linux-kernel - Re: Detecting if you are running in a container

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Date:	Tue, 11 Oct 2011 21:26:32 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	david@...g.hm
Cc:	Theodore Tso <tytso@....EDU>, Matt Helsley <matthltc@...ibm.com>,
	Lennart Poettering <mzxreary@...inter.de>,
	Kay Sievers <kay.sievers@...y.org>,
	linux-kernel@...r.kernel.org, harald@...hat.com, david@...ar.dk,
	greg@...ah.com, Linux Containers <containers@...ts.osdl.org>,
	Linux Containers <lxc-devel@...ts.sourceforge.net>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Daniel Lezcano <daniel.lezcano@...e.fr>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: Detecting if you are running in a container

david@...g.hm writes:

> On Tue, 11 Oct 2011, Eric W. Biederman wrote:
>
>> Theodore Tso <tytso@....EDU> writes:
>>
>>> On Oct 11, 2011, at 2:42 AM, Eric W. Biederman wrote:
>>>
>>>> I am totally in favor of not starting the entire world.  But just
>>>> like I find it convienient to loopback mount an iso image to see
>>>> what is on a disk image.  It would be handy to be able to just
>>>> download a distro image and play with it, without doing anything
>>>> special.
>>>
>>> Agreed, but what's wrong with firing up KVM to play with a distro
>>> image?  Personally, I don't consider that "doing something special".
>>
>> Then let me flip this around and give a much more practical use case.
>> Testing.  A very interesting number of cases involve how multiple
>> machines interact.  You can test a lot more logical machines interacting
>> with containers than you can with vms.  And you can test on all the
>> aritectures and platforms linux supports not just the handful that are
>> well supported by hardware virtualization.
>
> but in containers, you are not really testing lots of machines, you are testing
> lots of processes on the same machine (they share the same kernel)

True.  But usually that is the interesting part.

>> I admit for a lot of test cases that it makes sense not to use a full
>> set of userspace daemons.  At the same time there is not particularly
>> good reason to have a design that doesn't allow you to run a full
>> userspace.
>
> how do you share the display between all the different containers if they are
> trying to run the X server?

Either X does not start because the hardware it needs is not present or
Xnest or similar gets started.

> how do you avoid all the containers binding to the same port on the default IP
> address?

Network namespaces.

> how do you arbitrate dbus across the containers.

Why should you?

> when a new USB device gets plugged in, which container gets control of
> it?

None of them.  Although today they may all get the uevent.  None of the
containers should have permission to call mknod to mess with it.

> there are a LOT of hard questions when you start talking about running a full
> system inside a container that do not apply for other use of
> containers.

Not really mostly the answer is that you say no.

Eric
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