Re: [PATCH bpf-next v9 02/10] bpf: Return false for bpf_prog_check_recur() default case

[Yonghong Song <yonghong.song@linux.dev>]

On 11/4/24 5:21 PM, Alexei Starovoitov wrote:
> On Mon, Nov 4, 2024 at 11:35 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>> The bpf_prog_check_recur() funciton is currently used by trampoline
>> and tracing programs (also using trampoline) to check whether a
>> particular prog supports recursion checking or not. The default case
>> (non-trampoline progs) return true in the current implementation.
>>
>> Let us make the non-trampoline prog recursion check return false
>> instead. It does not impact any existing use cases and allows the
>> function to be used outside the trampoline context in the next patch.
> Does not impact ?! But it does.
> This patch removes recursion check from fentry progs.
> This cannot be right.

The original bpf_prog_check_recur() implementation:

static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
{
         switch (resolve_prog_type(prog)) {
         case BPF_PROG_TYPE_TRACING:
                 return prog->expected_attach_type != BPF_TRACE_ITER;
         case BPF_PROG_TYPE_STRUCT_OPS:
         case BPF_PROG_TYPE_LSM:
                 return false;
         default:
                 return true;
         }
}

fentry prog is a TRACING prog, so it is covered. Did I miss anything?

>
> pw-bot: cr
>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   include/linux/bpf_verifier.h | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
>> index 4513372c5bc8..ad887c68d3e1 100644
>> --- a/include/linux/bpf_verifier.h
>> +++ b/include/linux/bpf_verifier.h
>> @@ -889,9 +889,8 @@ static inline bool bpf_prog_check_recur(const struct bpf_prog *prog)
>>                  return prog->expected_attach_type != BPF_TRACE_ITER;
>>          case BPF_PROG_TYPE_STRUCT_OPS:
>>          case BPF_PROG_TYPE_LSM:
>> -               return false;
>>          default:
>> -               return true;
>> +               return false;
>>          }
>>   }
>>
>> --
>> 2.43.5
>>