2024-05-09, 15:25:21 +0200, Antonio Quartulli wrote:

By the way, thank you very much for taking the time to have this
constructive discussion. I really appreciate it!

Cheers :)

On 09/05/2024 14:16, Sabrina Dubroca wrote:

quoted

2024-05-09, 12:35:32 +0200, Antonio Quartulli wrote:

quoted

On 09/05/2024 12:09, Sabrina Dubroca wrote:
Hence I decided to fully trigger the unregistration.

That's the bit that doesn't make sense to me: the device is going
away, so you trigger a manual unregister. Cleaning up some additional
resources (peers etc), that makes sense. But calling
unregister_netdevice (when you're most likely getting called from
unregister_netdevice already, because I don't see other spots setting
dev->reg_state = NETREG_UNREGISTERING) is what I don't get. And I
wonder why you're not hitting the BUG_ON in
unregister_netdevice_many_notify:

     BUG_ON(dev->reg_state != NETREG_REGISTERED);

I think because we have our ovpn->registered check.

It ensures that we don't call ovpn_iface_destruct more than once.

Ah, probably, yes.

But now, that I implemented the rtnl_link_ops I can confirm I am hitting the
BUG_ON. And now it makes sense.

I presume that now I can I simply remove the call to unregister_netdevice()
from ovpn_iface_destruct() and move it to ovpn_nl_del_iface_doit().

Sounds good.

quoted

quoted

quoted

Because you create your devices via genl (which I'm not a fan
of, even if it's a bit nicer for userspace having a single netlink api
to deal with),

Originally I had implemented the rtnl_link_ops, but the (meaningful)
objection was that a user is never supposed to create an ovpn iface by
himself, but there should always be an openvpn process running in userspace.
Hence the restriction to genl only.

Sorry, but how does genl prevent a user from creating the ovpn
interface manually? Whatever API you define, anyone who manages to
come up with the right netlink message will be able to create an
interface. You can't stop people from using your API without your
official client.

I don't want to prevent people from creating ovpn ifaces the way they like.
I just don't see how the rtnl_link API can be useful, other than allowing
users to execute 'ip link add/del..'.

And by design that is not a usecase we want to support, because once the
iface is created, nothing will happen if there is no userspace software
driving it (no matter if it is openvpn or anything else).

When explaining this decision, I like to make a comparison to virtual
802.11/wifi ifaces.
They also lack rtnl_link (AFAIR) as they also require some userspace
software to handle them in order to be useful.

All this said, having everything in one place looks cleaner too :)

From an API point of view, maybe. But for the kernel implementation,
using rtnl_link_ops->newlink is easier.

quoted

quoted

quoted

default_device_exit_batch/default_device_exit_net think
ovpn devices are real NICs and move them back to init_net instead of
destroying them.

Maybe we can extend the condition in default_device_exit_net with a
new flag so that ovpn devices get destroyed by the core, even without
rtnl_link_ops?

Thanks for pointing out the function responsible for this decision.
How would you extend the check though?

Alternatively, what if ovpn simply registers an empty rtnl_link_ops with
netns_fund set to false? That should make the condition happy, while keeping
ovpn genl-only

Yes. I was thinking about adding a flag to the device, because I
wasn't sure an almost empty rtnl_link_ops could be handled safely, but
it seems ok. ovs does it, see commit 5b9e7e160795 ("openvswitch:
introduce rtnl ops stub"). And, as that commit message says, "ip -d
link show" would also show that the device is of type openvpn (or
ovpn, whatever you put in ops->kind), which would be nice.

I just coded something along those lines.

Great, thanks.

It seems pretty clean and we don't need to touch core (+ the bonus of having
the name in "ip -d link")....and the iface does get destroyed upon netns
exit! :-)

I am grasping much better how all these APIs work together now.

Nice :)

-- 
Sabrina