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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, May 15, 2014 at 09:42:17AM -0400, Michael H. Warfield wrote:
> On Wed, 2014-05-14 at 21:00 -0700, Greg Kroah-Hartman wrote:
> > On Wed, May 14, 2014 at 10:15:27PM -0500, Seth Forshee wrote:
> > > On Wed, May 14, 2014 at 10:17:31PM -0400, Michael H. Warfield wrote:
> > > > > > 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.
> > > > 
> > > > I was just mentioning that to Serge just a week or so ago reminding him
> > > > of what you told all of us face to face back then.  We were having a
> > > > discussion over loop devices into containers and this topic came up.
> > > 
> > > It was the loop device use case that got me started down this path in
> > > the first place, so I don't personally have any interest in physical
> > > devices right now (though I was sure others would).
> 
> > Why do you want to give access to a loop device to a container?
> > Shouldn't you set up the loop devices before creating the container and
> > then pass those mount points into the container?  I thought that was how
> > things worked today, or am I missing something?
> 
> Ah, you keep feeding me easy ones.  I need raw access to loop devices
> and loop-control because I'm using containers to build NST (Network
> Security Toolkit) distribution iso images (one container is x86_64 while
> the other is i686).  Each requires 2 loop devices.  You can't set up the
> loop devices in advance since the containers will be creating the images
> and building them.  NST tinkers with the base build engine
> configuration, so I really DON'T want it running on a hard iron host. 
> There may be other cases where I need other specialized containers for
> building distros.  I'm also looking at custom builds of Kali (another
> security distribution).

Then don't use a container to build such a thing, or fix the build
scripts to not do that :)

That is not a "normal" use case for a container at all.  Containers are
not for "everything", use a virtual machine for some tasks (like this
one).

> Serge mentioned something to me about a loopdevfs (?) thing that someone
> else is working on.  That would seem to be a better solution in this
> particular case but I don't know much about it or where it's at.

Ok, let's see those patches then.

thanks,

greg k-h