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

[David Ahern <dsahern@kernel.org>]

On 7/1/26 5:02 AM, Toke Høiland-Jørgensen wrote:
> David Ahern <dsahern@kernel.org> 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.