netdev - Re: [PATCH bpf 1/2] bpf_redirect_neigh: Support supplying the nexthop as a helper parameter

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Date:   Mon, 19 Oct 2020 20:23:35 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf 1/2] bpf_redirect_neigh: Support supplying the nexthop
 as a helper parameter

On 10/19/20 6:04 PM, Toke Høiland-Jørgensen wrote:
> From: Toke Høiland-Jørgensen <toke@...hat.com>
> 
> Based on the discussion in [0], update the bpf_redirect_neigh() helper to
> accept an optional parameter specifying the nexthop information. This makes
> it possible to combine bpf_fib_lookup() and bpf_redirect_neigh() without
> incurring a duplicate FIB lookup - since the FIB lookup helper will return
> the nexthop information even if no neighbour is present, this can simply be
> passed on to bpf_redirect_neigh() if bpf_fib_lookup() returns
> BPF_FIB_LKUP_RET_NO_NEIGH.
> 
> [0] https://lore.kernel.org/bpf/393e17fc-d187-3a8d-2f0d-a627c7c63fca@iogearbox.net/
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

Looks good to me, thanks! I'll wait till David gets a chance as well to review.
One thing that would have made sense to me (probably worth a v2) is to keep the
fib lookup flag you had back then, meaning sth like BPF_FIB_SKIP_NEIGH which
would then return a BPF_FIB_LKUP_RET_NO_NEIGH before doing the neigh lookup inside
the bpf_ipv{4,6}_fib_lookup() so that programs can just unconditionally use the
combination of bpf_fib_lookup(skb, [...], BPF_FIB_SKIP_NEIGH) with the
bpf_redirect_neigh([...]) extension in that case and not do this bpf_redirect()
vs bpf_redirect_neigh() dance as you have in the selftest in patch 2/2.

Thanks,
Daniel