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

[Martin KaFai Lau <kafai@fb.com>]

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?