netdev - Re: [PATCH bpf-next] bpf_fib_lookup: return target ifindex even if neighbour lookup fails

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Date:   Thu, 08 Oct 2020 22:59:17 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Daniel Borkmann <daniel@...earbox.net>, ast@...com
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH bpf-next] bpf_fib_lookup: return target ifindex even if
 neighbour lookup fails

Daniel Borkmann <daniel@...earbox.net> writes:

> On 10/8/20 4:53 PM, Toke Høiland-Jørgensen wrote:
>> The bpf_fib_lookup() helper performs a neighbour lookup for the destination
>> IP and returns BPF_FIB_LKUP_NO_NEIGH if this fails, with the expectation
>> that the BPF program will pass the packet up the stack in this case.
>> However, with the addition of bpf_redirect_neigh() that can be used instead
>> to perform the neighbour lookup.
>> 
>> However, for that we still need the target ifindex, and since
>> bpf_fib_lookup() already has that at the time it performs the neighbour
>> lookup, there is really no reason why it can't just return it in any case.
>> With this fix, a BPF program can do the following to perform a redirect
>> based on the routing table that will succeed even if there is no neighbour
>> entry:
>> 
>> 	ret = bpf_fib_lookup(skb, &fib_params, sizeof(fib_params), 0);
>> 	if (ret == BPF_FIB_LKUP_RET_SUCCESS) {
>> 		__builtin_memcpy(eth->h_dest, fib_params.dmac, ETH_ALEN);
>> 		__builtin_memcpy(eth->h_source, fib_params.smac, ETH_ALEN);
>> 
>> 		return bpf_redirect(fib_params.ifindex, 0);
>> 	} else if (ret == BPF_FIB_LKUP_RET_NO_NEIGH) {
>> 		return bpf_redirect_neigh(fib_params.ifindex, 0);
>> 	}
>> 
>> Cc: David Ahern <dsahern@...il.com>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>
> ACK, this looks super useful! Could you also add a new flag which would skip
> neighbor lookup in the helper while at it (follow-up would be totally fine from
> my pov since both are independent from each other)?

Sure, can do. Thought about adding it straight away, but wasn't sure if
it would be useful, since the fib lookup has already done quite a lot of
work by then. But if you think it'd be useful, I can certainly add it.
I'll look at this tomorrow; if you merge this before then I'll do it as
a follow-up, and if not I'll respin with it added. OK? :)

-Toke