netdev - Re: [PATCH bpf-next v5 2/5] bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push

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Date:   Thu, 31 Jan 2019 17:47:19 -0800
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Peter Oskolkov <posk@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        Peter Oskolkov <posk.devel@...il.com>,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH bpf-next v5 2/5] bpf: implement BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap

On Thu, Jan 31, 2019 at 5:04 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 01/31/2019 12:51 AM, Peter Oskolkov wrote:
> > This patch implements BPF_LWT_ENCAP_IP mode in bpf_lwt_push_encap
> > BPF helper. It enables BPF programs (specifically, BPF_PROG_TYPE_LWT_IN
> > and BPF_PROG_TYPE_LWT_XMIT prog types) to add IP encapsulation headers
> > to packets (e.g. IP/GRE, GUE, IPIP).
> >
> > This is useful when thousands of different short-lived flows should be
> > encapped, each with different and dynamically determined destination.
> > Although lwtunnels can be used in some of these scenarios, the ability
> > to dynamically generate encap headers adds more flexibility, e.g.
> > when routing depends on the state of the host (reflected in global bpf
> > maps).
> >
> > Signed-off-by: Peter Oskolkov <posk@...gle.com>

> > +int bpf_lwt_push_ip_encap(struct sk_buff *skb, void *hdr, u32 len, bool ingress)
> > +{
> > +     struct iphdr *iph;
> > +     bool ipv4;
> > +     int err;
> > +
> > +     if (unlikely(len < sizeof(struct iphdr) || len > LWT_BPF_MAX_HEADROOM))
> > +             return -EINVAL;
> > +
> > +     /* validate protocol and length */
> > +     iph = (struct iphdr *)hdr;
> > +     if (iph->version == 4) {
> > +             ipv4 = true;
> > +             if (unlikely(len < iph->ihl * 4))
> > +                     return -EINVAL;
> > +     } else if (iph->version == 6) {
> > +             ipv4 = false;
> > +             if (unlikely(len < sizeof(struct ipv6hdr)))
> > +                     return -EINVAL;
> > +     } else {
> > +             return -EINVAL;
> > +     }
> > +
> > +     if (ingress)
> > +             err = skb_cow_head(skb, len + skb->mac_len);
> > +     else
> > +             err = skb_cow_head(skb,
> > +                                len + LL_RESERVED_SPACE(skb_dst(skb)->dev));
> > +     if (unlikely(err))
> > +             return err;
> > +
> > +     /* push the encap headers and fix pointers */
> > +     skb_reset_inner_headers(skb);
> > +     skb->encapsulation = 1;
> > +     skb_push(skb, len);
> > +     if (ingress)
> > +             skb_postpush_rcsum(skb, iph, len);
> > +     skb_reset_network_header(skb);
> > +     memcpy(skb_network_header(skb), hdr, len);
> > +     bpf_compute_data_pointers(skb);
>
> Does this work transparently with GSO as well or would we need to
> update shared info for this (like in nat64 case, for example)?

Good point. It does need to update the gso_type to include the tunnel
type, similar to iptunnel_handle_offloads.

Only, the nice feature of this interface is that it is encap protocol
independent, which implies that it does not know the correct type.

I don't think that we want to allow programs to write the gso_type themselves.

With GSO_PARTIAL, perhaps specifying the exact tunnel type can be
avoided as long as it is a fixed prefix to replicate?

The transport layer size does not change, so no need to recompute gso_segs?

Either way, this seems non-trivial enough to me to do in a separate
follow-on patch. For now just fail if skb_is_gso.