Re: [PATCH 4/6] user namespaces: add user_ns to super block

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Quoting Eric W. Biederman (ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org):
> Matt Helsley <matthltc-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> writes:
> 
> > 	Would this require passing the vfsmount to the filesystems themselves,
> > or would they be within the VFS code only? 
> 
> The interesting bit is the user_namespace contained in the vfsmount.  We
> can pass that down.  I think semantically it makes sense for a filesystem
> mount to only operate in a single mount namespace.
> 
> > If not wholly within the VFS
> > I wonder if Al Viro would object to this. He's resisted past attempts to
> > pass the vfsmount structs into more filesystem code paths and I'm
> > guessing that could affect whether or not this approach can be
> > implemented.
> 
> Dave Hansen raised that concern when we were talking about it earlier.  Since
> we just care about a property of the mount it isn't a big deal.
> 
> Actually thinking about this a little farther it may be simplest to have the
> mnt_namespace capture the user_namespace, although that doesn't seem to map
> semantically very well with cloning of the filesystem.

Interesting idea.  I'm going to pursue that.

So at a do_new_mount(), mnt->user_ns = current->user_ns.  At
do_loopback(), we ask the fs whether the new_mnt->user_ns can be set to
current->user_ns.  If not, it keeps the original, meaning that current
will always receive user nobody access to the fs.  Otherwise, the
fs is saying that it knows how to properly convert userids from
current->user->user_ns to ones which make sense in the
original_mnt->user_ns.

> This is very much a question of how do we map the uid/gids store in the filesystem
> into the uids/gids in the kernel.  Which user namespace do they belong in.
> 
> Especially in the case of read only mounts we can safely share a filesystem between
> user_namespaces with no changes to the filesystem.    Which I suspect is the
> first case we want to allow as that is a tremendous savings in space if you have
> lots of instances of the same distro, and people have been doing it with /usr
> for years.
> 
> Eric