linux-kernel - Re: [PATCH 0/5] RFC: connector: Add network namespace awareness

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Date:   Tue, 14 Jul 2020 15:03:40 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Matt Bennett <Matt.Bennett@...iedtelesis.co.nz>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "christian.brauner@...ntu.com" <christian.brauner@...ntu.com>,
        "containers@...ts.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zbr@...emap.net" <zbr@...emap.net>
Subject: Re: [PATCH 0/5] RFC: connector: Add network namespace awareness

On 2020-07-13, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Matt Bennett <Matt.Bennett@...iedtelesis.co.nz> writes:
> 
> > On Thu, 2020-07-02 at 21:10 +0200, Christian Brauner wrote:
> >> On Thu, Jul 02, 2020 at 08:17:38AM -0500, Eric W. Biederman wrote:
> >> > Matt Bennett <matt.bennett@...iedtelesis.co.nz> writes:
> >> > 
> >> > > Previously the connector functionality could only be used by processes running in the
> >> > > default network namespace. This meant that any process that uses the connector functionality
> >> > > could not operate correctly when run inside a container. This is a draft patch series that
> >> > > attempts to now allow this functionality outside of the default network namespace.
> >> > > 
> >> > > I see this has been discussed previously [1], but am not sure how my changes relate to all
> >> > > of the topics discussed there and/or if there are any unintended side effects from my draft
> >> > > changes.
> >> > 
> >> > Is there a piece of software that uses connector that you want to get
> >> > working in containers?
> >
> > We have an IPC system [1] where processes can register their socket
> > details (unix, tcp, tipc, ...) to a 'monitor' process. Processes can
> > then get notified when other processes they are interested in
> > start/stop their servers and use the registered details to connect to
> > them. Everything works unless a process crashes, in which case the
> > monitoring process never removes their details. Therefore the
> > monitoring process uses the connector functionality with
> > PROC_EVENT_EXIT to detect when a process crashes and removes the
> > details if it is a previously registered PID.
> >
> > This was working for us until we tried to run our system in a container.
> >
> >> > 
> >> > I am curious what the motivation is because up until now there has been
> >> > nothing very interesting using this functionality.  So it hasn't been
> >> > worth anyone's time to make the necessary changes to the code.
> >> 
> >> Imho, we should just state once and for all that the proc connector will
> >> not be namespaced. This is such a corner-case thing and has been
> >> non-namespaced for such a long time without consistent push for it to be
> >> namespaced combined with the fact that this needs quite some code to
> >> make it work correctly that I fear we end up buying more bugs than we're
> >> selling features. And realistically, you and I will end up maintaining
> >> this and I feel this is not worth the time(?). Maybe I'm being too
> >> pessimistic though.
> >> 
> >
> > Fair enough. I can certainly look for another way to detect process
> > crashes. Interestingly I found a patch set [2] on the mailing list
> > that attempts to solve the problem I wish to solve, but it doesn't
> > look like the patches were ever developed further. From reading the
> > discussion thread on that patch set it appears that I should be doing
> > some form of polling on the /proc files.
> 
> Recently Christian Brauner implemented pidfd complete with a poll
> operation that reports when a process terminates.
> 
> If you are willing to change your userspace code switching to pidfd
> should be all that you need.

While this does solve the problem of getting exit notifications in
general, you cannot get the exit code. But if they don't care about that
then we can solve that problem another time. :D

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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