[PATCH bpf-next v2 03/10] bpf: add kfunc bpf_fsession_cookie for TRACING SESSION

Menglong Dong
2025-10-22T08:02:26Z

Add the kfunc bpf_fsession_cookie(), which is similar to
bpf_session_cookie() and return the address of the session cookie. The
address of the session cookie is stored after session flags, which means
ctx[nr_args + 2].

Inline this kfunc in the verifier too.

Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
---
include/linux/bpf.h | 1 +
kernel/bpf/verifier.c | 20 ++++++++++++++++++--
kernel/trace/bpf_trace.c | 10 ++++++++++
3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 3f6cad5df2db..83d5d4d3120d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1735,6 +1735,7 @@ struct bpf_prog {
enforce_expected_attach_type:1, /* Enforce expected_attach_type checking at attach time */
call_get_stack:1, /* Do we call bpf_get_stack() or bpf_get_stackid() */
call_get_func_ip:1, /* Do we call get_func_ip() */
+ call_session_cookie:1, /* Do we call bpf_fsession_cookie() */
tstamp_type_access:1, /* Accessed __sk_buff->tstamp_type */
sleepable:1; /* BPF program is sleepable */
enum bpf_prog_type type; /* Type of BPF program */
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a4d0dd4440fd..ab46e5fbc7a6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12294,6 +12294,7 @@ enum special_kfunc_type {
KF_bpf_task_work_schedule_signal,
KF_bpf_task_work_schedule_resume,
KF_bpf_tracing_is_exit,
+ KF_bpf_fsession_cookie,
};

BTF_ID_LIST(special_kfunc_list)
@@ -12367,6 +12368,7 @@ BTF_ID(func, __bpf_trap)
BTF_ID(func, bpf_task_work_schedule_signal)
BTF_ID(func, bpf_task_work_schedule_resume)
BTF_ID(func, bpf_tracing_is_exit)
+BTF_ID(func, bpf_fsession_cookie)

static bool is_task_work_add_kfunc(u32 func_id)
{
@@ -12422,7 +12424,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
bool arg_mem_size = false;

if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||
- meta->func_id == special_kfunc_list[KF_bpf_tracing_is_exit])
+ meta->func_id == special_kfunc_list[KF_bpf_tracing_is_exit] ||
+ meta->func_id == special_kfunc_list[KF_bpf_fsession_cookie])
return KF_ARG_PTR_TO_CTX;

/* In this function, we verify the kfunc's BTF as per the argument type,
@@ -13915,7 +13918,8 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
}
}

- if (meta.func_id == special_kfunc_list[KF_bpf_session_cookie]) {
+ if (meta.func_id == special_kfunc_list[KF_bpf_session_cookie] ||
+ meta.func_id == special_kfunc_list[KF_bpf_fsession_cookie]) {
meta.r0_size = sizeof(u64);
meta.r0_rdonly = false;
}
@@ -14196,6 +14200,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
return err;
}

+ if (meta.func_id == special_kfunc_list[KF_bpf_fsession_cookie])
+ env->prog->call_session_cookie = true;
+
return 0;
}

@@ -22007,6 +22014,15 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
insn_buf[5] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
*cnt = 6;
+ } else if (desc->func_id == special_kfunc_list[KF_bpf_fsession_cookie]) {
+ /* Load nr_args from ctx - 8 */
+ insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
+ /* add rax, 2 */
+ insn_buf[1] = BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 2);
+ insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
+ insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1);
+ insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
+ *cnt = 5;
}

if (env->insn_aux_data[insn_idx].arg_prog) {
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d0720d850621..4a8568bd654d 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -3370,10 +3370,20 @@ __bpf_kfunc bool bpf_tracing_is_exit(void *ctx)
return ((u64 *)ctx)[nr_args + 1] & 1;
}

+__bpf_kfunc u64 *bpf_fsession_cookie(void *ctx)
+{
+ /* This helper call is inlined by verifier. */
+ u64 nr_args = ((u64 *)ctx)[-1];
+
+ /* ctx[nr_args + 2] is the session cookie address */
+ return (u64 *)((u64 *)ctx)[nr_args + 2];
+}
+
__bpf_kfunc_end_defs();

BTF_KFUNCS_START(tracing_kfunc_set_ids)
BTF_ID_FLAGS(func, bpf_tracing_is_exit, KF_FASTCALL)
+BTF_ID_FLAGS(func, bpf_fsession_cookie, KF_FASTCALL)
BTF_KFUNCS_END(tracing_kfunc_set_ids)

static int bpf_tracing_filter(const struct bpf_prog *prog, u32 kfunc_id)
--
2.51.1.dirty