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

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Message-ID: <alpine.LRH.2.20.1910021540270.24629@dhcp-10-175-191-98.vpn.oracle.com>
Date:   Wed, 2 Oct 2019 16:10:02 +0100 (BST)
From:   Alan Maguire <alan.maguire@...cle.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Marek Majkowski <marek@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        David Miller <davem@...emloft.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        netdev@...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 Wed, 2 Oct 2019, Toke Høiland-Jørgensen wrote:

> This series adds support for executing multiple XDP programs on a single
> interface in sequence, through the use of chain calls, as discussed at the Linux
> Plumbers Conference last month:
> 
> https://linuxplumbersconf.org/event/4/contributions/460/
> 
> # HIGH-LEVEL IDEA
> 
> The basic idea is to express the chain call sequence through a special map type,
> which contains a mapping from a (program, return code) tuple to another program
> to run in next in the sequence. Userspace can populate this map to express
> arbitrary call sequences, and update the sequence by updating or replacing the
> map.
> 
> The actual execution of the program sequence is done in bpf_prog_run_xdp(),
> which will lookup the chain sequence map, and if found, will loop through calls
> to BPF_PROG_RUN, looking up the next XDP program in the sequence based on the
> previous program ID and return code.
> 
> An XDP chain call map can be installed on an interface by means of a new netlink
> attribute containing an fd pointing to a chain call map. This can be supplied
> along with the XDP prog fd, so that a chain map is always installed together
> with an XDP program.
> 

This is great stuff Toke! One thing that wasn't immediately clear to me -
and this may be just me - is the relationship between program 
behaviour for the XDP_DROP case and chain call execution.  My initial
thought was that a program in the chain XDP_DROP'ping the packet would
terminate the call chain, but on looking at patch #4 it seems that
the only way the call chain execution is terminated is if

- XDP_ABORTED is returned from a program in the call chain; or
- the map entry for the next program (determined by the return value
  of the current program) is empty; or
- we run out of entries in the map

The return value of the last-executed program in the chain seems
to be what determines packet processing behaviour after executing
the chain (_DROP, _TX, _PASS, etc).  So there's no way to both XDP_PASS 
and XDP_TX a packet from the same chain, right? Just want to make
sure I've got the semantics correct. Thanks!

Alan