Merge branch 'net-smc-introduce-smc_hs_ctrl'

D. Wythe says:

====================
net/smc: Introduce smc_hs_ctrl

This patch aims to introduce BPF injection capabilities for SMC and
includes a self-test to ensure code stability.

Since the SMC protocol isn't ideal for every situation, especially
short-lived ones, most applications can't guarantee the absence of
such scenarios. Consequently, applications may need specific strategies
to decide whether to use SMC. For example, an application might limit SMC
usage to certain IP addresses or ports.

To maintain the principle of transparent replacement, we want applications
to remain unaffected even if they need specific SMC strategies. In other
words, they should not require recompilation of their code.

Additionally, we need to ensure the scalability of strategy implementation.
While using socket options or sysctl might be straightforward, it could
complicate future expansions.

Fortunately, BPF addresses these concerns effectively. Users can write
their own strategies in eBPF to determine whether to use SMC, and they can
easily modify those strategies in the future.

This is a rework of the series from [1]. Changes since [1] are limited to
the SMC parts:

1. Rename smc_ops to smc_hs_ctrl and change interface name.
2. Squash SMC patches, removing standalone non-BPF hook capability.
3. Fix typos

[1]: https://lore.kernel.org/bpf/20250123015942.94810-1-alibuda@linux.alibaba.com/#t

v2 -> v1:
  - Removed the fixes patch, which have already been merged on current branch.
  - Fixed compilation warning of smc_call_hsbpf() when CONFIG_SMC_HS_CTRL_BPF
    is not enabled.
  - Changed the default value of CONFIG_SMC_HS_CTRL_BPF to Y.
  - Fix typo and renamed some variables

v3 -> v2:
  - Removed the libbpf patch, which have already been merged on current branch.
  - Fixed sparse warning of smc_call_hsbpf() and xchg().

v4 -> v3:
   - Rebased on latest bpf-next, updated SMC loopback config from SMC_LO to DIBS_LO
     per upstream changes.

v5 -> v4:
    - Removed the redundant sk parameter from smc_call_hsbpf
    - Reject registration when bpf_link is set, link support will be added in the
      future.
    - Updated selftests with new test heplers.
====================

Link: https://patch.msgid.link/20251107035632.115950-1-alibuda@linux.alibaba.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

include/net/netns/smc.h

@@ -17,6 +17,9 @@ struct netns_smc {
 #ifdef CONFIG_SYSCTL
 	struct ctl_table_header		*smc_hdr;
 #endif
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+	struct smc_hs_ctrl __rcu	*hs_ctrl;
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
 	unsigned int			sysctl_autocorking_size;
 	unsigned int			sysctl_smcr_buf_type;
 	int				sysctl_smcr_testlink_time;

include/net/smc.h

@@ -17,6 +17,8 @@
 #include <linux/wait.h>
 #include <linux/dibs.h>
 
+struct tcp_sock;
+struct inet_request_sock;
 struct sock;
 
 #define SMC_MAX_PNETID_LEN	16	/* Max. length of PNET id */
@@ -50,4 +52,55 @@ struct smcd_dev {
 	u8 going_away : 1;
 };
 
+#define SMC_HS_CTRL_NAME_MAX 16
+
+enum {
+	/* ops can be inherit from init_net */
+	SMC_HS_CTRL_FLAG_INHERITABLE = 0x1,
+
+	SMC_HS_CTRL_ALL_FLAGS = SMC_HS_CTRL_FLAG_INHERITABLE,
+};
+
+struct smc_hs_ctrl {
+	/* private */
+
+	struct list_head list;
+	struct module *owner;
+
+	/* public */
+
+	/* unique name */
+	char name[SMC_HS_CTRL_NAME_MAX];
+	int flags;
+
+	/* Invoked before computing SMC option for SYN packets.
+	 * We can control whether to set SMC options by returning various value.
+	 * Return 0 to disable SMC, or return any other value to enable it.
+	 */
+	int (*syn_option)(struct tcp_sock *tp);
+
+	/* Invoked before Set up SMC options for SYN-ACK packets
+	 * We can control whether to respond SMC options by returning various
+	 * value. Return 0 to disable SMC, or return any other value to enable
+	 * it.
+	 */
+	int (*synack_option)(const struct tcp_sock *tp,
+			     struct inet_request_sock *ireq);
+};
+
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+#define smc_call_hsbpf(init_val, tp, func, ...) ({				\
+	typeof(init_val) __ret = (init_val);					\
+	struct smc_hs_ctrl *ctrl;						\
+	rcu_read_lock();							\
+	ctrl = rcu_dereference(sock_net((struct sock *)(tp))->smc.hs_ctrl);	\
+	if (ctrl && ctrl->func)							\
+		__ret = ctrl->func(tp, ##__VA_ARGS__);				\
+	rcu_read_unlock();							\
+	__ret;									\
+})
+#else
+#define smc_call_hsbpf(init_val, tp, ...)  ({ (void)(tp); (init_val); })
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
+
 #endif	/* _SMC_H */

kernel/bpf/bpf_struct_ops.c

@@ -1162,6 +1162,7 @@ bool bpf_struct_ops_get(const void *kdata)
 	map = __bpf_map_inc_not_zero(&st_map->map, false);
 	return !IS_ERR(map);
 }
+EXPORT_SYMBOL_GPL(bpf_struct_ops_get);
 
 void bpf_struct_ops_put(const void *kdata)
 {
@@ -1173,6 +1174,7 @@ void bpf_struct_ops_put(const void *kdata)
 
 	bpf_map_put(&st_map->map);
 }
+EXPORT_SYMBOL_GPL(bpf_struct_ops_put);
 
 u32 bpf_struct_ops_id(const void *kdata)
 {

kernel/bpf/syscall.c

@@ -1234,6 +1234,7 @@ int bpf_obj_name_cpy(char *dst, const char *src, unsigned int size)
 
 	return src - orig_src;
 }
+EXPORT_SYMBOL_GPL(bpf_obj_name_cpy);
 
 int map_check_no_btf(const struct bpf_map *map,
 		     const struct btf *btf,

net/ipv4/tcp_output.c

@@ -40,6 +40,7 @@
 #include <net/tcp.h>
 #include <net/tcp_ecn.h>
 #include <net/mptcp.h>
+#include <net/smc.h>
 #include <net/proto_memory.h>
 #include <net/psp.h>
 
@@ -802,34 +803,36 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 	mptcp_options_write(th, ptr, tp, opts);
 }
 
-static void smc_set_option(const struct tcp_sock *tp,
+static void smc_set_option(struct tcp_sock *tp,
 			   struct tcp_out_options *opts,
 			   unsigned int *remaining)
 {
 #if IS_ENABLED(CONFIG_SMC)
-	if (static_branch_unlikely(&tcp_have_smc)) {
-		if (tp->syn_smc) {
-			if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
-				opts->options |= OPTION_SMC;
-				*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
-			}
+	if (static_branch_unlikely(&tcp_have_smc) && tp->syn_smc) {
+		tp->syn_smc = !!smc_call_hsbpf(1, tp, syn_option);
+		/* re-check syn_smc */
+		if (tp->syn_smc &&
+		    *remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
+			opts->options |= OPTION_SMC;
+			*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
 		}
 	}
 #endif
 }
 
 static void smc_set_option_cond(const struct tcp_sock *tp,
-				const struct inet_request_sock *ireq,
+				struct inet_request_sock *ireq,
 				struct tcp_out_options *opts,
 				unsigned int *remaining)
 {
 #if IS_ENABLED(CONFIG_SMC)
-	if (static_branch_unlikely(&tcp_have_smc)) {
-		if (tp->syn_smc && ireq->smc_ok) {
-			if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
-				opts->options |= OPTION_SMC;
-				*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
-			}
+	if (static_branch_unlikely(&tcp_have_smc) && tp->syn_smc && ireq->smc_ok) {
+		ireq->smc_ok = !!smc_call_hsbpf(1, tp, synack_option, ireq);
+		/* re-check smc_ok */
+		if (ireq->smc_ok &&
+		    *remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
+			opts->options |= OPTION_SMC;
+			*remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
 		}
 	}
 #endif

net/smc/Kconfig

@@ -19,3 +19,13 @@ config SMC_DIAG
 	  smcss.
 
 	  if unsure, say Y.
+
+config SMC_HS_CTRL_BPF
+	bool "Generic eBPF hook for SMC handshake flow"
+	depends on SMC && BPF_SYSCALL
+	default y
+	help
+	  SMC_HS_CTRL_BPF enables support to register generic eBPF hook for SMC
+	  handshake flow, which offer much greater flexibility in modifying the behavior
+	  of the SMC protocol stack compared to a complete kernel-based approach. Select
+	  this option if you want filtring the handshake process via eBPF programs.

net/smc/Makefile

@@ -6,3 +6,4 @@ smc-y := af_smc.o smc_pnet.o smc_ib.o smc_clc.o smc_core.o smc_wr.o smc_llc.o
 smc-y += smc_cdc.o smc_tx.o smc_rx.o smc_close.o smc_ism.o smc_netlink.o smc_stats.o
 smc-y += smc_tracepoint.o smc_inet.o
 smc-$(CONFIG_SYSCTL) += smc_sysctl.o
+smc-$(CONFIG_SMC_HS_CTRL_BPF) += smc_hs_bpf.o

net/smc/af_smc.c

@@ -58,6 +58,7 @@
 #include "smc_tracepoint.h"
 #include "smc_sysctl.h"
 #include "smc_inet.h"
+#include "smc_hs_bpf.h"
 
 static DEFINE_MUTEX(smc_server_lgr_pending);	/* serialize link group
 						 * creation on server
@@ -3600,8 +3601,16 @@ static int __init smc_init(void)
 		pr_err("%s: smc_inet_init fails with %d\n", __func__, rc);
 		goto out_ulp;
 	}
+	rc = bpf_smc_hs_ctrl_init();
+	if (rc) {
+		pr_err("%s: bpf_smc_hs_ctrl_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_ib:

net/smc/smc_hs_bpf.c

@@ -0,0 +1,140 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ *  Shared Memory Communications over RDMA (SMC-R) and RoCE
+ *
+ *  Generic hook for SMC handshake flow.
+ *
+ *  Copyright IBM Corp. 2016
+ *  Copyright (c) 2025, Alibaba Inc.
+ *
+ *  Author: D. Wythe <alibuda@linux.alibaba.com>
+ */
+
+#include <linux/bpf_verifier.h>
+#include <linux/bpf.h>
+#include <linux/btf.h>
+#include <linux/rculist.h>
+
+#include "smc_hs_bpf.h"
+
+static DEFINE_SPINLOCK(smc_hs_ctrl_list_lock);
+static LIST_HEAD(smc_hs_ctrl_list);
+
+static int smc_hs_ctrl_reg(struct smc_hs_ctrl *ctrl)
+{
+	int ret = 0;
+
+	spin_lock(&smc_hs_ctrl_list_lock);
+	/* already exist or duplicate name */
+	if (smc_hs_ctrl_find_by_name(ctrl->name))
+		ret = -EEXIST;
+	else
+		list_add_tail_rcu(&ctrl->list, &smc_hs_ctrl_list);
+	spin_unlock(&smc_hs_ctrl_list_lock);
+	return ret;
+}
+
+static void smc_hs_ctrl_unreg(struct smc_hs_ctrl *ctrl)
+{
+	spin_lock(&smc_hs_ctrl_list_lock);
+	list_del_rcu(&ctrl->list);
+	spin_unlock(&smc_hs_ctrl_list_lock);
+
+	/* Ensure that all readers to complete */
+	synchronize_rcu();
+}
+
+struct smc_hs_ctrl *smc_hs_ctrl_find_by_name(const char *name)
+{
+	struct smc_hs_ctrl *ctrl;
+
+	list_for_each_entry_rcu(ctrl, &smc_hs_ctrl_list, list) {
+		if (strcmp(ctrl->name, name) == 0)
+			return ctrl;
+	}
+	return NULL;
+}
+
+static int __smc_bpf_stub_set_tcp_option(struct tcp_sock *tp) { return 1; }
+static int __smc_bpf_stub_set_tcp_option_cond(const struct tcp_sock *tp,
+					      struct inet_request_sock *ireq)
+{
+	return 1;
+}
+
+static struct smc_hs_ctrl __smc_bpf_hs_ctrl = {
+	.syn_option	= __smc_bpf_stub_set_tcp_option,
+	.synack_option	= __smc_bpf_stub_set_tcp_option_cond,
+};
+
+static int smc_bpf_hs_ctrl_init(struct btf *btf) { return 0; }
+
+static int smc_bpf_hs_ctrl_reg(void *kdata, struct bpf_link *link)
+{
+	if (link)
+		return -EOPNOTSUPP;
+
+	return smc_hs_ctrl_reg(kdata);
+}
+
+static void smc_bpf_hs_ctrl_unreg(void *kdata, struct bpf_link *link)
+{
+	smc_hs_ctrl_unreg(kdata);
+}
+
+static int smc_bpf_hs_ctrl_init_member(const struct btf_type *t,
+				       const struct btf_member *member,
+				       void *kdata, const void *udata)
+{
+	const struct smc_hs_ctrl *u_ctrl;
+	struct smc_hs_ctrl *k_ctrl;
+	u32 moff;
+
+	u_ctrl = (const struct smc_hs_ctrl *)udata;
+	k_ctrl = (struct smc_hs_ctrl *)kdata;
+
+	moff = __btf_member_bit_offset(t, member) / 8;
+	switch (moff) {
+	case offsetof(struct smc_hs_ctrl, name):
+		if (bpf_obj_name_cpy(k_ctrl->name, u_ctrl->name,
+				     sizeof(u_ctrl->name)) <= 0)
+			return -EINVAL;
+		return 1;
+	case offsetof(struct smc_hs_ctrl, flags):
+		if (u_ctrl->flags & ~SMC_HS_CTRL_ALL_FLAGS)
+			return -EINVAL;
+		k_ctrl->flags = u_ctrl->flags;
+		return 1;
+	default:
+		break;
+	}
+
+	return 0;
+}
+
+static const struct bpf_func_proto *
+bpf_smc_hs_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
+{
+	return bpf_base_func_proto(func_id, prog);
+}
+
+static const struct bpf_verifier_ops smc_bpf_verifier_ops = {
+	.get_func_proto		= bpf_smc_hs_func_proto,
+	.is_valid_access	= bpf_tracing_btf_ctx_access,
+};
+
+static struct bpf_struct_ops bpf_smc_hs_ctrl_ops = {
+	.name		= "smc_hs_ctrl",
+	.init		= smc_bpf_hs_ctrl_init,
+	.reg		= smc_bpf_hs_ctrl_reg,
+	.unreg		= smc_bpf_hs_ctrl_unreg,
+	.cfi_stubs	= &__smc_bpf_hs_ctrl,
+	.verifier_ops	= &smc_bpf_verifier_ops,
+	.init_member	= smc_bpf_hs_ctrl_init_member,
+	.owner		= THIS_MODULE,
+};
+
+int bpf_smc_hs_ctrl_init(void)
+{
+	return register_bpf_struct_ops(&bpf_smc_hs_ctrl_ops, smc_hs_ctrl);
+}

net/smc/smc_hs_bpf.h

@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ *  Shared Memory Communications over RDMA (SMC-R) and RoCE
+ *
+ *  Generic hook for SMC handshake flow.
+ *
+ *  Copyright IBM Corp. 2016
+ *  Copyright (c) 2025, Alibaba Inc.
+ *
+ *  Author: D. Wythe <alibuda@linux.alibaba.com>
+ */
+
+#ifndef __SMC_HS_CTRL
+#define __SMC_HS_CTRL
+
+#include <net/smc.h>
+
+/* Find hs_ctrl by the target name, which required to be a c-string.
+ * Return NULL if no such ctrl was found,otherwise, return a valid ctrl.
+ *
+ * Note: Caller MUST ensure it's was invoked under rcu_read_lock.
+ */
+struct smc_hs_ctrl *smc_hs_ctrl_find_by_name(const char *name);
+
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+int bpf_smc_hs_ctrl_init(void);
+#else
+static inline int bpf_smc_hs_ctrl_init(void) { return 0; }
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
+
+#endif /* __SMC_HS_CTRL */

net/smc/smc_sysctl.c

@@ -12,12 +12,14 @@
 
 #include <linux/init.h>
 #include <linux/sysctl.h>
+#include <linux/bpf.h>
 #include <net/net_namespace.h>
 
 #include "smc.h"
 #include "smc_core.h"
 #include "smc_llc.h"
 #include "smc_sysctl.h"
+#include "smc_hs_bpf.h"
 
 static int min_sndbuf = SMC_BUF_MIN_SIZE;
 static int min_rcvbuf = SMC_BUF_MIN_SIZE;
@@ -32,6 +34,69 @@ static int conns_per_lgr_max = SMC_CONN_PER_LGR_MAX;
 static unsigned int smcr_max_wr_min = 2;
 static unsigned int smcr_max_wr_max = 2048;
 
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+static int smc_net_replace_smc_hs_ctrl(struct net *net, const char *name)
+{
+	struct smc_hs_ctrl *ctrl = NULL;
+
+	rcu_read_lock();
+	/* null or empty name ask to clear current ctrl */
+	if (name && name[0]) {
+		ctrl = smc_hs_ctrl_find_by_name(name);
+		if (!ctrl) {
+			rcu_read_unlock();
+			return -EINVAL;
+		}
+		/* no change, just return */
+		if (ctrl == rcu_dereference(net->smc.hs_ctrl)) {
+			rcu_read_unlock();
+			return 0;
+		}
+		if (!bpf_try_module_get(ctrl, ctrl->owner)) {
+			rcu_read_unlock();
+			return -EBUSY;
+		}
+	}
+	/* xhcg old ctrl with the new one atomically */
+	ctrl = unrcu_pointer(xchg(&net->smc.hs_ctrl, RCU_INITIALIZER(ctrl)));
+	/* release old ctrl */
+	if (ctrl)
+		bpf_module_put(ctrl, ctrl->owner);
+
+	rcu_read_unlock();
+	return 0;
+}
+
+static int proc_smc_hs_ctrl(const struct ctl_table *ctl, int write,
+			    void *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct net *net = container_of(ctl->data, struct net, smc.hs_ctrl);
+	char val[SMC_HS_CTRL_NAME_MAX];
+	const struct ctl_table tbl = {
+		.data = val,
+		.maxlen = SMC_HS_CTRL_NAME_MAX,
+	};
+	struct smc_hs_ctrl *ctrl;
+	int ret;
+
+	rcu_read_lock();
+	ctrl = rcu_dereference(net->smc.hs_ctrl);
+	if (ctrl)
+		memcpy(val, ctrl->name, sizeof(ctrl->name));
+	else
+		val[0] = '\0';
+	rcu_read_unlock();
+
+	ret = proc_dostring(&tbl, write, buffer, lenp, ppos);
+	if (ret)
+		return ret;
+
+	if (write)
+		ret = smc_net_replace_smc_hs_ctrl(net, val);
+	return ret;
+}
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
+
 static struct ctl_table smc_table[] = {
 	{
 		.procname       = "autocorking_size",
@@ -119,6 +184,15 @@ static struct ctl_table smc_table[] = {
 		.extra1		= &smcr_max_wr_min,
 		.extra2		= &smcr_max_wr_max,
 	},
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+	{
+		.procname	= "hs_ctrl",
+		.data		= &init_net.smc.hs_ctrl,
+		.mode		= 0644,
+		.maxlen		= SMC_HS_CTRL_NAME_MAX,
+		.proc_handler	= proc_smc_hs_ctrl,
+	},
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
 };
 
 int __net_init smc_sysctl_net_init(struct net *net)
@@ -129,6 +203,16 @@ int __net_init smc_sysctl_net_init(struct net *net)
 	table = smc_table;
 	if (!net_eq(net, &init_net)) {
 		int i;
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+		struct smc_hs_ctrl *ctrl;
+
+		rcu_read_lock();
+		ctrl = rcu_dereference(init_net.smc.hs_ctrl);
+		if (ctrl && ctrl->flags & SMC_HS_CTRL_FLAG_INHERITABLE &&
+		    bpf_try_module_get(ctrl, ctrl->owner))
+			rcu_assign_pointer(net->smc.hs_ctrl, ctrl);
+		rcu_read_unlock();
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
 
 		table = kmemdup(table, sizeof(smc_table), GFP_KERNEL);
 		if (!table)
@@ -161,6 +245,9 @@ err_reg:
 	if (!net_eq(net, &init_net))
 		kfree(table);
 err_alloc:
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+	smc_net_replace_smc_hs_ctrl(net, NULL);
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
 	return -ENOMEM;
 }
 
@@ -170,6 +257,10 @@ void __net_exit smc_sysctl_net_exit(struct net *net)
 
 	table = net->smc.smc_hdr->ctl_table_arg;
 	unregister_net_sysctl_table(net->smc.smc_hdr);
+#if IS_ENABLED(CONFIG_SMC_HS_CTRL_BPF)
+	smc_net_replace_smc_hs_ctrl(net, NULL);
+#endif /* CONFIG_SMC_HS_CTRL_BPF */
+
 	if (!net_eq(net, &init_net))
 		kfree(table);
 }

tools/testing/selftests/bpf/config

@@ -123,3 +123,8 @@ CONFIG_XDP_SOCKETS=y
 CONFIG_XFRM_INTERFACE=y
 CONFIG_TCP_CONG_DCTCP=y
 CONFIG_TCP_CONG_BBR=y
+CONFIG_INFINIBAND=y
+CONFIG_SMC=y
+CONFIG_SMC_HS_CTRL_BPF=y
+CONFIG_DIBS=y
+CONFIG_DIBS_LO=y

tools/testing/selftests/bpf/prog_tests/test_bpf_smc.c

@@ -0,0 +1,390 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <linux/genetlink.h>
+#include "network_helpers.h"
+#include "bpf_smc.skel.h"
+
+#ifndef IPPROTO_SMC
+#define IPPROTO_SMC 256
+#endif
+
+#define CLIENT_IP			"127.0.0.1"
+#define SERVER_IP			"127.0.1.0"
+#define SERVER_IP_VIA_RISK_PATH	"127.0.2.0"
+
+#define SERVICE_1	80
+#define SERVICE_2	443
+#define SERVICE_3	8443
+
+#define TEST_NS	"bpf_smc_netns"
+
+static struct netns_obj *test_netns;
+
+struct smc_policy_ip_key {
+	__u32  sip;
+	__u32  dip;
+};
+
+struct smc_policy_ip_value {
+	__u8	mode;
+};
+
+#if defined(__s390x__)
+/* s390x has default seid  */
+static bool setup_ueid(void) { return true; }
+static void cleanup_ueid(void) {}
+#else
+enum {
+	SMC_NETLINK_ADD_UEID = 10,
+	SMC_NETLINK_REMOVE_UEID
+};
+
+enum {
+	SMC_NLA_EID_TABLE_UNSPEC,
+	SMC_NLA_EID_TABLE_ENTRY,    /* string */
+};
+
+struct msgtemplate {
+	struct nlmsghdr n;
+	struct genlmsghdr g;
+	char buf[1024];
+};
+
+#define GENLMSG_DATA(glh)	((void *)(NLMSG_DATA(glh) + GENL_HDRLEN))
+#define GENLMSG_PAYLOAD(glh)	(NLMSG_PAYLOAD(glh, 0) - GENL_HDRLEN)
+#define NLA_DATA(na)		((void *)((char *)(na) + NLA_HDRLEN))
+#define NLA_PAYLOAD(len)	((len) - NLA_HDRLEN)
+
+#define SMC_GENL_FAMILY_NAME	"SMC_GEN_NETLINK"
+#define SMC_BPFTEST_UEID	"SMC-BPFTEST-UEID"
+
+static uint16_t smc_nl_family_id = -1;
+
+static int send_cmd(int fd, __u16 nlmsg_type, __u32 nlmsg_pid,
+		    __u16 nlmsg_flags, __u8 genl_cmd, __u16 nla_type,
+		    void *nla_data, int nla_len)
+{
+	struct nlattr *na;
+	struct sockaddr_nl nladdr;
+	int r, buflen;
+	char *buf;
+
+	struct msgtemplate msg = {0};
+
+	msg.n.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);
+	msg.n.nlmsg_type = nlmsg_type;
+	msg.n.nlmsg_flags = nlmsg_flags;
+	msg.n.nlmsg_seq = 0;
+	msg.n.nlmsg_pid = nlmsg_pid;
+	msg.g.cmd = genl_cmd;
+	msg.g.version = 1;
+	na = (struct nlattr *)GENLMSG_DATA(&msg);
+	na->nla_type = nla_type;
+	na->nla_len = nla_len + 1 + NLA_HDRLEN;
+	memcpy(NLA_DATA(na), nla_data, nla_len);
+	msg.n.nlmsg_len += NLMSG_ALIGN(na->nla_len);
+
+	buf = (char *)&msg;
+	buflen = msg.n.nlmsg_len;
+	memset(&nladdr, 0, sizeof(nladdr));
+	nladdr.nl_family = AF_NETLINK;
+
+	while ((r = sendto(fd, buf, buflen, 0, (struct sockaddr *)&nladdr,
+			   sizeof(nladdr))) < buflen) {
+		if (r > 0) {
+			buf += r;
+			buflen -= r;
+		} else if (errno != EAGAIN) {
+			return -1;
+		}
+	}
+	return 0;
+}
+
+static bool get_smc_nl_family_id(void)
+{
+	struct sockaddr_nl nl_src;
+	struct msgtemplate msg;
+	struct nlattr *nl;
+	int fd, ret;
+	pid_t pid;
+
+	fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
+	if (!ASSERT_OK_FD(fd, "nl_family socket"))
+		return false;
+
+	pid = getpid();
+
+	memset(&nl_src, 0, sizeof(nl_src));
+	nl_src.nl_family = AF_NETLINK;
+	nl_src.nl_pid = pid;
+
+	ret = bind(fd, (struct sockaddr *)&nl_src, sizeof(nl_src));
+	if (!ASSERT_OK(ret, "nl_family bind"))
+		goto fail;
+
+	ret = send_cmd(fd, GENL_ID_CTRL, pid,
+		       NLM_F_REQUEST, CTRL_CMD_GETFAMILY,
+		       CTRL_ATTR_FAMILY_NAME, (void *)SMC_GENL_FAMILY_NAME,
+		       strlen(SMC_GENL_FAMILY_NAME));
+	if (!ASSERT_OK(ret, "nl_family query"))
+		goto fail;
+
+	ret = recv(fd, &msg, sizeof(msg), 0);
+	if (!ASSERT_FALSE(msg.n.nlmsg_type == NLMSG_ERROR || ret < 0 ||
+			  !NLMSG_OK(&msg.n, ret), "nl_family response"))
+		goto fail;
+
+	nl = (struct nlattr *)GENLMSG_DATA(&msg);
+	nl = (struct nlattr *)((char *)nl + NLA_ALIGN(nl->nla_len));
+	if (!ASSERT_EQ(nl->nla_type, CTRL_ATTR_FAMILY_ID, "nl_family nla type"))
+		goto fail;
+
+	smc_nl_family_id = *(uint16_t *)NLA_DATA(nl);
+	close(fd);
+	return true;
+fail:
+	close(fd);
+	return false;
+}
+
+static bool smc_ueid(int op)
+{
+	struct sockaddr_nl nl_src;
+	struct msgtemplate msg;
+	struct nlmsgerr *err;
+	char test_ueid[32];
+	int fd, ret;
+	pid_t pid;
+
+	/* UEID required */
+	memset(test_ueid, '\x20', sizeof(test_ueid));
+	memcpy(test_ueid, SMC_BPFTEST_UEID, strlen(SMC_BPFTEST_UEID));
+	fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
+	if (!ASSERT_OK_FD(fd, "ueid socket"))
+		return false;
+
+	pid = getpid();
+	memset(&nl_src, 0, sizeof(nl_src));
+	nl_src.nl_family = AF_NETLINK;
+	nl_src.nl_pid = pid;
+
+	ret = bind(fd, (struct sockaddr *)&nl_src, sizeof(nl_src));
+	if (!ASSERT_OK(ret, "ueid bind"))
+		goto fail;
+
+	ret = send_cmd(fd, smc_nl_family_id, pid,
+		       NLM_F_REQUEST | NLM_F_ACK, op, SMC_NLA_EID_TABLE_ENTRY,
+		       (void *)test_ueid, sizeof(test_ueid));
+	if (!ASSERT_OK(ret, "ueid cmd"))
+		goto fail;
+
+	ret = recv(fd, &msg, sizeof(msg), 0);
+	if (!ASSERT_FALSE(ret < 0 ||
+			  !NLMSG_OK(&msg.n, ret), "ueid response"))
+		goto fail;
+
+	if (msg.n.nlmsg_type == NLMSG_ERROR) {
+		err = NLMSG_DATA(&msg);
+		switch (op) {
+		case SMC_NETLINK_REMOVE_UEID:
+			if (!ASSERT_FALSE((err->error && err->error != -ENOENT),
+					  "ueid remove"))
+				goto fail;
+			break;
+		case SMC_NETLINK_ADD_UEID:
+			if (!ASSERT_OK(err->error, "ueid add"))
+				goto fail;
+			break;
+		default:
+			break;
+		}
+	}
+	close(fd);
+	return true;
+fail:
+	close(fd);
+	return false;
+}
+
+static bool setup_ueid(void)
+{
+	/* get smc nl id */
+	if (!get_smc_nl_family_id())
+		return false;
+	/* clear old ueid for bpftest */
+	smc_ueid(SMC_NETLINK_REMOVE_UEID);
+	/* smc-loopback required ueid */
+	return smc_ueid(SMC_NETLINK_ADD_UEID);
+}
+
+static void cleanup_ueid(void)
+{
+	smc_ueid(SMC_NETLINK_REMOVE_UEID);
+}
+#endif /* __s390x__ */
+
+static bool setup_netns(void)
+{
+	test_netns = netns_new(TEST_NS, true);
+	if (!ASSERT_OK_PTR(test_netns, "open net namespace"))
+		goto fail_netns;
+
+	SYS(fail_ip, "ip addr add 127.0.1.0/8 dev lo");
+	SYS(fail_ip, "ip addr add 127.0.2.0/8 dev lo");
+
+	return true;
+fail_ip:
+	netns_free(test_netns);
+fail_netns:
+	return false;
+}
+
+static void cleanup_netns(void)
+{
+	netns_free(test_netns);
+}
+
+static bool setup_smc(void)
+{
+	if (!setup_ueid())
+		return false;
+
+	if (!setup_netns())
+		goto fail_netns;
+
+	return true;
+fail_netns:
+	cleanup_ueid();
+	return false;
+}
+
+static int set_client_addr_cb(int fd, void *opts)
+{
+	const char *src = (const char *)opts;
+	struct sockaddr_in localaddr;
+
+	localaddr.sin_family = AF_INET;
+	localaddr.sin_port = htons(0);
+	localaddr.sin_addr.s_addr = inet_addr(src);
+	return !ASSERT_OK(bind(fd, &localaddr, sizeof(localaddr)), "client bind");
+}
+
+static void run_link(const char *src, const char *dst, int port)
+{
+	struct network_helper_opts opts = {0};
+	int server, client;
+
+	server = start_server_str(AF_INET, SOCK_STREAM, dst, port, NULL);
+	if (!ASSERT_OK_FD(server, "start service_1"))
+		return;
+
+	opts.proto = IPPROTO_TCP;
+	opts.post_socket_cb = set_client_addr_cb;
+	opts.cb_opts = (void *)src;
+
+	client = connect_to_fd_opts(server, &opts);
+	if (!ASSERT_OK_FD(client, "start connect"))
+		goto fail_client;
+
+	close(client);
+fail_client:
+	close(server);
+}
+
+static void block_link(int map_fd, const char *src, const char *dst)
+{
+	struct smc_policy_ip_value val = { .mode = /* block */ 0 };
+	struct smc_policy_ip_key key = {
+		.sip = inet_addr(src),
+		.dip = inet_addr(dst),
+	};
+
+	bpf_map_update_elem(map_fd, &key, &val, BPF_ANY);
+}
+
+/*
+ * This test describes a real-life service topology as follows:
+ *
+ *                             +-------------> service_1
+ *            link 1           |                     |
+ *   +--------------------> server                   |  link 2
+ *   |                         |                     V
+ *   |                         +-------------> service_2
+ *   |        link 3
+ *  client -------------------> server_via_unsafe_path -> service_3
+ *
+ * Among them,
+ * 1. link-1 is very suitable for using SMC.
+ * 2. link-2 is not suitable for using SMC, because the mode of this link is
+ *    kind of short-link services.
+ * 3. link-3 is also not suitable for using SMC, because the RDMA link is
+ *    unavailable and needs to go through a long timeout before it can fallback
+ *    to TCP.
+ * To achieve this goal, we use a customized SMC ip strategy via smc_hs_ctrl.
+ */
+static void test_topo(void)
+{
+	struct bpf_smc *skel;
+	int rc, map_fd;
+
+	skel = bpf_smc__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "bpf_smc__open_and_load"))
+		return;
+
+	rc = bpf_smc__attach(skel);
+	if (!ASSERT_OK(rc, "bpf_smc__attach"))
+		goto fail;
+
+	map_fd = bpf_map__fd(skel->maps.smc_policy_ip);
+	if (!ASSERT_OK_FD(map_fd, "bpf_map__fd"))
+		goto fail;
+
+	/* Mock the process of transparent replacement, since we will modify
+	 * protocol to ipproto_smc accropding to it via
+	 * fmod_ret/update_socket_protocol.
+	 */
+	write_sysctl("/proc/sys/net/smc/hs_ctrl", "linkcheck");
+
+	/* Configure ip strat */
+	block_link(map_fd, CLIENT_IP, SERVER_IP_VIA_RISK_PATH);
+	block_link(map_fd, SERVER_IP, SERVER_IP);
+
+	/* should go with smc */
+	run_link(CLIENT_IP, SERVER_IP, SERVICE_1);
+	/* should go with smc fallback */
+	run_link(SERVER_IP, SERVER_IP, SERVICE_2);
+
+	ASSERT_EQ(skel->bss->smc_cnt, 2, "smc count");
+	ASSERT_EQ(skel->bss->fallback_cnt, 1, "fallback count");
+
+	/* should go with smc */
+	run_link(CLIENT_IP, SERVER_IP, SERVICE_2);
+
+	ASSERT_EQ(skel->bss->smc_cnt, 3, "smc count");
+	ASSERT_EQ(skel->bss->fallback_cnt, 1, "fallback count");
+
+	/* should go with smc fallback */
+	run_link(CLIENT_IP, SERVER_IP_VIA_RISK_PATH, SERVICE_3);
+
+	ASSERT_EQ(skel->bss->smc_cnt, 4, "smc count");
+	ASSERT_EQ(skel->bss->fallback_cnt, 2, "fallback count");
+
+fail:
+	bpf_smc__destroy(skel);
+}
+
+void test_bpf_smc(void)
+{
+	if (!setup_smc()) {
+		printf("setup for smc test failed, test SKIP:\n");
+		test__skip();
+		return;
+	}
+
+	if (test__start_subtest("topo"))
+		test_topo();
+
+	cleanup_ueid();
+	cleanup_netns();
+}

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

@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_tracing_net.h"
+
+char _license[] SEC("license") = "GPL";
+
+enum {
+	BPF_SMC_LISTEN	= 10,
+};
+
+struct smc_sock___local {
+	struct sock sk;
+	struct smc_sock *listen_smc;
+	bool use_fallback;
+} __attribute__((preserve_access_index));
+
+int smc_cnt = 0;
+int fallback_cnt = 0;
+
+SEC("fentry/smc_release")
+int BPF_PROG(bpf_smc_release, struct socket *sock)
+{
+	/* only count from one side (client) */
+	if (sock->sk->__sk_common.skc_state == BPF_SMC_LISTEN)
+		return 0;
+	smc_cnt++;
+	return 0;
+}
+
+SEC("fentry/smc_switch_to_fallback")
+int BPF_PROG(bpf_smc_switch_to_fallback, struct smc_sock___local *smc)
+{
+	/* only count from one side (client) */
+	if (smc && !smc->listen_smc)
+		fallback_cnt++;
+	return 0;
+}
+
+/* go with default value if no strat was found */
+bool default_ip_strat_value = true;
+
+struct smc_policy_ip_key {
+	__u32	sip;
+	__u32	dip;
+};
+
+struct smc_policy_ip_value {
+	__u8	mode;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(key_size, sizeof(struct smc_policy_ip_key));
+	__uint(value_size, sizeof(struct smc_policy_ip_value));
+	__uint(max_entries, 128);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+} smc_policy_ip SEC(".maps");
+
+static bool smc_check(__u32 src, __u32 dst)
+{
+	struct smc_policy_ip_value *value;
+	struct smc_policy_ip_key key = {
+		.sip = src,
+		.dip = dst,
+	};
+
+	value = bpf_map_lookup_elem(&smc_policy_ip, &key);
+	return value ? value->mode : default_ip_strat_value;
+}
+
+SEC("fmod_ret/update_socket_protocol")
+int BPF_PROG(smc_run, int family, int type, int protocol)
+{
+	struct task_struct *task;
+
+	if (family != AF_INET && family != AF_INET6)
+		return protocol;
+
+	if ((type & 0xf) != SOCK_STREAM)
+		return protocol;
+
+	if (protocol != 0 && protocol != IPPROTO_TCP)
+		return protocol;
+
+	task = bpf_get_current_task_btf();
+	/* Prevent from affecting other tests */
+	if (!task || !task->nsproxy->net_ns->smc.hs_ctrl)
+		return protocol;
+
+	return IPPROTO_SMC;
+}
+
+SEC("struct_ops")
+int BPF_PROG(bpf_smc_set_tcp_option_cond, const struct tcp_sock *tp,
+	     struct inet_request_sock *ireq)
+{
+	return smc_check(ireq->req.__req_common.skc_daddr,
+			 ireq->req.__req_common.skc_rcv_saddr);
+}
+
+SEC("struct_ops")
+int BPF_PROG(bpf_smc_set_tcp_option, struct tcp_sock *tp)
+{
+	return smc_check(tp->inet_conn.icsk_inet.sk.__sk_common.skc_rcv_saddr,
+			 tp->inet_conn.icsk_inet.sk.__sk_common.skc_daddr);
+}
+
+SEC(".struct_ops")
+struct smc_hs_ctrl  linkcheck = {
+	.name		= "linkcheck",
+	.syn_option	= (void *)bpf_smc_set_tcp_option,
+	.synack_option	= (void *)bpf_smc_set_tcp_option_cond,
+};