Re: [PATCH bpf v5 2/2] selftests/bpf: strengthen bpf_kfunc_implicit_arg to verify aux injection

[Ihor Solodrai]

On 6/8/26 7:26 AM, chenyuan_fl@xxxxxxx wrote:
> From: Yuan Chen <chenyuan@xxxxxxxxxx>
> 
> Verify that the KF_IMPLICIT_ARGS injection path correctly passes
> the bpf_prog_aux pointer by checking aux->name in
> bpf_kfunc_implicit_arg() for the expected program name prefix.
> If the verifier incorrectly skipped injection (as could happen
> with pahole 1.30's BTF mismatch), the stale register would not
> contain a valid aux pointer and the name check would fail.
> 
> This is a positive test exercised by the existing kfunc_implicit_args
> selftest, which calls bpf_kfunc_implicit_arg(5) and expects a return
> value of 5.
> 
> Signed-off-by: Yuan Chen <chenyuan@xxxxxxxxxx>

Acked-by: Ihor Solodrai <ihor.solodrai@xxxxxxxxx>

> ---
>  tools/testing/selftests/bpf/test_kmods/bpf_testmod.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> index 30f1cd23093c..624d57a5c79a 100644
> --- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> +++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> @@ -1906,7 +1906,11 @@ int bpf_kfunc_multi_st_ops_test_1_assoc(struct st_ops_args *args, struct bpf_pro
>  
>  int bpf_kfunc_implicit_arg(int a, struct bpf_prog_aux *aux)
>  {
> -	if (aux && a > 0)
> +	/* Verify the kernel injected the correct bpf_prog_aux pointer
> +	 * rather than leaving a stale register value. */
> +	if (!aux || strncmp(aux->name, "test_kfunc", sizeof("test_kfunc") - 1))
> +		return -EINVAL;
> +	if (a > 0)
>  		return a;
>  	return -EINVAL;
>  }