netdev - Re: [PATCH ipsec-next 2/3] xfrm: interface: support collect metadata mode

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Date:   Wed, 24 Aug 2022 19:06:32 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Eyal Birger <eyal.birger@...il.com>, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
        steffen.klassert@...unet.com, herbert@...dor.apana.org.au,
        pablo@...filter.org, contact@...elbtn.com, dsahern@...nel.org
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, devel@...ux-ipsec.org
Subject: Re: [PATCH ipsec-next 2/3] xfrm: interface: support collect metadata
 mode

Hi Eyal,

On 8/23/22 5:45 PM, 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_XFRM
> which holds the if_id property.
> 
> 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@...il.com>

Can be done in follow-up (once merged back from net-next into bpf-next),
but it would be nice to also have a BPF CI selftest for it to make sure
the ipsec+collect_md with BPF is consistently tested for incoming patches.

Thanks,
Daniel