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

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

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.

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.

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.

>
> > On the receive side, xfrmi_rcv_cb() populates a dst metadata for each
> > packet received and attaches it to the skb. The if_id used in this case is
> > fetched from the xfrm state. This can later be used by upper layers such
> > as tc, ebpf, and ip rules.
> >
> > Because the skb is scrubed in xfrmi_rcv_cb(), the attachment of the dst
> > metadata is postponed until after scrubing. Similarly, xfrm_input() is
> > adapted to avoid dropping metadata dsts by only dropping 'valid'
> > (skb_valid_dst(skb) == true) dsts.
> >
> > Policy matching on packets arriving from collect_md xfrmi devices is
> > done by using the xfrm state existing in the skb's sec_path.
> > The xfrm_if_cb.decode_cb() interface implemented by xfrmi_decode_session()
> > is changed to keep the details of the if_id extraction tucked away
> > in xfrm_interface.c.
> >
> > Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
>
> The rest of the patch looks good.

Thanks for the review!
Eyal.