[PATCH bpf-next v9 01/11] bpf: add fsession support

From: Menglong Dong <hidden>
Date: 2026-01-10 14:11:42
Also in: bpf, lkml
Subsystem: bpf [core], bpf [general] (safe dynamic programs and tools), bpf [selftests] (test runners & infrastructure), kernel selftest framework, networking [general], the rest · Maintainers: Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, Shuah Khan, "David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds

The fsession is something that similar to kprobe session. It allow to
attach a single BPF program to both the entry and the exit of the target
functions.

Introduce the struct bpf_fsession_link, which allows to add the link to
both the fentry and fexit progs_hlist of the trampoline.

Signed-off-by: Menglong Dong <redacted>
Co-developed-by: Leon Hwang <redacted>
Signed-off-by: Leon Hwang <redacted>
---
v5:
- unify the name to "fsession"
- use more explicit way in __bpf_trampoline_link_prog()

v4:
- instead of adding a new hlist to progs_hlist in trampoline, add the bpf
  program to both the fentry hlist and the fexit hlist.
---
 include/linux/bpf.h                           | 19 +++++++++
 include/uapi/linux/bpf.h                      |  1 +
 kernel/bpf/btf.c                              |  2 +
 kernel/bpf/syscall.c                          | 18 ++++++++-
 kernel/bpf/trampoline.c                       | 40 ++++++++++++++++---
 kernel/bpf/verifier.c                         | 12 ++++--
 net/bpf/test_run.c                            |  1 +
 net/core/bpf_sk_storage.c                     |  1 +
 tools/include/uapi/linux/bpf.h                |  1 +
 .../bpf/prog_tests/tracing_failure.c          |  2 +-
 10 files changed, 87 insertions(+), 10 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 5936f8e2996f..41228b0add52 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1309,6 +1309,7 @@ enum bpf_tramp_prog_type {
 	BPF_TRAMP_MODIFY_RETURN,
 	BPF_TRAMP_MAX,
 	BPF_TRAMP_REPLACE, /* more than MAX */
+	BPF_TRAMP_FSESSION,
 };
 
 struct bpf_tramp_image {

@@ -1875,6 +1876,11 @@ struct bpf_tracing_link {
 	struct bpf_prog *tgt_prog;
 };
 
+struct bpf_fsession_link {
+	struct bpf_tracing_link link;
+	struct bpf_tramp_link fexit;
+};
+
 struct bpf_raw_tp_link {
 	struct bpf_link link;
 	struct bpf_raw_event_map *btp;

@@ -2169,6 +2175,19 @@ static inline void bpf_struct_ops_desc_release(struct bpf_struct_ops_desc *st_op
 
 #endif
 
+static inline int bpf_fsession_cnt(struct bpf_tramp_links *links)
+{
+	struct bpf_tramp_links fentries = links[BPF_TRAMP_FENTRY];
+	int cnt = 0;
+
+	for (int i = 0; i < links[BPF_TRAMP_FENTRY].nr_links; i++) {
+		if (fentries.links[i]->link.prog->expected_attach_type == BPF_TRACE_FSESSION)
+			cnt++;
+	}
+
+	return cnt;
+}
+
 int bpf_prog_ctx_arg_info_init(struct bpf_prog *prog,
 			       const struct bpf_ctx_arg_aux *info, u32 cnt);

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 2a2ade4be60f..44e7dbc278e3 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -1145,6 +1145,7 @@ enum bpf_attach_type {
 	BPF_NETKIT_PEER,
 	BPF_TRACE_KPROBE_SESSION,
 	BPF_TRACE_UPROBE_SESSION,
+	BPF_TRACE_FSESSION,
 	__MAX_BPF_ATTACH_TYPE
 };

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 539c9fdea41d..8b1dcd440356 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -6107,6 +6107,7 @@ static int btf_validate_prog_ctx_type(struct bpf_verifier_log *log, const struct
 		case BPF_TRACE_FENTRY:
 		case BPF_TRACE_FEXIT:
 		case BPF_MODIFY_RETURN:
+		case BPF_TRACE_FSESSION:
 			/* allow u64* as ctx */
 			if (btf_is_int(t) && t->size == 8)
 				return 0;

@@ -6704,6 +6705,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 			fallthrough;
 		case BPF_LSM_CGROUP:
 		case BPF_TRACE_FEXIT:
+		case BPF_TRACE_FSESSION:
 			/* When LSM programs are attached to void LSM hooks
 			 * they use FEXIT trampolines and when attached to
 			 * int LSM hooks, they use MODIFY_RETURN trampolines.

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ecc0929ce462..c65e7a70cb78 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -3577,6 +3577,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 	case BPF_PROG_TYPE_TRACING:
 		if (prog->expected_attach_type != BPF_TRACE_FENTRY &&
 		    prog->expected_attach_type != BPF_TRACE_FEXIT &&
+		    prog->expected_attach_type != BPF_TRACE_FSESSION &&
 		    prog->expected_attach_type != BPF_MODIFY_RETURN) {
 			err = -EINVAL;
 			goto out_put_prog;

@@ -3626,7 +3627,21 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
 		key = bpf_trampoline_compute_key(tgt_prog, NULL, btf_id);
 	}
 
-	link = kzalloc(sizeof(*link), GFP_USER);
+	if (prog->expected_attach_type == BPF_TRACE_FSESSION) {
+		struct bpf_fsession_link *fslink;
+
+		fslink = kzalloc(sizeof(*fslink), GFP_USER);
+		if (fslink) {
+			bpf_link_init(&fslink->fexit.link, BPF_LINK_TYPE_TRACING,
+				      &bpf_tracing_link_lops, prog, attach_type);
+			fslink->fexit.cookie = bpf_cookie;
+			link = &fslink->link;
+		} else {
+			link = NULL;
+		}
+	} else {
+		link = kzalloc(sizeof(*link), GFP_USER);
+	}
 	if (!link) {
 		err = -ENOMEM;
 		goto out_put_prog;

@@ -4350,6 +4365,7 @@ attach_type_to_prog_type(enum bpf_attach_type attach_type)
 	case BPF_TRACE_RAW_TP:
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
+	case BPF_TRACE_FSESSION:
 	case BPF_MODIFY_RETURN:
 		return BPF_PROG_TYPE_TRACING;
 	case BPF_LSM_MAC:

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 2a125d063e62..11e043049d68 100644
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c

@@ -111,7 +111,7 @@ bool bpf_prog_has_trampoline(const struct bpf_prog *prog)
 
 	return (ptype == BPF_PROG_TYPE_TRACING &&
 		(eatype == BPF_TRACE_FENTRY || eatype == BPF_TRACE_FEXIT ||
-		 eatype == BPF_MODIFY_RETURN)) ||
+		 eatype == BPF_MODIFY_RETURN || eatype == BPF_TRACE_FSESSION)) ||
 		(ptype == BPF_PROG_TYPE_LSM && eatype == BPF_LSM_MAC);
 }

@@ -559,6 +559,8 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog)
 		return BPF_TRAMP_MODIFY_RETURN;
 	case BPF_TRACE_FEXIT:
 		return BPF_TRAMP_FEXIT;
+	case BPF_TRACE_FSESSION:
+		return BPF_TRAMP_FSESSION;
 	case BPF_LSM_MAC:
 		if (!prog->aux->attach_func_proto->type)
 			/* The function returns void, we cannot modify its

@@ -596,6 +598,8 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 {
 	enum bpf_tramp_prog_type kind;
 	struct bpf_tramp_link *link_exiting;
+	struct bpf_fsession_link *fslink;
+	struct hlist_head *prog_list;
 	int err = 0;
 	int cnt = 0, i;

@@ -621,24 +625,44 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
 					  BPF_MOD_JUMP, NULL,
 					  link->link.prog->bpf_func);
 	}
+	if (kind == BPF_TRAMP_FSESSION) {
+		prog_list = &tr->progs_hlist[BPF_TRAMP_FENTRY];
+		cnt++;
+	} else {
+		prog_list = &tr->progs_hlist[kind];
+	}
 	if (cnt >= BPF_MAX_TRAMP_LINKS)
 		return -E2BIG;
 	if (!hlist_unhashed(&link->tramp_hlist))
 		/* prog already linked */
 		return -EBUSY;
-	hlist_for_each_entry(link_exiting, &tr->progs_hlist[kind], tramp_hlist) {
+	hlist_for_each_entry(link_exiting, prog_list, tramp_hlist) {
 		if (link_exiting->link.prog != link->link.prog)
 			continue;
 		/* prog already linked */
 		return -EBUSY;
 	}
 
-	hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
-	tr->progs_cnt[kind]++;
+	hlist_add_head(&link->tramp_hlist, prog_list);
+	if (kind == BPF_TRAMP_FSESSION) {
+		tr->progs_cnt[BPF_TRAMP_FENTRY]++;
+		fslink = container_of(link, struct bpf_fsession_link, link.link);
+		hlist_add_head(&fslink->fexit.tramp_hlist,
+			       &tr->progs_hlist[BPF_TRAMP_FEXIT]);
+		tr->progs_cnt[BPF_TRAMP_FEXIT]++;
+	} else {
+		tr->progs_cnt[kind]++;
+	}
 	err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
 	if (err) {
 		hlist_del_init(&link->tramp_hlist);
-		tr->progs_cnt[kind]--;
+		if (kind == BPF_TRAMP_FSESSION) {
+			tr->progs_cnt[BPF_TRAMP_FENTRY]--;
+			hlist_del_init(&fslink->fexit.tramp_hlist);
+			tr->progs_cnt[BPF_TRAMP_FEXIT]--;
+		} else {
+			tr->progs_cnt[kind]--;
+		}
 	}
 	return err;
 }

@@ -659,6 +683,7 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
 					struct bpf_trampoline *tr,
 					struct bpf_prog *tgt_prog)
 {
+	struct bpf_fsession_link *fslink;
 	enum bpf_tramp_prog_type kind;
 	int err;

@@ -672,6 +697,11 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
 		guard(mutex)(&tgt_prog->aux->ext_mutex);
 		tgt_prog->aux->is_extended = false;
 		return err;
+	} else if (kind == BPF_TRAMP_FSESSION) {
+		fslink = container_of(link, struct bpf_fsession_link, link.link);
+		hlist_del_init(&fslink->fexit.tramp_hlist);
+		tr->progs_cnt[BPF_TRAMP_FEXIT]--;
+		kind = BPF_TRAMP_FENTRY;
 	}
 	hlist_del_init(&link->tramp_hlist);
 	tr->progs_cnt[kind]--;

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 53635ea2e41b..774c9b0aafa3 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -17403,6 +17403,7 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
 		switch (env->prog->expected_attach_type) {
 		case BPF_TRACE_FENTRY:
 		case BPF_TRACE_FEXIT:
+		case BPF_TRACE_FSESSION:
 			range = retval_range(0, 0);
 			break;
 		case BPF_TRACE_RAW_TP:

@@ -23300,6 +23301,7 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 		if (prog_type == BPF_PROG_TYPE_TRACING &&
 		    insn->imm == BPF_FUNC_get_func_ret) {
 			if (eatype == BPF_TRACE_FEXIT ||
+			    eatype == BPF_TRACE_FSESSION ||
 			    eatype == BPF_MODIFY_RETURN) {
 				/* Load nr_args from ctx - 8 */
 				insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);

@@ -24244,7 +24246,8 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 		if (tgt_prog->type == BPF_PROG_TYPE_TRACING &&
 		    prog_extension &&
 		    (tgt_prog->expected_attach_type == BPF_TRACE_FENTRY ||
-		     tgt_prog->expected_attach_type == BPF_TRACE_FEXIT)) {
+		     tgt_prog->expected_attach_type == BPF_TRACE_FEXIT ||
+		     tgt_prog->expected_attach_type == BPF_TRACE_FSESSION)) {
 			/* Program extensions can extend all program types
 			 * except fentry/fexit. The reason is the following.
 			 * The fentry/fexit programs are used for performance

@@ -24259,7 +24262,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 			 * beyond reasonable stack size. Hence extending fentry
 			 * is not allowed.
 			 */
-			bpf_log(log, "Cannot extend fentry/fexit\n");
+			bpf_log(log, "Cannot extend fentry/fexit/fsession\n");
 			return -EINVAL;
 		}
 	} else {

@@ -24343,6 +24346,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 	case BPF_LSM_CGROUP:
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
+	case BPF_TRACE_FSESSION:
 		if (!btf_type_is_func(t)) {
 			bpf_log(log, "attach_btf_id %u is not a function\n",
 				btf_id);

@@ -24509,6 +24513,7 @@ static bool can_be_sleepable(struct bpf_prog *prog)
 		case BPF_TRACE_FEXIT:
 		case BPF_MODIFY_RETURN:
 		case BPF_TRACE_ITER:
+		case BPF_TRACE_FSESSION:
 			return true;
 		default:
 			return false;

@@ -24590,9 +24595,10 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
 			tgt_info.tgt_name);
 		return -EINVAL;
 	} else if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
+		   prog->expected_attach_type == BPF_TRACE_FSESSION ||
 		   prog->expected_attach_type == BPF_MODIFY_RETURN) &&
 		   btf_id_set_contains(&noreturn_deny, btf_id)) {
-		verbose(env, "Attaching fexit/fmod_ret to __noreturn function '%s' is rejected.\n",
+		verbose(env, "Attaching fexit/fsession/fmod_ret to __noreturn function '%s' is rejected.\n",
 			tgt_info.tgt_name);
 		return -EINVAL;
 	}

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 655efac6f133..3b0d9bd039de 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c

@@ -685,6 +685,7 @@ int bpf_prog_test_run_tracing(struct bpf_prog *prog,
 	switch (prog->expected_attach_type) {
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
+	case BPF_TRACE_FSESSION:
 		if (bpf_fentry_test1(1) != 2 ||
 		    bpf_fentry_test2(2, 3) != 5 ||
 		    bpf_fentry_test3(4, 5, 6) != 15 ||

diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
index 850dd736ccd1..de111818f3a0 100644
--- a/net/core/bpf_sk_storage.c
+++ b/net/core/bpf_sk_storage.c

@@ -365,6 +365,7 @@ static bool bpf_sk_storage_tracing_allowed(const struct bpf_prog *prog)
 		return true;
 	case BPF_TRACE_FENTRY:
 	case BPF_TRACE_FEXIT:
+	case BPF_TRACE_FSESSION:
 		return !!strncmp(prog->aux->attach_func_name, "bpf_sk_storage",
 				 strlen("bpf_sk_storage"));
 	default:

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index b816bc53d2e1..3ca7d76e05f0 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h

@@ -1145,6 +1145,7 @@ enum bpf_attach_type {
 	BPF_NETKIT_PEER,
 	BPF_TRACE_KPROBE_SESSION,
 	BPF_TRACE_UPROBE_SESSION,
+	BPF_TRACE_FSESSION,
 	__MAX_BPF_ATTACH_TYPE
 };

diff --git a/tools/testing/selftests/bpf/prog_tests/tracing_failure.c b/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
index 10e231965589..f9f9e1cb87bf 100644
--- a/tools/testing/selftests/bpf/prog_tests/tracing_failure.c
+++ b/tools/testing/selftests/bpf/prog_tests/tracing_failure.c

@@ -73,7 +73,7 @@ static void test_tracing_deny(void)
 static void test_fexit_noreturns(void)
 {
 	test_tracing_fail_prog("fexit_noreturns",
-			       "Attaching fexit/fmod_ret to __noreturn function 'do_exit' is rejected.");
+			       "Attaching fexit/fsession/fmod_ret to __noreturn function 'do_exit' is rejected.");
 }
 
 void test_tracing_failure(void)

-- 
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