On Mon, Jul 09, 2018 at 04:56:04PM -0400, Jason Baron wrote:
On 07/05/2018 12:10 PM, Daniel P. Berrangé wrote:
On Thu, Jul 05, 2018 at 10:20:16AM -0400, Jason Baron wrote:
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.
Presumably the /dev/tapNN device name also changes when you rename the tap device interface using SIOCSIFNAME ?
I don't think so. the NN is the ifindex of the device- changing the device name does not affect the ifindex.
Ah right that makes sense.
eg if it was /dev/tap24 in the host and you called SIOCSIFNAME(eth0) when moving it into the container, it would be /dev/eth0 inside the container ?
When moving it into the container the ifindex can change since the ifindex range is per-namespace (not global).
Oh thats interesting, I hadn't realized that.
Anyway, given that this /dev/tapNN approach is what exists today, libvirt will likely want to implement support for this regardless in order to support existing kernels.
Ok, in this case whatever created the tap device outside of the container would pass the name of the device to libvirt and make sure that the /dev/tapNN device was setup correctly in the container. I believe this differs from how libvirt works today in that libvirt would need to be modified to open an existing device (I think it currently always creates new ones).
Libvirt can use a pre-created TAP device today, but not a pre-created MACVTAP, so supporting the latter is new code for us no matter what.
One slight complication with either of the solutions above is that libvirt won't know whether it is given a TAP or a MACVTAP device. It'll only be given the device name. So with code today we would probably have to first try /dev/tapNNN and if that doesn't exist then try /dev/net/tun with TUNSETIFF.
hmmm. doesn't libvirt make this distinction today?
No need to make the distinction yet, since we only support pre-created TAP devices right now. In cases where we create the devices ourselves, we already know what is what.
If adding a new /dev/net/tap, something could seemlessy accept either a TAP or MACTAP nic name would be nice.
I think if we added a new ioctl() as I proposed it could accept either type of nic.
ok that would be nice. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|