Theodore Ts'o <tytso@xxxxxxx> writes:
On Thu, Jul 13, 2017 at 07:11:36AM -0500, Eric W. Biederman wrote:Yep. That is what it looks like from here.
The concise summary:So correct me if I am wrong; in general, there will only be one
Today we have the xattr security.capable that holds a set of
capabilities that an application gains when executed. AKA setuid root exec
without actually being setuid root.
User namespaces have the concept of capabilities that are not global but
are limited to their user namespace. We do not currently have
filesystem support for this concept.
variant of the form:
It's not like there will be:
Except.... if you have an Distribution root directory which is shared
by many containers, you would need to put the xattrs in the overlay
inodes. Worse, each time you launch a new container, with a new
subuid allocation, you will have to iterate over all files with
capabilities and do a copy-up operations on the xattrs in overlayfs.
So that's actually a bit of a disaster.
So for distribution overlays, you will need to do things a different
way, which is to map the distro subdirectory so you know that the
capability with the global uid 0 should be used for the container
"root" uid, right?
So this hack of using security.foo@uid=1000 is *only* useful when the
subcontainer root wants to create the privileged executable. You
still have to do things the other way.
So can we make perhaps the assertion that *either*:
exists, but never both? And there BAR is exclusive to only one
Otherwise, I suspect that the architecture is going to turn around and
bite us in the *ss eventually, because someone will want to do
something crazy and the solution will not be scalable.
Which is why I asked the question about scalability of the xattr
implementations. It looks like trying to accomodate the general
case just gets us in trouble, and sets unrealistic expectations.
Which strongly suggests that Serge's previous version that
just reved the format of security.capable so that a uid field could
be added is likely to be the better approach.
I want to see what Serge and Stefan have to say but the case looks
pretty clear cut at the moment.