selftests/bpf: Add mptcp subflow example

Move Nicolas's patch into bpf selftests directory. This example added a
test that was adding a different mark (SO_MARK) on each subflow, and
changing the TCP CC only on the first subflow.

This example shows how it is possible to:

    Identify the parent msk of an MPTCP subflow.
    Put different sockopt for each subflow of a same MPTCP connection.

Here especially, we implemented two different behaviours:

    A socket mark (SOL_SOCKET SO_MARK) is put on each subflow of a same
    MPTCP connection. The order of creation of the current subflow defines
    its mark. The TCP CC algorithm of the very first subflow of an MPTCP
    connection is set to "reno".

The code comes from

    commit 4d120186e4d6 ("bpf:examples: update mptcp_set_mark_kern.c")

in MPTCP repo https://github.com/multipath-tcp/mptcp_net-next (the
"scripts" branch).

Closes: multipath-tcp/mptcp_net-next#76
Co-developed-by: Geliang Tang <[email protected]>
Signed-off-by: Geliang Tang <[email protected]>
Signed-off-by: Nicolas Rybowski <[email protected]>

tools/testing/selftests/bpf/progs/mptcp_subflow.c

@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020, Tessares SA. */
+/* Copyright (c) 2024, Kylin Software */
+
+#include <sys/socket.h> // SOL_SOCKET, SO_MARK, ...
+#include <linux/tcp.h>  // TCP_CONGESTION
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_tcp_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+
+#ifndef SOL_TCP
+#define SOL_TCP 6
+#endif
+
+#ifndef TCP_CA_NAME_MAX
+#define TCP_CA_NAME_MAX 16
+#endif
+
+char cc[TCP_CA_NAME_MAX] = "reno";
+
+/* Associate a subflow counter to each token */
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(key_size, sizeof(__u32));
+	__uint(value_size, sizeof(__u32));
+	__uint(max_entries, 100);
+} mptcp_sf SEC(".maps");
+
+SEC("sockops")
+int mptcp_subflow(struct bpf_sock_ops *skops)
+{
+	__u32 init = 1, key, mark, *cnt;
+	struct mptcp_sock *msk;
+	struct bpf_sock *sk;
+	int err;
+
+	if (skops->op != BPF_SOCK_OPS_TCP_CONNECT_CB)
+		return 1;
+
+	sk = skops->sk;
+	if (!sk)
+		return 1;
+
+	msk = bpf_skc_to_mptcp_sock(sk);
+	if (!msk)
+		return 1;
+
+	key = msk->token;
+	cnt = bpf_map_lookup_elem(&mptcp_sf, &key);
+	if (cnt) {
+		/* A new subflow is added to an existing MPTCP connection */
+		__sync_fetch_and_add(cnt, 1);
+		mark = *cnt;
+	} else {
+		/* A new MPTCP connection is just initiated and this is its primary subflow */
+		bpf_map_update_elem(&mptcp_sf, &key, &init, BPF_ANY);
+		mark = init;
+	}
+
+	/* Set the mark of the subflow's socket based on appearance order */
+	err = bpf_setsockopt(skops, SOL_SOCKET, SO_MARK, &mark, sizeof(mark));
+	if (err < 0)
+		return 1;
+	if (mark == 1)
+		err = bpf_setsockopt(skops, SOL_TCP, TCP_CONGESTION, cc, TCP_CA_NAME_MAX);
+
+	return 1;
+}