[PATCH bpf-next v8 04/11] bpf: support fsession for bpf_session_is_return

From: Menglong Dong <hidden>
Date: 2026-01-08 02:25:49
Also in: bpf, lkml
Subsystem: bpf [core], bpf [general] (safe dynamic programs and tools), bpf [security & lsm] (security audit and enforcement using bpf), bpf [tracing], the rest, tracing · Maintainers: Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, KP Singh, Matt Bobrowski, Song Liu, Linus Torvalds, Steven Rostedt, Masami Hiramatsu

If fsession exists, we will use the bit (1 << BPF_TRAMP_M_IS_RETURN) in
ctx[-1] to store the "is_return" flag.

Introduce the function bpf_fsession_is_return(), which is used to tell if
it is fexit currently. Meanwhile, inline it in the verifier. The calling
to bpf_session_is_return() will be changed to bpf_fsession_is_return() in
verifier for fsession.

Signed-off-by: Menglong Dong <redacted>
Co-developed-by: Leon Hwang <redacted>
Signed-off-by: Leon Hwang <redacted>
---
v7:
- reuse the kfunc bpf_session_is_return() instead of introduce new kfunc

v4:
- split out the bpf_fsession_cookie() to another patch

v3:
- merge the bpf_tracing_is_exit and bpf_fsession_cookie into a single
  patch

v2:
- store the session flags after return value, instead of before nr_args
- inline the bpf_tracing_is_exit, as Jiri suggested
---
 include/linux/bpf.h      |  5 +++++
 kernel/bpf/verifier.c    | 15 ++++++++++++++-
 kernel/trace/bpf_trace.c | 34 +++++++++++++++++++++++-----------
 3 files changed, 42 insertions(+), 12 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 41228b0add52..95248b0d28ab 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1229,6 +1229,9 @@ enum {
 #endif
 };
 
+#define BPF_TRAMP_M_NR_ARGS	0
+#define BPF_TRAMP_M_IS_RETURN	8
+
 struct bpf_tramp_links {
 	struct bpf_tramp_link *links[BPF_MAX_TRAMP_LINKS];
 	int nr_links;

@@ -3974,4 +3977,6 @@ static inline int bpf_map_check_op_flags(struct bpf_map *map, u64 flags, u64 all
 	return 0;
 }
 
+bool bpf_fsession_is_return(void *ctx);
+
 #endif /* _LINUX_BPF_H */

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index bfff3f84fd91..d3709edd0e51 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -12374,6 +12374,7 @@ enum special_kfunc_type {
 	KF_bpf_arena_alloc_pages,
 	KF_bpf_arena_free_pages,
 	KF_bpf_arena_reserve_pages,
+	KF_bpf_session_is_return,
 };
 
 BTF_ID_LIST(special_kfunc_list)

@@ -12451,6 +12452,7 @@ BTF_ID(func, bpf_task_work_schedule_resume_impl)
 BTF_ID(func, bpf_arena_alloc_pages)
 BTF_ID(func, bpf_arena_free_pages)
 BTF_ID(func, bpf_arena_reserve_pages)
+BTF_ID(func, bpf_session_is_return)
 
 static bool is_task_work_add_kfunc(u32 func_id)
 {

@@ -12505,7 +12507,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
 	struct bpf_reg_state *reg = &regs[regno];
 	bool arg_mem_size = false;
 
-	if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx])
+	if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||
+	    meta->func_id == special_kfunc_list[KF_bpf_session_is_return])
 		return KF_ARG_PTR_TO_CTX;
 
 	if (argno + 1 < nargs &&

@@ -22440,6 +22443,9 @@ static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc
 	} else if (func_id == special_kfunc_list[KF_bpf_arena_free_pages]) {
 		if (env->insn_aux_data[insn_idx].non_sleepable)
 			addr = (unsigned long)bpf_arena_free_pages_non_sleepable;
+	} else if (func_id == special_kfunc_list[KF_bpf_session_is_return]) {
+		if (prog->expected_attach_type == BPF_TRACE_FSESSION)
+			addr = (unsigned long)bpf_fsession_is_return;
 	}
 	desc->addr = addr;
 	return 0;

@@ -22558,6 +22564,13 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		   desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
 		insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
 		*cnt = 1;
+	} else if (desc->func_id == special_kfunc_list[KF_bpf_session_is_return] &&
+		   env->prog->expected_attach_type == BPF_TRACE_FSESSION) {
+		/* Load nr_args from ctx - 8 */
+		insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
+		insn_buf[1] = BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, BPF_TRAMP_M_IS_RETURN);
+		insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
+		*cnt = 3;
 	}
 
 	if (env->insn_aux_data[insn_idx].arg_prog) {

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 297dcafb2c55..056f30844de2 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -3314,6 +3314,12 @@ static u64 bpf_uprobe_multi_entry_ip(struct bpf_run_ctx *ctx)
 }
 #endif /* CONFIG_UPROBES */
 
+bool bpf_fsession_is_return(void *ctx)
+{
+	/* This helper call is inlined by verifier. */
+	return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));
+}
+
 __bpf_kfunc_start_defs();
 
 __bpf_kfunc bool bpf_session_is_return(void *ctx)

@@ -3334,34 +3340,40 @@ __bpf_kfunc __u64 *bpf_session_cookie(void *ctx)
 
 __bpf_kfunc_end_defs();
 
-BTF_KFUNCS_START(kprobe_multi_kfunc_set_ids)
+BTF_KFUNCS_START(session_kfunc_set_ids)
 BTF_ID_FLAGS(func, bpf_session_is_return)
 BTF_ID_FLAGS(func, bpf_session_cookie)
-BTF_KFUNCS_END(kprobe_multi_kfunc_set_ids)
+BTF_KFUNCS_END(session_kfunc_set_ids)
 
-static int bpf_kprobe_multi_filter(const struct bpf_prog *prog, u32 kfunc_id)
+static int bpf_session_filter(const struct bpf_prog *prog, u32 kfunc_id)
 {
-	if (!btf_id_set8_contains(&kprobe_multi_kfunc_set_ids, kfunc_id))
+	if (!btf_id_set8_contains(&session_kfunc_set_ids, kfunc_id))
 		return 0;
 
-	if (!is_kprobe_session(prog) && !is_uprobe_session(prog))
+	if (!is_kprobe_session(prog) && !is_uprobe_session(prog) &&
+	    prog->expected_attach_type != BPF_TRACE_FSESSION)
 		return -EACCES;
 
 	return 0;
 }
 
-static const struct btf_kfunc_id_set bpf_kprobe_multi_kfunc_set = {
+static const struct btf_kfunc_id_set bpf_session_kfunc_set = {
 	.owner = THIS_MODULE,
-	.set = &kprobe_multi_kfunc_set_ids,
-	.filter = bpf_kprobe_multi_filter,
+	.set = &session_kfunc_set_ids,
+	.filter = bpf_session_filter,
 };
 
-static int __init bpf_kprobe_multi_kfuncs_init(void)
+static int __init bpf_trace_kfuncs_init(void)
 {
-	return register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_kprobe_multi_kfunc_set);
+	int err = 0;
+
+	err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_session_kfunc_set);
+	err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_session_kfunc_set);
+
+	return err;
 }
 
-late_initcall(bpf_kprobe_multi_kfuncs_init);
+late_initcall(bpf_trace_kfuncs_init);
 
 typedef int (*copy_fn_t)(void *dst, const void *src, u32 size, struct task_struct *tsk);

-- 
2.52.0

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