Re: [PATCH v5 00/42] idmapped mounts

Christian Brauner
2021-01-16T00:28:29Z

On Fri, Jan 15, 2021 at 12:51:20PM -0500, Theodore Ts'o wrote:
> On Fri, Jan 15, 2021 at 04:24:23PM +0000, Christoph Hellwig wrote:
> >
> > That is what the capabilities are designed for and we already check
> > for them.
>
> So perhaps I'm confused, but my understanding is that in the
> containers world, capabilities are a lot more complicated. There is:
>
> 1) The initial namespace capability set
>
> 2) The container's user-namespace capability set
>
> 3) The namespace in which the file system is mounted --- which is
> "usually, but not necessarily the initial namespace" and
> presumably could potentially not necessarily be the current
> container's user name space, is namespaces can be hierarchically
> arranged.
>
> Is that correct? If so, how does this patch set change things (if
> any), and and how does this interact with quota administration
> operations?

The cases you listed are correct. The patchset doesn't change them.
Simply put, the patchset doesn't alter capability checking in any way.

>
> On a related note, ext4 specifies a "reserved user" or "reserved
> group" which can access the reserved blocks. If we have a file system
> which is mounted in a namespace running a container which is running
> RHEL or SLES, and in that container, we have a file system mounted (so
> it was not mounted in the initial namespace), with id-mapping --- and
> then there is a further sub-container created with its own user
> sub-namespace further mapping uids/gids --- will the right thing
> happen? For that matter, how *is* the "right thing" defined?

In short, nothing changes. Whatever happened before happens now.

Specifically s_resuid/s_resgid are superblock mount options and so never
change on a per-mount basis and thus also aren't affected by idmapped
mounts.

>
> Sorry if this is a potentially stupid question, but I find user
> namespaces and id and capability mapping to be hopefully confusing for
> my tiny brain. :-)

No, I really appreciate the questions. :) My brain can most likely
handle less. :)

Christian