netdev - Re: [PATCH bpf-next 0/9] xdp: Support multiple programs on a single interface through chain calls

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Date:   Thu, 3 Oct 2019 07:03:06 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        Marek Majkowski <marek@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        David Miller <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 0/9] xdp: Support multiple programs on a single
 interface through chain calls

On Thu, Oct 3, 2019 at 1:53 AM Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>> The xdpcap use-case is to trap any XDP return action code (and tcpdump
> via perf event ring_buffer).  For system administrators the xdpcap
> use-case is something we hear about all the time, so one of the missing
> features for XDP.  As Toke also wrote, we want to extend this to ALSO
> be-able to see/dump the packet BEFORE a given XDP program.

It sounds to me that 'xdpdump/xdpcap' (tcpdump equivalent) is
the only use case both you and Toke are advocating for.
I think such case we can do already without new kernel code:
- retrieve prog_id of the program attached to given xdp ifindex
- convert to fd
- create prog_array of one element and store that prog_fd
- create xdpump bpf prog that prints to ring buffer
  and tail_calls into that prog_array
- replace xdp prog on that ifindex

Now it see all the traffic first and existing xdp progs keep working.
What am I missing?