[RFC PATCH net-next v2] net/smc: Introduce a hook to modify syn_smc at runtime

["D. Wythe" <alibuda@linux.alibaba.com>]

From: "D. Wythe" <alibuda@linux.alibaba.com>

The introduction of IPPROTO_SMC enables eBPF programs to determine
whether to use SMC based on the context of socket creation, such as
network namespaces, PID and comm name, etc.

As a subsequent enhancement, this patch introduces a new hook for eBPF
programs that allows decisions on whether to use SMC or not at runtime,
including but not limited to local/remote IP address or ports. In
simpler words, this feature allows modifications to syn_smc through eBPF
programs before the TCP three-way handshake got established.

Thanks to kfunc for making it easier for us to implement this feature in
SMC.

Signed-off-by: D. Wythe <alibuda@linux.alibaba.com>

---
v1 -> v2:
1. Fix wrong use of ireq->smc_ok, should be rx_opt->smc_ok.
2. Fix compile error when CONFIG_IPV6 or CONFIG_BPF_SYSCALL was not set.

---
 include/linux/tcp.h  |  4 ++-
 net/ipv4/tcp_input.c |  4 +--
 net/smc/af_smc.c     | 75 ++++++++++++++++++++++++++++++++++++++++++++++------
 3 files changed, 72 insertions(+), 11 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 6a5e08b..d028d76 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -478,7 +478,9 @@ struct tcp_sock {
 #endif
 #if IS_ENABLED(CONFIG_SMC)
 	bool	syn_smc;	/* SYN includes SMC */
-	bool	(*smc_hs_congested)(const struct sock *sk);
+	void	(*smc_openreq_init)(struct request_sock *req,
+			     const struct tcp_options_received *rx_opt,
+			     struct sk_buff *skb, const struct sock *sk);
 #endif
 
 #if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9f314df..99f34f5 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -7036,8 +7036,8 @@ static void tcp_openreq_init(struct request_sock *req,
 	ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
 	ireq->ir_mark = inet_request_mark(sk, skb);
 #if IS_ENABLED(CONFIG_SMC)
-	ireq->smc_ok = rx_opt->smc_ok && !(tcp_sk(sk)->smc_hs_congested &&
-			tcp_sk(sk)->smc_hs_congested(sk));
+	if (rx_opt->smc_ok && tcp_sk(sk)->smc_openreq_init)
+		tcp_sk(sk)->smc_openreq_init(req, rx_opt, skb, sk);
 #endif
 }
 
diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index 0316217..fdac7e2b 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -70,6 +70,15 @@
 static void smc_tcp_listen_work(struct work_struct *);
 static void smc_connect_work(struct work_struct *);
 
+__bpf_hook_start();
+
+__weak noinline int select_syn_smc(const struct sock *sk, struct sockaddr *peer)
+{
+	return 1;
+}
+
+__bpf_hook_end();
+
 int smc_nl_dump_hs_limitation(struct sk_buff *skb, struct netlink_callback *cb)
 {
 	struct smc_nl_dmp_ctx *cb_ctx = smc_nl_dmp_ctx(cb);
@@ -156,19 +165,43 @@ static struct sock *smc_tcp_syn_recv_sock(const struct sock *sk,
 	return NULL;
 }
 
-static bool smc_hs_congested(const struct sock *sk)
+static void smc_openreq_init(struct request_sock *req,
+			     const struct tcp_options_received *rx_opt,
+			     struct sk_buff *skb, const struct sock *sk)
 {
+	struct inet_request_sock *ireq = inet_rsk(req);
+	struct sockaddr_storage rmt_sockaddr = {0};
 	const struct smc_sock *smc;
 
 	smc = smc_clcsock_user_data(sk);
 
 	if (!smc)
-		return true;
+		return;
 
-	if (workqueue_congested(WORK_CPU_UNBOUND, smc_hs_wq))
-		return true;
+	if (smc->limit_smc_hs && workqueue_congested(WORK_CPU_UNBOUND, smc_hs_wq))
+		goto out_no_smc;
 
-	return false;
+	rmt_sockaddr.ss_family = sk->sk_family;
+
+	if (rmt_sockaddr.ss_family == AF_INET) {
+		struct sockaddr_in *rmt4_sockaddr =  (struct sockaddr_in *)&rmt_sockaddr;
+
+		rmt4_sockaddr->sin_addr.s_addr = ireq->ir_rmt_addr;
+		rmt4_sockaddr->sin_port	= ireq->ir_rmt_port;
+#if IS_ENABLED(CONFIG_IPV6)
+	} else {
+		struct sockaddr_in6 *rmt6_sockaddr =  (struct sockaddr_in6 *)&rmt_sockaddr;
+
+		rmt6_sockaddr->sin6_addr = ireq->ir_v6_rmt_addr;
+		rmt6_sockaddr->sin6_port = ireq->ir_rmt_port;
+#endif /* CONFIG_IPV6 */
+	}
+
+	ireq->smc_ok = select_syn_smc(sk, (struct sockaddr *)&rmt_sockaddr);
+	return;
+out_no_smc:
+	ireq->smc_ok = 0;
+	return;
 }
 
 struct smc_hashinfo smc_v4_hashinfo = {
@@ -1671,7 +1704,7 @@ int smc_connect(struct socket *sock, struct sockaddr *addr,
 	}
 
 	smc_copy_sock_settings_to_clc(smc);
-	tcp_sk(smc->clcsock->sk)->syn_smc = 1;
+	tcp_sk(smc->clcsock->sk)->syn_smc = select_syn_smc(sk, addr);
 	if (smc->connect_nonblock) {
 		rc = -EALREADY;
 		goto out;
@@ -2650,8 +2683,7 @@ int smc_listen(struct socket *sock, int backlog)
 
 	inet_csk(smc->clcsock->sk)->icsk_af_ops = &smc->af_ops;
 
-	if (smc->limit_smc_hs)
-		tcp_sk(smc->clcsock->sk)->smc_hs_congested = smc_hs_congested;
+	tcp_sk(smc->clcsock->sk)->smc_openreq_init = smc_openreq_init;
 
 	rc = kernel_listen(smc->clcsock, backlog);
 	if (rc) {
@@ -3475,6 +3507,24 @@ static void __net_exit smc_net_stat_exit(struct net *net)
 	.exit = smc_net_stat_exit,
 };
 
+#if IS_ENABLED(CONFIG_BPF_SYSCALL)
+BTF_SET8_START(bpf_smc_fmodret_ids)
+BTF_ID_FLAGS(func, select_syn_smc)
+BTF_SET8_END(bpf_smc_fmodret_ids)
+
+static const struct btf_kfunc_id_set bpf_smc_fmodret_set = {
+	.owner = THIS_MODULE,
+	.set   = &bpf_smc_fmodret_ids,
+};
+
+static int bpf_smc_kfunc_init(void)
+{
+	return register_btf_fmodret_id_set(&bpf_smc_fmodret_set);
+}
+#else
+static inline int bpf_smc_kfunc_init(void) { return 0; }
+#endif /* CONFIG_BPF_SYSCALL */
+
 static int __init smc_init(void)
 {
 	int rc;
@@ -3574,8 +3624,17 @@ static int __init smc_init(void)
 		pr_err("%s: smc_inet_init fails with %d\n", __func__, rc);
 		goto out_ulp;
 	}
+
+	rc = bpf_smc_kfunc_init();
+	if (rc) {
+		pr_err("%s: bpf_smc_kfunc_init fails with %d\n", __func__, rc);
+		goto out_inet;
+	}
+
 	static_branch_enable(&tcp_have_smc);
 	return 0;
+out_inet:
+	smc_inet_exit();
 out_ulp:
 	tcp_unregister_ulp(&smc_ulp_ops);
 out_lo:
-- 
1.8.3.1