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

[Toke Høiland-Jørgensen]

David Ahern <dsahern@xxxxxxxxxx> writes:

> On 7/1/26 5:02 AM, Toke Høiland-Jørgensen wrote:
>> David Ahern <dsahern@xxxxxxxxxx> writes:
>>> Seems to me the fib_lookup for xdp needs to return the bottom device,
>>> not the vlan device, for forwarding to work. That's why I added the
>>> fields to the struct. That allows the program to push the vlan header if
>>> required. My preference (dream?) was that Tx path had support to tell
>>> the redirect the vlan and h/w added it on send.
>> 
>> Sure, returning the bottom device index with the VLAN tag makes sense,
>> and that's basically what this series does (but bails out on stacked
>> VLANs). However, that's not what the helper does today, which is why the
>> flag is there, to opt-in to the new behaviour. I don't think we can just
>> change the ifindex without breaking existing applications (as noted
>> up-thread).
>
> I do not see it as breaking existing programs which is why I chimed in
> on the thread.
>
>> 
>>> But really, once stacked devices come into play, I just wanted to make
>>> sure thought is given to different use cases. As you know the lookup
>>> struct if hard bound to 64B and it is trying to cover a lot of use cases.
>> 
>> Agreed, I don't think we can handle stacked devices in this helper. But
>> we could split it out into a new one. Something like:
>> 
>> struct lower_device_info {
>> 	enum device_type type;
>> 	struct {
>> 		__be16	h_vlan_proto;
>> 		__be16	h_vlan_TCI;
>> 	} vlan;
>>         /* add other types here */
>> };
>> 
>> int xdp_get_lower_device(int ifindex, struct lower_device_info *info);
>> 
>> called like:
>> 
>> int xdp_program(struct xdp_md *ctx)
>> {
>>         struct lower_device_info dev_info = {};
>> 	int ifindex, ret;
>> 
>>         ifindex = find_destination(ctx); /* does fib lookup, or something else */
>> 
>>         while ((ret = xdp_get_lower_device_info(ifindex, &dev_info)) > 0) {
>>         	if (dev_info.type == VLAN) {
>>                       	push_vlan_tag(ctx, &dev_info.vlan);
>>                         ifindex = ret;
>>                 } else {
>>                 	return XDP_PASS; /* we only handle VLAN devices */
>>                 }
>>         }
>> 
>>         return bpf_redirect(ifindex, 0);
>> }
>> 
>> 
>> With a helper like this, we obviously don't strictly speaking need to
>> change the fib lookup helper at all. However, for the single-tagged VLAN
>> case, I think supporting it directly in the fib lookup could still have
>> value, as an optimisation: it saves an extra call for resolving the
>> ifindex, and the fields are already there. So I think my preference
>> would be to merge this series as-is, and then follow up with a new kfunc
>> to handle the stacked case. But we could also just drop this series and
>> go straight to the new kfunc.
>> 
>> WDYT?
>
> no preference. I only chimed in because of the added flag to the uapi
> which I do not see as needed. If the consensus is that it is in fact
> needed, all good then.

Alright, cool - care to provide an ACK, then? :)

-Toke