Re: Trying the bpf trace a bpf xdp program

["Eelco Chaudron" <echaudro@redhat.com>]

On 4 Dec 2019, at 19:01, Yonghong Song wrote:

<SNIP>

>>> I’ve put my code on GitHub, maybe it’s just something stupid…
>
> Thanks for the test case. This indeed a kernel bug.
> The following change fixed the issue:
>
>
> -bash-4.4$ git diff
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index a0482e1c4a77..034ef81f935b 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9636,7 +9636,10 @@ static int check_attach_btf_id(struct
> bpf_verifier_env *env)
>                                  ret = -EINVAL;
>                                  goto out;
>                          }
> -                       addr = (long)
> tgt_prog->aux->func[subprog]->bpf_func;
> +                       if (subprog == 0)
> +                               addr = (long) tgt_prog->bpf_func;
> +                       else
> +                               addr = (long)
> tgt_prog->aux->func[subprog]->bpf_func;
>                  } else {
>                          addr = kallsyms_lookup_name(tname);
>                          if (!addr) {
> -bash-4.4$
>
> The reason is for a bpf program without any additional subprogram
> (callees), tgt_prog->aux->func is not populated and is a NULL pointer,
> so the access tgt_prog->aux->func[0]->bpf_func will segfault.
>
> With the above change, your test works properly.

Thanks for the quick response, and as you mention the test passes with 
the patch above.

I will continue my experiments later this week, and let you know if I 
run into any other problems.

Cheers,

Eelco