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Message-ID: <bf190e76-b178-d915-8d0d-811905d38fd2@iogearbox.net>
Date: Thu, 8 Oct 2020 17:08:43 +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 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)?
Thanks,
Daniel
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