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

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Message-ID: <87d01se8qc.fsf@toke.dk>
Date:   Thu, 08 Oct 2020 22:57:47 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     David Ahern <dsahern@...il.com>, daniel@...earbox.net, ast@...com
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf_fib_lookup: return target ifindex even if
 neighbour lookup fails

David Ahern <dsahern@...il.com> writes:

> On 10/8/20 7:53 AM, 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);
>> 	}
>> 
>
> There are a lot of assumptions in this program flow and redundant work.
> fib_lookup is generic and allows the caller to control the input
> parameters. direct_neigh does a fib lookup based on network header data
> from the skb.
>
> I am fine with the patch, but users need to be aware of the subtle details.

Yeah, I'm aware they are not equivalent; the code above was just meant
as a minimal example motivating the patch. If you think it's likely to
confuse people to have this example in the commit message, I can remove
it?

-Toke