Eric W. Biederman wrote:

Pavel Emelyanov [off-list ref] writes:

quoted

Thanks. What's the problem with setns?

joining a preexisting namespace is roughly the same problem as
unsharing a namespace.  We simply haven't figure out how to do it
safely for the pid and the uid namespaces.

The pid may change after this for sure. What problems do you know
about it? What if we try to allocate the same PID in a new space
or return -EBUSY? This will be a good starting point. If we manage
to fix it later this will not break the API at all.

quoted

quoted

I have designed these file descriptors to pin the namespaces, so
waiting for them to exit isn't something they can do now.  It makes a
lot of sense to have similar ones that take  weak references to the namespaces
that we can use to wait for a namespace to exit.

Yes, I saw this from patches. Eric, I'd very much appreciate if we
workout a solution that will allow us to kill two birds with one stone.
I do not want to invent yet another bunch of system calls for "taking
weak reference".

Definitely.  I only consider the current interface to be a mushy not
set in stone.

OK. The interface is good. I just don't want you to send it for an inclusion
until we decide what to do with waiting.

quoted

As a "brain storm" start up. Can we use inotify/dnotify for this? 
Or maybe we should better equip the nsfd call with flags argument and 
add a flag for weak reference? In that case - how shall we get a 
notification about namespace is dead? With poll? Maybe worth making
the sys_close return only when the namespace is dead (by providing a
proper ->release callback of a file)?

We would want poll to work, anything else is a weird work-around.
The challenging part is that we don't have any infrastructure for
notifying when a namespace goes away.  So that has to be built before
we can wire it up to userspace.  I don't expect it is too difficult
but there is work to be done.

Poll is OK with me. As far as the notification is concerned - that's also
done in OpenVZ. If you are OK to wait for a week or two I can do it for net
namespaces.

Eric