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 10:22:59 -0700
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.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

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.