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

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Eric W. Biederman (ebiederm@xmission.com):
> Glauber Costa <glommer@parallels.com> 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