Re: [RFC PATCH v2 0/7] Add a root_dir option to nfs.conf

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2019-05-28 at 11:25 -0400, Steve Dickson wrote:
> 
> On 5/21/19 3:58 PM, Trond Myklebust wrote:
> > On Tue, 2019-05-21 at 15:06 -0400, Chuck Lever wrote:
> > > > On May 21, 2019, at 2:17 PM, Trond Myklebust <
> > > > trondmy@hammerspace.com> wrote:
> > > > 
> > > > On Tue, 2019-05-21 at 13:40 -0400, Chuck Lever wrote:
> > > > > Hi Trond -
> > > > > 
> > > > > > On May 21, 2019, at 8:46 AM, Trond Myklebust <
> > > > > > trondmy@gmail.com
> > > > > > wrote:
> > > > > > 
> > > > > > The following patchset adds support for the 'root_dir'
> > > > > > configuration
> > > > > > option for nfsd in nfs.conf. If a user sets this option to
> > > > > > a
> > > > > > valid
> > > > > > directory path, then nfsd will act as if it is confined to
> > > > > > a
> > > > > > chroot
> > > > > > jail based on that directory. All paths in /etc/exporfs and
> > > > > > from
> > > > > > exportfs are then resolved relative to that directory.
> > > > > 
> > > > > What about files under /proc that mountd might access? I
> > > > > assume
> > > > > these
> > > > > pathnames are not affected.
> > > > > 
> > > > That's why we have 2 threads. One thread is root jailed using
> > > > chroot,
> > > > and is used to talk to knfsd. The other thread is not root
> > > > jailed
> > > > (or
> > > > at least not by root_dir) and so has full access to /etc,
> > > > /proc,
> > > > /var,
> > > > ...
> > > > 
> > > > > Aren't there also one or two other files that maintain export
> > > > > state
> > > > > like /var/lib/nfs/rmtab? Are those affected?
> > > > 
> > > > See above. They are not affected.
> > > > 
> > > > > IMHO it could be less confusing to administrators to make
> > > > > root_dir an
> > > > > [exportfs] option instead of a [mountd] option, if this is
> > > > > not a
> > > > > true
> > > > > chroot of mountd.
> > > > 
> > > > It is neither. I made in a [nfsd] option, since it governs the
> > > > way
> > > > that
> > > > both exportfs and mountd talk to nfsd.
> > > 
> > > My point is not about implementation, it's about how this
> > > functionality
> > > is presented to administrators.
> > > 
> > > In nfs.conf, [nfsd] looks like it controls what options are
> > > passed
> > > via
> > > rpc.nfsd. That still seems like a confusing admin interface.
> > > 
> > > IMO admins won't care about who is talking to whom. They will
> > > care
> > > about
> > > how the export pathnames are interpreted. That seems like it
> > > belongs
> > > squarely with the exportfs interface.
> > > 
> > 
> > With the exportfs interface, yes. However it is not specific to the
> > exportfs utility, so to me [exportfs] is more confusing than what
> > exists now.
> > 
> > OK, so what if we put it in [general] instead, and perhaps rename
> > it
> > "export_rootdir"?
> > 
> I'm just catching up... my apologies tartness...
> 
> So setting root_dir effects *all* exports in /etc/exports? 
> If that is the case, that one variable can change hundreds
> of export... is that what we really want?
> 
> Wouldn't be better to have a little more granularity? 

Can you explain what you mean? The intention here is that if you have
all your exported filesystems set up in a subtree under
/mnt/my/exports, then you can remove that unnecessary prefix.

So, for instance, if I'm trying to export /mnt/my/exports/foo and
/mnt/my/exports/bar, then I can make those two filesystems appear as
/foo, and /bar to the remote clients.

If an admin wants to rearrange all the paths in /etc/exports, and make
a custom namespace, then that is possible using bind mounts: just
create a directory /my_exports, and use mount --bind to attach the
necessary mountpoints into the right spots in /my_exports, then use
export_rootdir to remove the /my_exports prefix.

> As for where root_dir should go, I think it makes senses
> to create a new [exportfs] section and have mountd read it
> from there. I think that would be more straightforward if
> we continue with the big hammer approach where any and all
> exports are effected. 
> 

Fair enough, I can add the [exports] section if you all agree that is
an appropriate place.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com