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Date:   Fri, 18 Sep 2020 15:26:26 -0700
From:   Song Liu <song@...nel.org>
To:     Nicolas Rybowski <nicolas.rybowski@...sares.net>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Matthieu Baerts <matthieu.baerts@...sares.net>,
        Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 1/5] bpf: expose is_mptcp flag to bpf_tcp_sock

On Fri, Sep 18, 2020 at 5:13 AM Nicolas Rybowski
<nicolas.rybowski@...sares.net> wrote:
>
> is_mptcp is a field from struct tcp_sock used to indicate that the
> current tcp_sock is part of the MPTCP protocol.
>
> In this protocol, a first socket (mptcp_sock) is created with
> sk_protocol set to IPPROTO_MPTCP (=262) for control purpose but it
> isn't directly on the wire. This is the role of the subflow (kernel)
> sockets which are classical tcp_sock with sk_protocol set to
> IPPROTO_TCP. The only way to differentiate such sockets from plain TCP
> sockets is the is_mptcp field from tcp_sock.
>
> Such an exposure in BPF is thus required to be able to differentiate
> plain TCP sockets from MPTCP subflow sockets in BPF_PROG_TYPE_SOCK_OPS
> programs.
>
> The choice has been made to silently pass the case when CONFIG_MPTCP is
> unset by defaulting is_mptcp to 0 in order to make BPF independent of
> the MPTCP configuration. Another solution is to make the verifier fail
> in 'bpf_tcp_sock_is_valid_ctx_access' but this will add an additional
> '#ifdef CONFIG_MPTCP' in the BPF code and a same injected BPF program
> will not run if MPTCP is not set.
>
> An example use-case is provided in
> https://github.com/multipath-tcp/mptcp_net-next/tree/scripts/bpf/examples
>
> Suggested-by: Matthieu Baerts <matthieu.baerts@...sares.net>
> Acked-by: Matthieu Baerts <matthieu.baerts@...sares.net>
> Acked-by: Mat Martineau <mathew.j.martineau@...ux.intel.com>
> Signed-off-by: Nicolas Rybowski <nicolas.rybowski@...sares.net>

Acked-by: Song Liu <songliubraving@...com>
[...]

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