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:   Tue, 29 Jan 2019 20:29:13 -0700
From:   David Ahern <dsahern@...il.com>
To:     Peter Oskolkov <posk@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org
Cc:     Peter Oskolkov <posk.devel@...il.com>,
        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 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.