From: ebiederm@xmission.com (Eric W. Biederman)
To: Ian Kent <raven@themaw.net>
Cc: Oleg Nesterov <oleg@redhat.com>,
        Stanislav Kinsbursky <skinsbursky@parallels.com>,
        Jeff Layton <jlayton@redhat.com>, Greg KH <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-nfs@vger.kernel.org, devel@openvz.org, bfields@fieldses.org,
        bharrosh@panasas.com,
        Linux Containers <containers@lists.linux-foundation.org>
References: <20131111071825.62da01d1@tlielax.poochiereds.net>
	<20131112004703.GB15377@kroah.com>
	<20131112061201.04cf25ab@tlielax.poochiereds.net>
	<528226EC.4050701@parallels.com>
	<20131112083043.0ab78e67@tlielax.poochiereds.net>
	<5285FA0A.2080802@parallels.com> <871u2incyo.fsf@xmission.com>
	<20131118172844.GA10005@redhat.com>
	<1455149857.2903.9.camel@themaw.net>
Date: Wed, 17 Feb 2016 20:57:48 -0600
In-Reply-To: <1455149857.2903.9.camel@themaw.net> (Ian Kent's message of "Thu,
	11 Feb 2016 08:17:37 +0800")
Message-ID: <8737sq4teb.fsf@x220.int.ebiederm.org>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: call_usermodehelper in containers
Sender: linux-nfs-owner@vger.kernel.org


Ccing The containers list because a related discussion is happening there
and somehow this thread has never made it there.

Ian Kent <raven@themaw.net> writes:

> On Mon, 2013-11-18 at 18:28 +0100, Oleg Nesterov wrote:
>> On 11/15, Eric W. Biederman wrote:
>> > 
>> > I don't understand that one.  Having a preforked thread with the
>> > proper
>> > environment that can act like kthreadd in terms of spawning user
>> > mode
>> > helpers works and is simple.
>
> Forgive me replying to such an old thread but ...
>
> After realizing workqueues can't be used to pre-create threads to run
> usermode helpers I've returned to look at this.

If someone can wind up with a good implementation I will be happy.

>> Can't we ask ->child_reaper to create the non-daemonized kernel thread
>> with the "right" ->nsproxy, ->fs, etc?
>
> Eric, do you think this approach would be sufficient too?
>
> Probably wouldn't be quite right for user namespaces but should provide
> what's needed for other cases?
>
> It certainly has the advantage of not having to maintain a plague of
> processes waiting around to execute helpers.

That certainly sounds attractive.  Especially for the case of everyone
who wants to set a core pattern in a container.

I am fuzzy on all of the details right now, but what I do remember is
that in the kernel the user mode helper concepts when they attempted to
scrub a processes environment were quite error prone until we managed to
get kthreadd(pid 2) on the scene which always had a clean environment.

If we are going to tie this kind of thing to the pid namespace I
recommend simplying denying it if you are in a user namespace without
an approrpriate pid namespace.  AKA simply not allowing thigns to be setup
if current->pid_ns->user_ns != current->user_ns.

That still leaves things a little hand-wavy but I hope that helps
conceptually.

Eric