linux-kernel - Re: call_usermodehelper in containers

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Message-ID: <8737sq4teb.fsf@x220.int.ebiederm.org>
Date:	Wed, 17 Feb 2016 20:57:48 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ian Kent <raven@...maw.net>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	Jeff Layton <jlayton@...hat.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, devel@...nvz.org, bfields@...ldses.org,
	bharrosh@...asas.com,
	Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: call_usermodehelper in containers

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

Ian Kent <raven@...maw.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