Re: [PATCH 0/4] fix depvpts in user namespaces

From: Serge Hallyn <serge.hallyn-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
To: "Eric W. Biederman" <ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>
Cc: containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org,
	mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org,
	linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Andrew Morton
	<akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Subject: Re: [PATCH 0/4] fix depvpts in user namespaces
Date: Sun, 17 Mar 2013 22:20:52 -0500	[thread overview]
Message-ID: <20130318032052.GA5958__12870.3099130236$1363576896$gmane$org@sergelap> (raw)
In-Reply-To: <87txoce5qy.fsf-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org>

Quoting Eric W. Biederman (ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org):
> Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org> writes:
...
> > What it a /dev/ptmx already exist? will it use it? That would be bad,
> > since that /dev/ptmx could be a host-side one. I actually believe
> > linking to $rootfs/dev/pts/ptmx is more robust than my solution against
> > remounts. So provided it can guarantee that the ptmx is not ever the
> > root ptmx, I would ack that.
> 
> For those playing with udev, especially older udev where udev is still
> udev and creates devices you can use the following udev rule to create
> the pts/ptmx symlink.
> 
> KERNEL=="ptmx" NAME:="pts/ptmx" SYMLINK="ptmx"
> 
> Before we do anything clever in the kernel it is definitely worth seeing
> how far we can take that little udev rule.

Before it was decided that it was ok to modify core packages to
accomodate containers, we had to install (non-standard) init jobs to
detect it was in a container and if so modify some behavior - for
instance to bind-mount a smaller /lib/init/fstab so that mountall
wouldn't try to mount some things.  That way the rootfs had to be
updated to run in a container, but could then still be used as a
rootfs for non-containers.

...

> As much as I hate the notion I suspect for most of device management
> what we want is to act like devtmpfs, and run all of the device node
> creation etc outside of the container (possibly even with bind mounts).

So you mean a task which is unprivileged on the host, privileged wrt the
container, and on host fs namespace, which bind mounts the host /dev
files into the container?

> Acting like devtmpfs should be something that is possible with no kernel
> changes.   Whereas allowing unprivileged processes to create device
> nodes probably has issues I haven't thought of yet.

Not sure what 'acting like devtmpfs' means (especially in contrast to
acting like udev) - maybe i need to go look at the code.

-serge