Re: [PATCH ipsec-next] xfrm: interface: support collect metadata mode

[Eyal Birger <eyal.birger@gmail.com>]

Hi Steffen,

On Mon, Dec 7, 2020 at 11:55 AM Steffen Klassert
<steffen.klassert@secunet.com> wrote:
>
> On Fri, Nov 27, 2020 at 02:32:44PM +0200, Eyal Birger wrote:
> > Hi Steffen,
> >
> > On Fri, Nov 27, 2020 at 11:44 AM Steffen Klassert
> > <steffen.klassert@secunet.com> wrote:
> > >
> > > On Sat, Nov 21, 2020 at 04:28:23PM +0200, Eyal Birger wrote:
> > > > This commit adds support for 'collect_md' mode on xfrm interfaces.
> > > >
> > > > Each net can have one collect_md device, created by providing the
> > > > IFLA_XFRM_COLLECT_METADATA flag at creation. This device cannot be
> > > > altered and has no if_id or link device attributes.
> > > >
> > > > On transmit to this device, the if_id is fetched from the attached dst
> > > > metadata on the skb. The dst metadata type used is METADATA_IP_TUNNEL
> > > > since the only needed property is the if_id stored in the tun_id member
> > > > of the ip_tunnel_info->key.
> > >
> > > Can we please have a separate metadata type for xfrm interfaces?
> > >
> > > Sharing such structures turned already out to be a bad idea
> > > on vti interfaces, let's try to avoid that misstake with
> > > xfrm interfaces.
> >
> > My initial thought was to do that, but it looks like most of the constructs
> > surrounding this facility - tc, nft, ovs, ebpf, ip routing - are built around
> > struct ip_tunnel_info and don't regard other possible metadata types.
>
> That is likely because most objects that have a collect_md mode are
> tunnels. We have already a second metadata type, and I don't see
> why we can't have a third one. Maybe we can create something more
> generic so that it can have other users too.
>
> > For xfrm interfaces, the only metadata used is the if_id, which is stored
> > in the metadata tun_id, so I think other than naming consideration, the use
> > of struct ip_tunnel_info does not imply tunneling and does not limit the
> > use of xfrmi to a specific mode of operation.
>
> I agree that this can work, but it is a first step into a wrong direction.
> Using a __be64 field of a completely unrelated structure as an u32 if_id
> is bad style IMO.
>
> > On the other hand, adding a new metadata type would require changing all
> > other places to regard the new metadata type, with a large number of
> > userspace visible changes.
>
> I admit that this might have some disadvantages too, but I'm not convinced
> that this justifies the 'ip_tunnel_info' hack.
>

I understand. I'll try to come up with something more generic.
I hope I can find a way to still utilize the existing userspace
constructs.

Thanks!
Eyal.