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

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Date:   Thu, 8 Oct 2020 23:02:58 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        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

On 10/8/20 10:59 PM, Toke Høiland-Jørgensen wrote:
> 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? :)

Sounds good to me; merge depending on David's final verdict in the other thread
wrt commit description.