Re: [PATCH review 0/11] General unprivileged mount support

Jan Kara [off-list ref] writes:

On Wed 06-07-16 08:54:46, Seth Forshee wrote:

quoted

On Wed, Jul 06, 2016 at 10:54:40AM +0200, Jan Kara wrote:

quoted

On Mon 04-07-16 11:27:46, Eric W. Biederman wrote:
I don't remember the indented uses for user-ns mounts so I may be just
wrong. But my experience tells me that external data (such as user
namespace ID mappings in your case) that modify meaning of on-disk format
tend to cause maintenance difficulties in the long run... Because someone
*will* have the idea of migrating these fs images between containers /
machines and then they have to make sure mappings get migrated as well and
it all becomes cumbersome.

The intended use case for this is containers, with the idea being that I
as a user will get the same behavior in the container as I would in
init_user_ns without needing any userspace modifications to achieve
that.

So if I have a filesystem that contains uid 0 and I mount it in my
container, I should see uid 0. If I mount the same bits in another
container with a different uid mapping I should also see uid 0.

If I mkfs a new filesystem in my container then mount it, the root
directory of the fs is owned by uid 0 in my container without any
modifications to mkfs.

I'd argue that this makes it easier to migrate a disk between containers
because the ids in the disk show up the same within the container
regardless of the id mapping. If someone wants to mount a filesystem in
one container and also access it in another container with a completely
different id mapping, well I don't think that's ever going to work well.

OK, I see how this is supposed to work. However you assume here that both
containers have the same set of valid UIDs, don't you? If that is not the
case, the mounted image will not be usable in the other container, right?

In practice most uids and gids are going to be < 2^16.  We currently
have a 32bit id space so it isn't hard to map all of the ids.

In practice this is exactly the same problem as moving filesystems
between systems and ensuring that the /etc/passwd and /etc/group entries
that describe the users and groups on the two systems are similiar
enough not to cause problems.

The difference between the classic case of moving filesystems between
systems and the case with user namespaces is that when there are
differences they can be encapuslated in a user namespace.

Eric