linux-kernel - Re: [lxc-devel] Detecting if you are running in a container

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Message-ID: <4EB06D27.4020507@msgid.tls.msk.ru>
Date:	Wed, 02 Nov 2011 02:05:27 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Kay Sievers <kay.sievers@...y.org>
CC:	Lennart Poettering <mzxreary@...inter.de>, greg@...ah.com,
	Paul Menage <paul@...lmenage.org>,
	linux-kernel@...r.kernel.org, david@...ar.dk,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux Containers <lxc-devel@...ts.sourceforge.net>,
	Linux Containers <containers@...ts.osdl.org>,
	"Serge E. Hallyn" <serge@...lyn.com>, harald@...hat.com
Subject: Re: [lxc-devel] Detecting if you are running in a container

[Replying to an oldish email...]

On 12.10.2011 20:59, Kay Sievers wrote:
> On Mon, Oct 10, 2011 at 23:41, Lennart Poettering <mzxreary@...inter.de> wrote:
>> On Mon, 10.10.11 13:59, Eric W. Biederman (ebiederm@...ssion.com) wrote:
> 
>>> - udev.  All of the kernel interfaces for udev should be supported in
>>>   current kernels.  However I believe udev is useless because container
>>>   start drops CAP_MKNOD so we can't do evil things.  So I would
>>>   recommend basing the startup of udev on presence of CAP_MKNOD.
>>
>> Using CAP_MKNOD as test here is indeed a good idea. I'll make sure udev
>> in a systemd world makes use of that.
> 
> Done.
> 
> http://git.kernel.org/?p=linux/hotplug/udev.git;a=commitdiff;h=9371e6f3e04b03692c23e392fdf005a08ccf1edb

Maybe CAP_MKNOD isn't actually a good idea, having in mind devtmpfs?

Without CAP_MKNOD, is devtmpfs still being populated internally by
the kernel, so that udev only needs to change ownership/permissions
and maintain symlinks in response to device changes, and perform
other duties (reacting to other types of events) normally?

In other words, provided devtmpfs works even without CAP_MKNOD,
I can easily imagine a whole system running without this capability
from the very early boot, with all functionality in place, including
udev and what not...

And having CAP_MKNOD in container may not be that bad either, while
cgroup device.permission is set correctly - some nodes may need to
be created still, even in an unprivileged containers.  Who filters
out CAP_MKNOD during container startup (I don't see it in the code,
which only removes CAP_SYS_BOOT, and even that due to current
limitation), and which evil things can be done if it is not filtered?

Thanks,

/mjt
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