linux-kernel - Re: [RFC PATCH 00/11] Add support for devtmpfs in user namespaces

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Date:	Wed, 14 May 2014 18:32:45 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
	Arnd Bergmann <arnd@...db.de>,
	Eric Biederman <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	lxc-devel@...ts.linuxcontainers.org
Subject: Re: [RFC PATCH 00/11] Add support for devtmpfs in user namespaces

On Wed, May 14, 2014 at 04:34:48PM -0500, Seth Forshee wrote:
> Unpriveleged containers cannot run mknod, making it difficult to support
> devices which appear at runtime.

Wait.

Why would you even want a container to see a "new" device?  That's the
whole point, your container should see a "clean" system, not the "this
USB device was just plugged in" system.  Otherwise, how are you going to
even tell that container a new device showed up?  Are you now going to
add udev support in containers?  Hah, no.

> Using devtmpfs is one possible
> solution, and it would have the added benefit of making container setup
> simpler. But simply letting containers mount devtmpfs isn't sufficient
> since the container may need to see a different, more limited set of
> devices, and because different environments making modifications to
> the filesystem could lead to conflicts.
> 
> This series solves these problems by assigning devices to user
> namespaces. Each device has an "owner" namespace which specifies which
> devtmpfs mount the device should appear in as well allowing priveleged
> operations on the device from that namespace. This defaults to
> init_user_ns. There's also an ns_global flag to indicate a device should
> appear in all devtmpfs mounts.

I'd strongly argue that this isn't even a "problem" at all.  And, as I
said at the Plumbers conference last year, adding namespaces to devices
isn't going to happen, sorry.  Please don't continue down this path.

greg k-h
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