Re: [PATCH 6/9] NFSv4: Convert the NFS client idmapper to use the container user namespace

["bfields@fieldses.org" <bfields@fieldses.org>]

On Thu, Apr 25, 2019 at 04:48:54PM +0000, Trond Myklebust wrote:
> On Thu, 2019-04-25 at 12:45 -0400, bfields@fieldses.org wrote:
> > On Thu, Apr 25, 2019 at 04:40:18PM +0000, Trond Myklebust wrote:
> > > On Thu, 2019-04-25 at 11:33 -0400, bfields@fieldses.org wrote:
> > > > On Thu, Apr 25, 2019 at 03:00:22PM +0000, Trond Myklebust wrote:
> > > > > The assumption is that if you have enough privileges to mount a
> > > > > filesystem using the NFS client, then you would also have
> > > > > enough
> > > > > privileges to run a userspace client, so there is little point
> > > > > in
> > > > > restricting the NFS client.
> > > > > 
> > > > > So the guiding principle is that a NFS client mount that is
> > > > > started
> > > > > in
> > > > > a container should behave as if it were started by a process in
> > > > > a
> > > > > "real
> > > > > VM". That means that the root uid/gid in the container maps to
> > > > > a
> > > > > root
> > > > > uid/gid on the wire.
> > > > > Ditto, if there is a need to run the idmapper in the container,
> > > > > then
> > > > > the expectation is that processes running as 'user' with uid
> > > > > 'u',
> > > > > will
> > > > > see their creds mapped correctly by the idmapper. Again, that's
> > > > > what
> > > > > you would see if the process were running in a VM instead of a
> > > > > container.
> > > > > 
> > > > > Does that all make sense?
> > > > 
> > > > Yes, thanks!
> > > > 
> > > > I thought there was a problem that the idmapper depended on
> > > > keyring usermodehelper calls that it was hard to pass namespace
> > > > information to.  Did that get fixed and I missed it or or forgot?
> > > 
> > > No, you are correct, and so we assume the container is using
> > > rpc.idmapd
> > > (and rpc.gssd) if it is running NFSv4 with RPCSEC_GSS.
> > > 
> > > If the keyring upcall gets fixed, then we may allow it to be used
> > > in
> > > future kernels.
> > 
> > OK, got it.  Is there anything we lose by using
> > idmapd?  (IDMAP_NAMESZ
> > might be a limitation?)
> 
> IDMAP_NAMESZ should be set to 128 by default, so should work for most
> cases. I don't think there are any further limitations.

Makes sense.  Looks good to me!

--b.