[PATCH bpf-next v1 1/6] bpf: implement KF_IMPLICIT_PROG_AUX_ARG flag

Ihor Solodrai
2025-09-24T21:17:40Z

Define KF_IMPLICIT_PROG_AUX_ARG and handle it in the BPF verifier.

The mechanism of patching is exactly the same as for __prog parameter
annotation: in check_kfunc_args() detect the relevant parameter and
remember regno in cur_aux(env)->arg_prog.

Then the (unchanged in this patch) fixup_kfunc_call() adds a mov
instruction to set the actual pointer to prog_aux.

The caveat for KF_IMPLICIT_PROG_AUX_ARG is in implicitness. We have to
separately check that the number of arguments is under
MAX_BPF_FUNC_REG_ARGS.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
include/linux/btf.h | 3 +++
kernel/bpf/verifier.c | 43 ++++++++++++++++++++++++++++++++++++-------
2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index f06976ffb63f..479ee96c2c97 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -79,6 +79,9 @@
#define KF_ARENA_RET (1 << 13) /* kfunc returns an arena pointer */
#define KF_ARENA_ARG1 (1 << 14) /* kfunc takes an arena pointer as its first argument */
#define KF_ARENA_ARG2 (1 << 15) /* kfunc takes an arena pointer as its second argument */
+/* kfunc takes a pointer to struct bpf_prog_aux as the last argument,
+ * passed implicitly in BPF */
+#define KF_IMPLICIT_PROG_AUX_ARG (1 << 16)

/*
* Tag marking a kernel function as a kfunc. This is meant to minimize the
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e892df386eed..f1f9ea21f99b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11948,6 +11948,11 @@ static bool is_kfunc_rcu_protected(struct bpf_kfunc_call_arg_meta *meta)
return meta->kfunc_flags & KF_RCU_PROTECTED;
}

+static bool is_kfunc_with_implicit_prog_aux_arg(struct bpf_kfunc_call_arg_meta *meta)
+{
+ return meta->kfunc_flags & KF_IMPLICIT_PROG_AUX_ARG;
+}
+
static bool is_kfunc_arg_mem_size(const struct btf *btf,
const struct btf_param *arg,
const struct bpf_reg_state *reg)
@@ -12029,6 +12034,18 @@ static bool is_kfunc_arg_prog(const struct btf *btf, const struct btf_param *arg
return btf_param_match_suffix(btf, arg, "__prog");
}

+static int set_kfunc_arg_prog_regno(struct bpf_verifier_env *env, struct bpf_kfunc_call_arg_meta *meta, u32 regno)
+{
+ if (meta->arg_prog) {
+ verifier_bug(env, "Only 1 prog->aux argument supported per-kfunc");
+ return -EFAULT;
+ }
+ meta->arg_prog = true;
+ cur_aux(env)->arg_prog = regno;
+
+ return 0;
+}
+
static bool is_kfunc_arg_scalar_with_name(const struct btf *btf,
const struct btf_param *arg,
const char *name)
@@ -13050,6 +13067,21 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return -EINVAL;
}

+ /* KF_IMPLICIT_PROG_AUX_ARG means that the kfunc has one less argument in BTF,
+ * so we have to set_kfunc_arg_prog_regno() outside the arg check loop.
+ */
+ if (is_kfunc_with_implicit_prog_aux_arg(meta)) {
+ if (nargs + 1 > MAX_BPF_FUNC_REG_ARGS) {
+ verifier_bug(env, "A kfunc with KF_IMPLICIT_PROG_AUX_ARG flag has %d > %d args",
+ nargs + 1, MAX_BPF_FUNC_REG_ARGS);
+ return -EFAULT;
+ }
+ u32 regno = nargs + 1;
+ ret = set_kfunc_arg_prog_regno(env, meta, regno);
+ if (ret)
+ return ret;
+ }
+
/* Check that BTF function arguments match actual types that the
* verifier sees.
*/
@@ -13066,14 +13098,11 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
if (is_kfunc_arg_ignore(btf, &args[i]))
continue;

+ /* __prog annotation check for backward compatibility */
if (is_kfunc_arg_prog(btf, &args[i])) {
- /* Used to reject repeated use of __prog. */
- if (meta->arg_prog) {
- verifier_bug(env, "Only 1 prog->aux argument supported per-kfunc");
- return -EFAULT;
- }
- meta->arg_prog = true;
- cur_aux(env)->arg_prog = regno;
+ ret = set_kfunc_arg_prog_regno(env, meta, regno);
+ if (ret)
+ return ret;
continue;
}

--
2.51.0