netdev - Re: [PATCH bpf-next v3 1/8] bpf: expose is_mptcp flag to bpf_tcp

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Message-ID: <CAEf4BzbnsdSAKoZhQbX8WPuNtnJBx9hNLS2ct8gBkSRg-=Meog@mail.gmail.com>
Date:   Tue, 10 May 2022 22:02:26 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     Mat Martineau <mathew.j.martineau@...ux.intel.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Nicolas Rybowski <nicolas.rybowski@...sares.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, mptcp@...ts.linux.dev,
        Matthieu Baerts <matthieu.baerts@...sares.net>
Subject: Re: [PATCH bpf-next v3 1/8] bpf: expose is_mptcp flag to bpf_tcp_sock

On Tue, May 10, 2022 at 5:48 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Mon, May 02, 2022 at 02:12:27PM -0700, Mat Martineau wrote:
> > From: Nicolas Rybowski <nicolas.rybowski@...sares.net>
> >
> > 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.
> There is already bpf_skc_to_tcp_sock() and its returned tcp_sock pointer
> can access all fields of the "struct tcp_sock" without extending
> the bpf_tcp_sock.
>
> iiuc, I believe the needs to extend bpf_tcp_sock here is to make the
> same bpf sockops prog works for kernel with and without CONFIG_MPTCP
> because tp->is_mptcp is not always available:
>
> struct tcp_sock {
>         /* ... */
>
> #if IS_ENABLED(CONFIG_MPTCP)
>         bool    is_mptcp;
> #endif
> };
>
> Andrii, do you think bpf_core_field_exists() can be used in
> the bpf prog to test if is_mptcp is available in the running kernel
> such that the same bpf prog can be used in kernel with and without
> CONFIG_MPTCP?

yep, absolutely:

bool is_mptcp = bpf_core_field_exists(struct tcp_sock, is_mptcp) ?
sock->is_mptcp : false;

One can also directly check if CONFIG_MPTCP is set with the following
in BPF-side code:

extern bool CONFIG_MPTCP __kconfig;