netdev - Re: [PATCH bpf-next v3 1/4] bpf: add plumbing for BPF_LWT_ENCAP_IP in bpf_lwt_push

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Date:   Wed, 30 Jan 2019 08:39:48 -0800
From:   Peter Oskolkov <posk.devel@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Peter Oskolkov <posk@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH bpf-next v3 1/4] bpf: add plumbing for BPF_LWT_ENCAP_IP in bpf_lwt_push_encap

On Tue, Jan 29, 2019 at 7:29 PM David Ahern <dsahern@...il.com> wrote:
>
> On 1/28/19 6:12 PM, Peter Oskolkov wrote
> > @@ -2583,7 +2594,15 @@ enum bpf_ret_code {
> >       BPF_DROP = 2,
> >       /* 3-6 reserved */
> >       BPF_REDIRECT = 7,
> > -     /* >127 are reserved for prog type specific return codes */
> > +     /* >127 are reserved for prog type specific return codes.
> > +      *
> > +      * BPF_LWT_REROUTE: used by BPF_PROG_TYPE_LWT_IN and
> > +      *    BPF_PROG_TYPE_LWT_XMIT to indicate that skb's dst
> > +      *    has changed and appropriate dst_input() or dst_output()
> > +      *    action has to be taken (this is an L3 redirect, as
> > +      *    opposed to L2 redirect represented by BPF_REDIRECT above).
> > +      */
> > +     BPF_LWT_REROUTE = 128,
> >  };
>
> What happens if a program pushes a new header onto the skb and does not
> return BPF_LWT_REROUTE?
>
> Might be better to move the route lookup and dst swap to run_lwt_bpf and
> only do it if the program returns BPF_LWT_REROUTE. That allows calling
> bpf_push_ip_encap without requiring a route lookup. That might be fine
> as long as their is not a protocol mismatch (ipv4 packet gets an ipv6
> header or vice versa). But then, I think you have the mismatch problem
> now if the program does not return BPF_LWT_REROUTE.

Makes sense - thanks for the suggestion I'll send a v4 later today.

Thanks,
Peter