Re: [PATCH bpf-next v5 1/3] bpf: Add BPF_FIB_LOOKUP_VLAN flag to bpf_fib_lookup() helper

[David Ahern]

On 6/29/26 9:08 AM, Toke Høiland-Jørgensen wrote:
> David Ahern <dsahern@xxxxxxxxxx> writes:
> 
>> On 6/23/26 9:05 PM, Avinash Duduskar wrote:
>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>> index 89b36de5fdbb..e00f0392e728 100644
>>> --- a/include/uapi/linux/bpf.h
>>> +++ b/include/uapi/linux/bpf.h
>>> @@ -3532,6 +3532,29 @@ union bpf_attr {
>>>   *			Use the mark present in *params*->mark for the fib lookup.
>>>   *			This option should not be used with BPF_FIB_LOOKUP_DIRECT,
>>>   *			as it only has meaning for full lookups.
>>> + *		**BPF_FIB_LOOKUP_VLAN**
>>
>> This flag should not be needed. Patches for vlan support were never
>> submitted (I have them in some old branch). Since the vlan params are
>> initialized to 0, no new flag should be needed. Besides, these are
>> output parameters.
> 
> There's no enforcement from the kernel side of the parameters being
> zero, though? So we do need the flag for feature detection; unless we
> expect applications to do that out of band? But then we'd need a
> mechanism to do that which could be... the presence of the flag in the
> ENUM (and thus in BTF)? :)
> 

This is output direction - return from the fib lookup. It does not make
sense to require a flag to get lookup output. vlan proto of 0 is not
valid, so it is a clear indication that the vlan output parameters were
not set during the lookup.