bpf: tcp: Support selected sock_ops callbacks as struct_ops

In LSFMMBPF 2025, I have talked about moving the BPF_PROG_TYPE_SOCK_OPS
to a struct_ops interface [1].

The BPF_SOCK_OPS_*_CB enum interface has grown over time as new TCP
callback points were added. A BPF_PROG_TYPE_SOCK_OPS program now
commonly needs a large switch on sock_ops->op, and the shared
bpf_sock_ops_kern context has become harder to extend because different
callbacks have different locking, argument, skb, and helper
requirements. The existing 'union { u32 args[4]; u32 replylong[4]; }' is
also not reliable in passing args to bpf prog when there are multiple
progs attached to a cgroup.

The above has already been solved in struct_ops. Add a TCP-specific
struct_ops type, bpf_tcp_ops, and support attaching it to cgroups.
This allows each callback have its own func signature and allows
the verifier to select kfuncs/helpers based on the specific
struct_ops member being implemented.

This patch wires up the following existing sock_ops callbacks:
- BPF_SOCK_OPS_TIMEOUT_INIT
- BPF_SOCK_OPS_RWND_INIT
- BPF_SOCK_OPS_RTT_CB
- BPF_SOCK_OPS_STATE_CB
- BPF_SOCK_OPS_RETRANS_CB
- BPF_SOCK_OPS_TCP_CONNECT_CB
- BPF_SOCK_OPS_TCP_LISTEN_CB
- BPF_SOCK_OPS_RTO_CB
- BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB
- BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB

BASE_RTT is ignored as it is not particularly useful. NEEDS_ECN should
be done in bpf-tcp-cc instead. The tstamp ones should be a separate
struct_ops (e.g. "bpf_sock_ops") that can work in both TCP and UDP.

timeout_init and rwnd_init could have a request_sock pointer. This patch
tries a different API and directly passes the request_sock pointer as
an arg.

Two other approaches were considered before settling on having
bpf_get_retval() read the dispatcher's run_ctx via saved_run_ctx. The
first was to inherit the retval in the trampoline itself: add a helper
in the four __bpf_prog_enter*() paths that, for struct_ops programs,
copies the chained value from the caller's run_ctx (now saved_run_ctx)
into the program's own run_ctx. It works but puts a per-enter
program-type check on the generic trampoline fast path, taxing all
fentry/fexit/lsm callers for a cgroup-struct_ops-only feature. The
second was to do that same inherit only for the int-returning members
via a gen_prologue that emits a hidden kfunc at the start of
timeout_init/rwnd_init; this keeps the cost off the generic path and
scoped to bpf_tcp_ops, but needs a kfunc + BTF_ID + prologue-emission
machinery. The chosen approach avoids both: it touches neither the
trampoline nor the program, since saved_run_ctx already points at the
dispatcher's run_ctx that carries the value.

[1], page 13: https://drive.google.com/file/d/1wjKZth6T0llLJ_ONPAL_6Q_jbxbAjByp/view?usp=sharing

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://patch.msgid.link/20260917200542.3689605-12-ameryhung@gmail.com

include/linux/bpf.h

@@ -2634,6 +2634,7 @@ struct bpf_trace_run_ctx {
 struct bpf_tramp_run_ctx {
 	struct bpf_run_ctx run_ctx;
 	u64 bpf_cookie;
+	int retval;
 	struct bpf_run_ctx *saved_run_ctx;
 };
 

include/net/tcp.h

@@ -2959,12 +2959,120 @@ static inline void tcp_clear_sock_ops_cb_flags(struct sock *sk)
 
 #endif
 
+#if defined(CONFIG_BPF_JIT) && defined(CONFIG_CGROUP_BPF)
+
+struct bpf_tcp_ops {
+	/* Should return the initial SYN (active open) or SYN-ACK (passive open)
+	 * retransmission timeout. Return the timeout in jiffies, or <= 0 for
+	 * the kernel default.
+	 *
+	 * @req: request_sock on the passive (synack) path; NULL otherwise.
+	 */
+	int (*timeout_init)(struct sock *sk, struct request_sock *req);
+
+	/* Should return the initial advertised receive window, in packets,
+	 * or < 0 for the kernel default. @req as in timeout_init().
+	 */
+	int (*rwnd_init)(struct sock *sk, struct request_sock *req);
+
+	/* Called when an active connection becomes established.
+	 * @skb is the SYNACK that completed the 3WHS, or NULL for a
+	 * TCP_REPAIR socket (tcp_finish_connect() with no skb).
+	 */
+	void (*active_established)(struct sock *sk, struct sk_buff *skb__nullable);
+
+	/* Called when a passive connection becomes established.
+	 * @skb is the ACK that completed the 3WHS.
+	 */
+	void (*passive_established)(struct sock *sk, struct sk_buff *skb);
+
+	/* Called when the retransmission timer fires. */
+	void (*rto)(struct sock *sk);
+
+	/* Called on every RTT sample.
+	 * @mrtt: the measured RTT, in microseconds.
+	 * @srtt: the updated smoothed RTT.
+	 */
+	void (*rtt)(struct sock *sk, long mrtt, u32 srtt);
+
+	/* Called when the connection changes TCP state.
+	 * @state: the new state (one of the TCP_* states).
+	 */
+	void (*set_state)(struct sock *sk, int state);
+
+	/* Called when an skb is retransmitted.
+	 * @skb: the retransmitted skb.
+	 * @err: tcp_transmit_skb() return value (0 on success).
+	 */
+	void (*retrans)(struct sock *sk, struct sk_buff *skb, int err);
+
+	/* Called right before an active connection is initialized. */
+	void (*connect)(struct sock *sk);
+
+	/* Called on listen(2), right after the socket enters TCP_LISTEN. */
+	void (*listen)(struct sock *sk);
+};
+
+#define bpf_tcp_ops_call(op, sk, ...)					\
+do {									\
+	if (cgroup_bpf_enabled(CGROUP_TCP_SOCK_OPS)) {			\
+		const struct bpf_prog_array_item *item;			\
+		const struct bpf_tcp_ops *tcp_ops;			\
+		struct cgroup *cgrp;					\
+									\
+		cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);		\
+		rcu_read_lock_dont_migrate();				\
+		bpf_cgroup_struct_ops_foreach(tcp_ops, item, cgrp,	\
+					      CGROUP_TCP_SOCK_OPS) {	\
+			if (tcp_ops->op)				\
+				tcp_ops->op(sk, ##__VA_ARGS__);		\
+		}							\
+		rcu_read_unlock_migrate();				\
+	}								\
+} while (0)
+
+#define bpf_tcp_ops_call_int(op, init_retval, sk, ...)			\
+({									\
+	int __retval = (init_retval);					\
+	if (cgroup_bpf_enabled(CGROUP_TCP_SOCK_OPS)) {			\
+		const struct bpf_prog_array_item *item;			\
+		const struct bpf_tcp_ops *tcp_ops;			\
+		struct bpf_tramp_run_ctx run_ctx;			\
+		struct bpf_run_ctx *old_run_ctx;			\
+		struct sock *__sk = sk_to_full_sk(sk);                  \
+		struct request_sock *req = NULL;			\
+		struct cgroup *cgrp;					\
+									\
+		if (__sk) {						\
+			run_ctx.retval = (init_retval);			\
+			cgrp = sock_cgroup_ptr(&__sk->sk_cgrp_data);	\
+			if (!sk_fullsock(sk))				\
+				req = (struct request_sock *)sk;	\
+			rcu_read_lock_dont_migrate();			\
+			old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);\
+			bpf_cgroup_struct_ops_foreach(tcp_ops, item, cgrp, \
+						      CGROUP_TCP_SOCK_OPS) { \
+				if (tcp_ops->op)			\
+					run_ctx.retval = tcp_ops->op(__sk, req, ##__VA_ARGS__); \
+			}						\
+			bpf_reset_run_ctx(old_run_ctx);			\
+			rcu_read_unlock_migrate();			\
+			__retval = run_ctx.retval;			\
+		}							\
+	}								\
+	__retval;							\
+})
+#else
+#define bpf_tcp_ops_call(op, sk, ...)		do { } while (0)
+#define bpf_tcp_ops_call_int(op, init_retval, sk, ...)	(init_retval)
+#endif
+
 static inline u32 tcp_timeout_init(struct sock *sk)
 {
 	int timeout;
 
 	timeout = tcp_call_bpf(sk, BPF_SOCK_OPS_TIMEOUT_INIT, 0, NULL);
-
+	timeout = bpf_tcp_ops_call_int(timeout_init, timeout, sk);
 	if (timeout <= 0)
 		timeout = TCP_TIMEOUT_INIT;
 	return min_t(int, timeout, TCP_RTO_MAX);
@@ -2975,7 +3083,7 @@ static inline u32 tcp_rwnd_init_bpf(struct sock *sk)
 	int rwnd;
 
 	rwnd = tcp_call_bpf(sk, BPF_SOCK_OPS_RWND_INIT, 0, NULL);
-
+	rwnd = bpf_tcp_ops_call_int(rwnd_init, rwnd, sk);
 	if (rwnd < 0)
 		rwnd = 0;
 	return rwnd;
@@ -2990,6 +3098,7 @@ static inline void tcp_bpf_rtt(struct sock *sk, long mrtt, u32 srtt)
 {
 	if (BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk), BPF_SOCK_OPS_RTT_CB_FLAG))
 		tcp_call_bpf_2arg(sk, BPF_SOCK_OPS_RTT_CB, mrtt, srtt);
+	bpf_tcp_ops_call(rtt, sk, mrtt, srtt);
 }
 
 #if IS_ENABLED(CONFIG_SMC)

net/ipv4/Makefile

@@ -70,6 +70,7 @@ obj-$(CONFIG_TCP_AO) += tcp_ao.o
 
 ifeq ($(CONFIG_BPF_JIT),y)
 obj-$(CONFIG_BPF_SYSCALL) += bpf_tcp_ca.o
+obj-$(CONFIG_CGROUP_BPF) += bpf_tcp_ops.o
 endif
 
 ifdef CONFIG_GCOV_PROFILE_NETFILTER

net/ipv4/af_inet.c

@@ -227,6 +227,7 @@ int __inet_listen_sk(struct sock *sk, int backlog)
 			return err;
 
 		tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_LISTEN_CB, 0, NULL);
+		bpf_tcp_ops_call(listen, sk);
 	}
 	return 0;
 }

net/ipv4/bpf_tcp_ops.c

@@ -0,0 +1,188 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <linux/bpf.h>
+#include <linux/btf_ids.h>
+#include <linux/bpf_verifier.h>
+#include <net/bpf_sk_storage.h>
+#include <net/tcp.h>
+
+static int timeout_init_stub(struct sock *sk, struct request_sock *req__nullable)
+{
+	struct bpf_tramp_run_ctx *ctx =
+		container_of(current->bpf_ctx, struct bpf_tramp_run_ctx, run_ctx);
+
+	return ctx->retval;
+}
+
+static int rwnd_init_stub(struct sock *sk, struct request_sock *req__nullable)
+{
+	struct bpf_tramp_run_ctx *ctx =
+		container_of(current->bpf_ctx, struct bpf_tramp_run_ctx, run_ctx);
+
+	return ctx->retval;
+}
+
+static void active_established_stub(struct sock *sk, struct sk_buff *skb__nullable)
+{
+}
+
+static void passive_established_stub(struct sock *sk, struct sk_buff *skb)
+{
+}
+
+static void rto_stub(struct sock *sk)
+{
+}
+
+static void rtt_stub(struct sock *sk, long mrtt, u32 srtt)
+{
+}
+
+static void set_state_stub(struct sock *sk, int state)
+{
+}
+
+static void retrans_stub(struct sock *sk, struct sk_buff *skb, int err)
+{
+}
+
+static void connect_stub(struct sock *sk)
+{
+}
+
+static void listen_stub(struct sock *sk)
+{
+}
+
+static struct bpf_tcp_ops __bpf_tcp_ops = {
+	.timeout_init = timeout_init_stub,
+	.rwnd_init = rwnd_init_stub,
+	.active_established = active_established_stub,
+	.passive_established = passive_established_stub,
+	.rto = rto_stub,
+	.rtt = rtt_stub,
+	.set_state = set_state_stub,
+	.retrans = retrans_stub,
+	.connect = connect_stub,
+	.listen = listen_stub,
+};
+
+BPF_CALL_0(bpf_tcp_ops_get_retval)
+{
+	struct bpf_tramp_run_ctx *ctx =
+		container_of(current->bpf_ctx, struct bpf_tramp_run_ctx, run_ctx);
+
+	/* bpf_get_retval() is only exposed to timeout_init/rwnd_init, which
+	 * always run via bpf_tcp_ops_call_int(). Its run_ctx carries the int
+	 * return value chained across the bpf_tcp_ops attached to the cgroup
+	 * and is this program's saved_run_ctx.
+	 */
+	if (WARN_ON_ONCE(!ctx->saved_run_ctx))
+		return 0;
+
+	return container_of(ctx->saved_run_ctx, struct bpf_tramp_run_ctx,
+			    run_ctx)->retval;
+}
+
+const struct bpf_func_proto bpf_tcp_ops_get_retval_proto = {
+	.func		= bpf_tcp_ops_get_retval,
+	.gpl_only	= false,
+	.ret_type	= RET_INTEGER,
+};
+
+static const struct bpf_func_proto *
+get_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
+{
+	u32 moff = prog->aux->attach_st_ops_member_off;
+
+	switch (func_id) {
+	case BPF_FUNC_sk_storage_get:
+		return &bpf_sk_storage_get_proto;
+	case BPF_FUNC_sk_storage_delete:
+		return &bpf_sk_storage_delete_proto;
+	case BPF_FUNC_setsockopt:
+		/* The listener is not locked. */
+		if (moff == offsetof(struct bpf_tcp_ops, rwnd_init) ||
+		    moff == offsetof(struct bpf_tcp_ops, timeout_init))
+			return NULL;
+		return &bpf_sk_setsockopt_proto;
+	case BPF_FUNC_getsockopt:
+		if (moff == offsetof(struct bpf_tcp_ops, rwnd_init) ||
+		    moff == offsetof(struct bpf_tcp_ops, timeout_init))
+			return NULL;
+		return &bpf_sk_getsockopt_proto;
+	case BPF_FUNC_get_retval:
+		if (moff == offsetof(struct bpf_tcp_ops, timeout_init) ||
+		    moff == offsetof(struct bpf_tcp_ops, rwnd_init))
+			return &bpf_tcp_ops_get_retval_proto;
+		return NULL;
+	default:
+		return bpf_base_func_proto(func_id, prog);
+	}
+}
+
+static bool is_valid_access(int off, int size, enum bpf_access_type type,
+			    const struct bpf_prog *prog, struct bpf_insn_access_aux *info)
+{
+	if (!bpf_tracing_btf_ctx_access(off, size, type, prog, info))
+		return false;
+
+	if (base_type(info->reg_type) == PTR_TO_BTF_ID &&
+	    !bpf_type_has_unsafe_modifiers(info->reg_type) &&
+	    info->btf_id == btf_sock_ids[BTF_SOCK_TYPE_SOCK])
+		/* promote it to tcp_sock */
+		info->btf_id = btf_sock_ids[BTF_SOCK_TYPE_TCP];
+
+	return true;
+}
+
+static int bpf_tcp_ops_init_member(const struct btf_type *t,
+				   const struct btf_member *member,
+				   void *kdata, const void *udata)
+{
+	return 0;
+}
+
+static int bpf_tcp_ops_check_member(const struct btf_type *t,
+				    const struct btf_member *member,
+				    const struct bpf_prog *prog)
+{
+	if (prog->sleepable)
+		return -EINVAL;
+
+	return 0;
+}
+
+static int bpf_tcp_ops_init(struct btf *btf)
+{
+	return 0;
+}
+
+static int bpf_tcp_ops_validate(void *kdata)
+{
+	return 0;
+}
+
+static const struct bpf_verifier_ops bpf_tcp_ops_verifier = {
+	.get_func_proto		= get_func_proto,
+	.is_valid_access	= is_valid_access,
+};
+
+static struct bpf_struct_ops bpf_tcp_ops = {
+	.verifier_ops = &bpf_tcp_ops_verifier,
+	.init_member = bpf_tcp_ops_init_member,
+	.check_member = bpf_tcp_ops_check_member,
+	.init = bpf_tcp_ops_init,
+	.validate = bpf_tcp_ops_validate,
+	.name = "bpf_tcp_ops",
+	.cgroup_atype = CGROUP_TCP_SOCK_OPS,
+	.cfi_stubs = &__bpf_tcp_ops,
+	.owner = THIS_MODULE,
+};
+
+static int __init __bpf_tcp_ops_init(void)
+{
+	return register_bpf_struct_ops(&bpf_tcp_ops, bpf_tcp_ops);
+}
+late_initcall(__bpf_tcp_ops_init);

net/ipv4/tcp.c

@@ -2997,6 +2997,7 @@ void tcp_set_state(struct sock *sk, int state)
 
 	if (BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk), BPF_SOCK_OPS_STATE_CB_FLAG))
 		tcp_call_bpf_2arg(sk, BPF_SOCK_OPS_STATE_CB, oldstate, state);
+	bpf_tcp_ops_call(set_state, sk, state);
 
 	switch (state) {
 	case TCP_ESTABLISHED:

net/ipv4/tcp_input.c

@@ -6724,6 +6724,10 @@ void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
 	tp->snd_cwnd_stamp = tcp_jiffies32;
 
 	bpf_skops_established(sk, bpf_op, skb);
+	if (bpf_op == BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB)
+		bpf_tcp_ops_call(active_established, sk, skb);
+	else
+		bpf_tcp_ops_call(passive_established, sk, skb);
 	/* Initialize congestion control unless BPF initialized it already: */
 	if (!icsk->icsk_ca_initialized)
 		tcp_init_congestion_control(sk);

net/ipv4/tcp_output.c

@@ -3678,6 +3678,7 @@ int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
 		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
 				  TCP_SKB_CB(skb)->seq, segs, err);
+	bpf_tcp_ops_call(retrans, sk, skb, err);
 
 	if (unlikely(err) && err != -EBUSY)
 		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);
@@ -4302,6 +4303,7 @@ int tcp_connect(struct sock *sk)
 	int err;
 
 	tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB, 0, NULL);
+	bpf_tcp_ops_call(connect, sk);
 
 #if defined(CONFIG_TCP_MD5SIG) && defined(CONFIG_TCP_AO)
 	/* Has to be checked late, after setting daddr/saddr/ops.

net/ipv4/tcp_timer.c

@@ -290,6 +290,7 @@ static int tcp_write_timeout(struct sock *sk)
 		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RTO_CB,
 				  icsk->icsk_retransmits,
 				  icsk->icsk_rto, (int)expired);
+	bpf_tcp_ops_call(rto, sk);
 
 	if (expired) {
 		/* Has it gone just too far? */