[PATCH bpf-next v5 03/10] bpf: add the kfunc bpf_fsession_is_return

From: Menglong Dong <hidden>
Date: 2025-12-24 13:08:32
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 kfunc bpf_fsession_is_return(), which is used to tell if it
is fexit currently. Meanwhile, inline it in the verifier.

Signed-off-by: Menglong Dong <redacted>
Co-developed-by: Leon Hwang <redacted>
Signed-off-by: Leon Hwang <redacted>
---
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      |  3 +++
 kernel/bpf/verifier.c    | 11 +++++++++-
 kernel/trace/bpf_trace.c | 43 +++++++++++++++++++++++++++++++++++++---
 3 files changed, 53 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 63e1bc29485e..dc6b4109f0bf 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -1213,6 +1213,9 @@ 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;

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a99e286a504d..8a5787e6ab0b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -12380,6 +12380,7 @@ enum special_kfunc_type {
 	KF___bpf_trap,
 	KF_bpf_task_work_schedule_signal_impl,
 	KF_bpf_task_work_schedule_resume_impl,
+	KF_bpf_fsession_is_return,
 };
 
 BTF_ID_LIST(special_kfunc_list)

@@ -12454,6 +12455,7 @@ BTF_ID(func, bpf_dynptr_file_discard)
 BTF_ID(func, __bpf_trap)
 BTF_ID(func, bpf_task_work_schedule_signal_impl)
 BTF_ID(func, bpf_task_work_schedule_resume_impl)
+BTF_ID(func, bpf_fsession_is_return)
 
 static bool is_task_work_add_kfunc(u32 func_id)
 {

@@ -12508,7 +12510,8 @@ 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_fsession_is_return])
 		return KF_ARG_PTR_TO_CTX;
 
 	/* In this function, we verify the kfunc's BTF as per the argument type,

@@ -22556,6 +22559,12 @@ 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_fsession_is_return]) {
+		/* 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 10c9992d2745..8a94a507bd51 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c

@@ -3356,12 +3356,49 @@ static const struct btf_kfunc_id_set bpf_kprobe_multi_kfunc_set = {
 	.filter = bpf_kprobe_multi_filter,
 };
 
-static int __init bpf_kprobe_multi_kfuncs_init(void)
+__bpf_kfunc_start_defs();
+
+__bpf_kfunc 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_end_defs();
+
+BTF_KFUNCS_START(tracing_kfunc_set_ids)
+BTF_ID_FLAGS(func, bpf_fsession_is_return, KF_FASTCALL)
+BTF_KFUNCS_END(tracing_kfunc_set_ids)
+
+static int bpf_tracing_filter(const struct bpf_prog *prog, u32 kfunc_id)
 {
-	return register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_kprobe_multi_kfunc_set);
+	if (!btf_id_set8_contains(&tracing_kfunc_set_ids, kfunc_id))
+		return 0;
+
+	if (prog->type != BPF_PROG_TYPE_TRACING ||
+	    prog->expected_attach_type != BPF_TRACE_FSESSION)
+		return -EINVAL;
+
+	return 0;
+}
+
+static const struct btf_kfunc_id_set bpf_tracing_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set = &tracing_kfunc_set_ids,
+	.filter = bpf_tracing_filter,
+};
+
+static int __init bpf_trace_kfuncs_init(void)
+{
+	int err = 0;
+
+	err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_kprobe_multi_kfunc_set);
+	err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_tracing_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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