[linux-cifs-client] [PATCH] cifs: make overriding of ownership conditional on new mount options

[simo]

On Tue, 2009-04-21 at 12:35 -0400, Jeff Layton wrote:
> We have a bit of a problem with the uid= option. The basic issue is
> that
> it means too many things and has too many side-effects.
> 
> It's possible to allow an unprivileged user to mount a filesystem if
> the
> user owns the mountpoint, /bin/mount is setuid root, and the mount is
> set up in /etc/fstab with the "user" option.
> 
> When doing this though, /bin/mount automatically adds the "uid=" and
> "gid=" options to the share. This is fortunate since the correct uid=
> option is needed in order to tell the upcall what user's credcache to
> use when generating the SPNEGO blob.
> 
> On a mount without unix extensions this is fine -- you generally will
> want the files to be owned by the "owner" of the mount. The problem
> comes in on a mount with unix extensions. With those enabled, the
> uid/gid options cause the ownership of files to be overriden even
> though
> the server is sending along the ownership info.
> 
> This means that it's not possible to have a mount by an unprivileged
> user that shows the server's file ownership info. The result is also
> inode permissions that have no reflection at all on the server. You
> simply cannot separate ownership from the mode in this fashion.
> 
> This behavior also makes MultiuserMount option less usable. Once you
> pass in the uid= option for a mount, then you can't use unix ownership
> info and allow someone to share the mount.
> 
> While I'm not thrilled with it, the only solution I can see is to stop
> making uid=/gid= force the overriding of ownership on mounts, and to
> add
> new mount options that turn this behavior on.

I too think we have no other choice.
I don't like the fact that existing users of cifs will see different
results with the same mount option on different kernels, but I also
don't see any other good way to do the change.

Ack, IMO.
Simo.

-- 
Simo Sorce
Samba Team GPL Compliance Officer <simo at samba.org>
Principal Software Engineer at Red Hat, Inc. <simo at redhat.com>