > > > 
> > > > > The first patch adds VFS support for walking the remote path, using a
> > > > > temporary mount namespace to represent the server's namespace, so that
> > > > > symlinks
> > > > 
> > > > I'm a bit confused about the symlink case--I take it you're assuming
> > > > that symlinks in the pseudofs should be interpreted as relative to the
> > > > server's namespace (in keeping with traditional implementations of
> > > > server exports), while symlinks elsewhere should continue to be
> > > > intepreted relative to the client's namespace.
> > 
> > Maybe I shouldn't have said "symlinks in the pseudofs", as that's not
> > entirely well defined--a complicated namespace may transition between
> > "pseudofs" and "real" filesystems multiple times.  So it's really a
> > statement about the client's mount behavior: symlinks found along the
> > mount path will be interpreted one way, symlinks found elsewhere
> > another.  Right?
> > 
> > Though put that way it's harder to decide what to store in a symlink,
> > since you can't necessarily control which paths a given client may
> > decide to mount.
> 
> That has been the nature of an NFS mount path string since it was first
> introduced in NFSv2: it refers to the server namespace.
> People haven't complained about this previously, so why should we
> start changing the meaning of the mount path when we move to NFSv4?

It wasn't previously possible for servers to expose symlinks in the
mount path to clients, so it's not clear to me how to apply precedent.

> 
> > > > Do the rfc's say anything about this?
> > > 
> > > No, the RFCs say nothing, but interpreting symlinks as being relative to
> > > the server namespace would be consistent with the mount behaviour of
> > > NFSv2/v3. It also makes me uncomfortable to have a remote mount path
> > > that could refer back to the client's namespace: that would not be an
> > > NFS mount, but a local bind mount...
> > 
> > Some may be surprised to find that /mntsymlink/ and /mnt/symlink/ will
> > be different after
> > 
> > 	mount file:/path/symlink/ /mntsymlink/
> > 	mount file:/path/	  /mnt/
> 
> So, what then if I do
> 
> 	ln -s ../foo /bar/baz/symlink
> 
> on the server, then compare
> 
> 	mount server:/bar/baz /mnt
> and
> 	mount server:/bar/baz/symlink /mnt
> 
> Would you argue that those two should produce the same result? My
> interpretation would be as follows:
> 
> In the first case, the symlink is visible as /mnt/symlink, and so
> 'cd /mnt/symlink' will take you to the local path '/foo' on the client.
> 
> In the second case, I'd be very surprised if the mount code did anything
> other than to follow /bar/baz/symlink to remote path /bar/foo, and then
> mount that on '/mnt'
> 
> If you agree that the above behaviour is correct, then how would you
> argue that replacing '/bar/baz/symlink' with an absolute symlink
> (i.e. 'ln -sf /bar/foo /bar/baz/symlink') should suddenly cause mount to
> do a bind mount?

I certainly agree that mount shouldn't do a bind mount in that case.

> > I see your point, though it might also be an argument for continuing to
> > error out on symlinks.
> 
> Again, why? We don't do that today with NFSv2/v3.

The question doesn't arise with NFSv2/v3, since the mount protocol can't
return a symlink to the client.

> > It could also be argued that if a given symlink is expected to be
> > interpreted on the server side, then the server should just go ahead and
> > do that for the client, rather than returning it as a symlink.
> 
> How would the server distinguish between a client that is doing a lookup
> of a mount path and one that is looking up a normal path?

Exactly, it can't--that's what worries me.  Under your proposal, the
server will return symlinks to the client which the client will
sometimes interpret relative to the server namespace, and sometimes
relative to the client namespace.

Since the server can't know which the client will do, I don't see how to
make any sensible decision about what the value of that symlink should
be.

In your example, if the intention of creating /bar/baz/symlink was
really to direct clients mounting that path to mount /bar/foo, I wonder
if the most helpful thing might just be for the server to return
a filehandle for the directory /bar/foo/ instead of for the symlink
/bar/foo/symlink.

> > Seems worth at least mentioning to the ietf group, as different behavior
> > across different clients would be confusing.
> 
> ...as would different behaviour across different versions of NFS.

Again, I don't understand how v2/v3 precedent applies here.

In any case, I think this is a subtle argument, and something that other
client implementers will come across, hence worth documenting on the
ietf list.

--b.
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