Re: [PATCH v3 2/3] nsproxy: attach to namespaces via pidfds

On Mon, May 04, 2020 at 12:09:46PM -0500, Eric W. Biederman wrote:

Christian Brauner [off-list ref] writes:

quoted

On Mon, May 04, 2020 at 11:25:07AM -0500, Eric W. Biederman wrote:

quoted

I am not thrilled about treating nstype as a flags fields when it is not
currently.  It was my hope when I designed the interface that not
treating nstype as a flags field would save us from the problem of bits
running out.

Hm, I researched the setns() syscall history before that and I didn't
see that reasoning anywhere. The "nstype" arg was originally advertised
on the list as "having a flags field is useful in general".

Take a look at the code.  At the end of the day nstype is not treated at
all like a flags field.

Oh, I wasn't trying to dispute that. I was just pointing at the history
where using it as a flags field in the future wasn't in principle out of
the question.

It isn't a very important point.  And it was certainly easier to use
the existing bits for essentially their existing meanings.  But it was
certainly something I was thinking at the time.

I think I left it as we can see either way, depending on how things
evolve.

I can imagine a use for a nstype being a single namespace from a pidfd.
Do you have any actual usecases for setting some but not all of the
namespaces from a pidfd?  If we don't have a compelling reason
I would like to kick that can down the road a ways farther.

Yeah, I think so. We already have a few use-cases. The syscall
interception stuff selectively attaches to subsets of namespaces
depending on what namespaces are needed to emulate a given syscall. And
the exec logic let's users select what namespaces to attach to. It's
common to setns to a subset of namespaces to perform operations with
privilege and then later attach others (often the userns).

I am also remembering that that setns freed the low 8 bits.  Which gave
some freedom beyond clone.

quoted

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That aside.  It would be very good if the default version of setting
everything from a pidfd would set the root directory from the process it
is copying everything else from.

I'm not sure I follow completely. If you specify CLONE_NEWNS then we do
set the root directory with set_fs_root() in commit_nsset(). Or are you
saying we should always do that independent of whether or not
CLONE_NEWNS is specified? And if so could you explain why we'd want
that? I'm sure I'm missing something!

I am suggesting that when we do:

"setns(pidfd, 0)" or "setns(pidfd, SETNS_PIDFD)"

That the result is not just the namespaces changing but also the root
directory changing to the pids root directory.  Something where the
whole is greater than the parts.

Ok, I can see that being useful. But If we do this, then a new flag
would be quite helpful. (I also think Michael had some reservations
against re-using 0 for something like this.)
But let me suggest moving your phrase from above down to here and
say that we could kick that can down the road for a follow-up extension?

Christian