Date:   Thu, 5 Jul 2018 10:20:16 -0400
From:   Jason Baron <jbaron@...mai.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:     "David S. Miller" <davem@...emloft.net>, libvir-list@...hat.com,
        rmohr@...hat.com, fabiand@...rs.noreply.github.com,
        berrange@...hat.com, "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: opening tap devices that are created in a container

Hi,

Opening tap devices, such as macvtap, that are created in containers is
problematic because the interface for opening tap devices is via
/dev/tapNN and devtmpfs is not typically mounted inside a container as
its not namespace aware. It is possible to do a mknod() in the
container, once the tap devices are created, however, since the tap
devices are created dynamically its not possible to apriori allow access
to certain major/minor numbers, since we don't know what these are going
to be. In addition, its desirable to not allow the mknod capability in
containers. This behavior, I think is somewhat inconsistent with the
tuntap driver where one can create tuntap devices inside a container by
first opening /dev/net/tun and then using them by supplying the tuntap
device name via the ioctl(TUNSETIFF). And since TUNSETIFF validates the
network namespace, one is limited to opening network devices that belong
to your current network namespace.

Here are some options to this issue, that I wanted to get feedback
about, and just wondering if anybody else has run into this.

1)

Don't create the tap device, such as macvtap in the container. Instead,
create the tap device outside of the container and then move it into the
desired container network namespace. In addition, do a mknod() for the
corresponding /dev/tapNN device from outside the container before doing
chroot().

This solution still doesn't allow tap devices to be created inside the
container. Thus, in the case of kubevirt, which runs libvirtd inside of
a container, it would mean changing libvirtd to open existing tap
devices (as opposed to the current behavior of creating new ones). This
would not require any kernel changes, but as mentioned seems
inconsistent with the tuntap interface.

2)

Add a new kernel interface for tap devices similar to how /dev/net/tun
currently works. It might be nice to use TUNSETIFF for tap devices, but
because tap devices have different fops they can't be easily switched
after open(). So the suggestion is a new ioctl (TUNGETFDBYNAME?), where
the tap device name is supplied and a new fd (distinct from the fd
returned by the open of /dev/net/tun) is returned as an output field as
part of the new ioctl parameter.

It may not make sense to have this new ioctl call for /dev/net/tun since
its really about opening a tap device, so it may make sense to introduce
it as part of a new device, such as /dev/net/tap. This new ioctl could
be used for macvtap and ipvtap (or any tap device). I think it might
also improve performance for tuntap devices themselves, if they are
opened this way since currently all tun operations such as read() and
write() take a reference count on the underlying tuntap device, since it
can be changed via TUNSETIFF. I tested this interface out, so I can
provide the kernel changes if that's helpful for clarification.

Thanks,

-Jason

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