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

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Martin KaFai Lau <kafai@fb.com>
Cc: Mat Martineau <mathew.j.martineau@linux.intel.com>,
	Andrii Nakryiko <andrii@kernel.org>,
	 Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	 Nicolas Rybowski <nicolas.rybowski@tessares.net>,
	Alexei Starovoitov <ast@kernel.org>,
	 Daniel Borkmann <daniel@iogearbox.net>,
	mptcp@lists.linux.dev,
	 Matthieu Baerts <matthieu.baerts@tessares.net>
Subject: Re: [PATCH bpf-next v3 1/8] bpf: expose is_mptcp flag to bpf_tcp_sock
Date: Tue, 10 May 2022 22:02:26 -0700	[thread overview]
Message-ID: <CAEf4BzbnsdSAKoZhQbX8WPuNtnJBx9hNLS2ct8gBkSRg-=Meog@mail.gmail.com> (raw)
In-Reply-To: <20220511004818.qnfpzgepmg7xufwd@kafai-mbp.dhcp.thefacebook.com>

On Tue, May 10, 2022 at 5:48 PM Martin KaFai Lau <kafai@fb.com> wrote:
>
> On Mon, May 02, 2022 at 02:12:27PM -0700, Mat Martineau wrote:
> > From: Nicolas Rybowski <nicolas.rybowski@tessares.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;