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:

quoted

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