# Re: \[PATCH\] bpf: Forbid trampoline attach for functions with variable arguments

- From: Daniel Borkmann \<daniel@iogearbox.net\>
- Date: 2021-05-06 23:32:00 
- List: netdev
- Page: https://ratatoskr.run/netdev/2021/05/7768558
- Thread: https://ratatoskr.run/netdev/2021/05/7768519/t.md

```text
On 5/5/21 8:45 PM, Andrii Nakryiko wrote:
> On Wed, May 5, 2021 at 6:42 AM Jiri Olsa [ref] wrote:
>>
>> We can't currently allow to attach functions with variable arguments.
>> The problem is that we should save all the registers for arguments,
>> which is probably doable, but if caller uses more than 6 arguments,
>> we need stack data, which will be wrong, because of the extra stack
>> frame we do in bpf trampoline, so we could crash.
>>
>> Also currently there's malformed trampoline code generated for such
>> functions at the moment as described in:
>>    https://lore.kernel.org/bpf/20210429212834.82621-1-jolsa@kernel.org/
>>
>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>> ---
>
> LGTM.
>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>
>>   kernel/bpf/btf.c | 13 +++++++++++++
>>   1 file changed, 13 insertions(+)
>>
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index 0600ed325fa0..161511bb3e51 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -5206,6 +5206,13 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
>>          m->ret_size = ret;
>>
>>          for (i = 0; i < nargs; i++) {
>> +               if (i == nargs - 1 && args[i].type == 0) {
>> +                       bpf_log(log,
>> +                               "The function %s with variable args is unsupported.\n",
>> +                               tname);
>> +                       return -EINVAL;
>> +

(Jiri, fyi, I removed this extra newline while applying. Please scan for such
things before submitting.)

>> +               }
>>                  ret = __get_type_size(btf, args[i].type, &t);
>>                  if (ret < 0) {
>>                          bpf_log(log,
>> @@ -5213,6 +5220,12 @@ int btf_distill_func_proto(struct bpf_verifier_log *log,
>>                                  tname, i, btf_kind_str[BTF_INFO_KIND(t->info)]);
>>                          return -EINVAL;
>>                  }
>> +               if (ret == 0) {
>> +                       bpf_log(log,
>> +                               "The function %s has malformed void argument.\n",
>> +                               tname);
>> +                       return -EINVAL;
>> +               }
>>                  m->arg_size[i] = ret;
>>          }
>>          m->nr_args = nargs;
>> --
>> 2.30.2
>>

```