Re: [PATCH bpf-next v1 05/10] selftests/bpf: Add tests for KF_IMPLICIT_ARGS

[Andrii Nakryiko]

On Fri, Jan 9, 2026 at 10:49 AM Ihor Solodrai <ihor.solodrai@xxxxxxxxx> wrote:
>
> Add trivial end-to-end tests to validate that KF_IMPLICIT_ARGS flag is
> properly handled by both resolve_btfids and the verifier.
>
> Declare kfuncs in bpf_testmod. Check that bpf_prog_aux pointer is set
> in the kfunc implementation. Verify that calls with implicit args and
> a legacy case all work.
>
> Signed-off-by: Ihor Solodrai <ihor.solodrai@xxxxxxxxx>
> ---
>  .../bpf/prog_tests/kfunc_implicit_args.c      | 10 +++++
>  .../selftests/bpf/progs/kfunc_implicit_args.c | 41 +++++++++++++++++++
>  .../selftests/bpf/test_kmods/bpf_testmod.c    | 26 ++++++++++++
>  3 files changed, 77 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args.c
>  create mode 100644 tools/testing/selftests/bpf/progs/kfunc_implicit_args.c
>

[...]

> @@ -0,0 +1,41 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
> +
> +#include <vmlinux.h>
> +#include <bpf/bpf_helpers.h>
> +#include "bpf_misc.h"
> +
> +extern int bpf_kfunc_implicit_arg(int a) __weak __ksym;
> +extern int bpf_kfunc_implicit_arg_impl(int a, void *aux__prog) __weak __ksym; // illegal

C++ comment

> +extern int bpf_kfunc_implicit_arg_legacy(int a, int b) __weak __ksym;
> +extern int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, void *aux__prog) __weak __ksym;
> +
> +char _license[] SEC("license") = "GPL";
> +

[...]

> diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> index 1c41d03bd5a1..503451875d33 100644
> --- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> +++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> @@ -1136,6 +1136,10 @@ __bpf_kfunc int bpf_kfunc_st_ops_inc10(struct st_ops_args *args)
>  __bpf_kfunc int bpf_kfunc_multi_st_ops_test_1(struct st_ops_args *args, u32 id);
>  __bpf_kfunc int bpf_kfunc_multi_st_ops_test_1_impl(struct st_ops_args *args, void *aux_prog);
>
> +__bpf_kfunc int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux);
> +__bpf_kfunc int bpf_kfunc_implicit_arg_legacy(int a, int b, struct bpf_prog_aux *aux);
> +__bpf_kfunc int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, void *aux__prog);
> +
>  BTF_KFUNCS_START(bpf_testmod_check_kfunc_ids)
>  BTF_ID_FLAGS(func, bpf_testmod_test_mod_kfunc)
>  BTF_ID_FLAGS(func, bpf_kfunc_call_test1)
> @@ -1178,6 +1182,9 @@ BTF_ID_FLAGS(func, bpf_kfunc_st_ops_test_pro_epilogue, KF_SLEEPABLE)
>  BTF_ID_FLAGS(func, bpf_kfunc_st_ops_inc10)
>  BTF_ID_FLAGS(func, bpf_kfunc_multi_st_ops_test_1)
>  BTF_ID_FLAGS(func, bpf_kfunc_multi_st_ops_test_1_impl)
> +BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy, KF_IMPLICIT_ARGS)
> +BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy_impl)

(irrelevant, now that I saw patch #8 discussion, but for the future
the point will stand and we can decide how resolve_btfids handles this
upfront)

I'm wondering, should we add KF_IMPLICIT_ARGS to legacy xxx_impl
kfuncs as well to explicitly mark them to resolve_btfids as legacy
implementations? And if we somehow find xxx_impl without it, then
resolve_btfids complains louds and fails, this should never happen?



>  BTF_KFUNCS_END(bpf_testmod_check_kfunc_ids)
>
>  static int bpf_testmod_ops_init(struct btf *btf)
> @@ -1669,6 +1676,25 @@ int bpf_kfunc_multi_st_ops_test_1_impl(struct st_ops_args *args, void *aux__prog
>         return ret;
>  }
>
> +int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux)
> +{
> +       if (aux && a > 0)
> +               return a;
> +       return -EINVAL;
> +}
> +
> +int bpf_kfunc_implicit_arg_legacy(int a, int b, struct bpf_prog_aux *aux)
> +{
> +       if (aux)
> +               return a + b;
> +       return -EINVAL;
> +}
> +
> +int bpf_kfunc_implicit_arg_legacy_impl(int a, int b, void *aux__prog)
> +{
> +       return bpf_kfunc_implicit_arg_legacy(a, b, aux__prog);
> +}
> +
>  static int multi_st_ops_reg(void *kdata, struct bpf_link *link)
>  {
>         struct bpf_testmod_multi_st_ops *st_ops =
> --
> 2.52.0
>