Re: [PATCH bpf-next 3/4] bpf: Add BPF_FIB_LOOKUP_SKIP_NEIGH for bpf_fib_lookup

Daniel Borkmann
2023-02-17T16:00:51Z

On 2/17/23 1:41 AM, Martin KaFai Lau wrote:
> From: Martin KaFai Lau <martin.lau@kernel.org>
>
> The bpf_fib_lookup() also looks up the neigh table.
> This was done before bpf_redirect_neigh() was added.
>
> In the use case that does not manage the neigh table
> and requires bpf_fib_lookup() to lookup a fib to
> decide if it needs to redirect or not, the bpf prog can
> depend only on using bpf_redirect_neigh() to lookup the
> neigh. It also keeps the neigh entries fresh and connected.
>
> This patch adds a bpf_fib_lookup flag, SKIP_NEIGH, to avoid
> the double neigh lookup when the bpf prog always call
> bpf_redirect_neigh() to do the neigh lookup.
>
> Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
> ---
> include/uapi/linux/bpf.h | 1 +
> net/core/filter.c | 33 +++++++++++++++++++++++----------
> tools/include/uapi/linux/bpf.h | 1 +
> 3 files changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 1503f61336b6..6c1956e36c97 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
[...]
> @@ -5838,21 +5836,28 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
> if (likely(nhc->nhc_gw_family != AF_INET6)) {
> if (nhc->nhc_gw_family)
> params->ipv4_dst = nhc->nhc_gw.ipv4;
> -
> - neigh = __ipv4_neigh_lookup_noref(dev,
> - (__force u32)params->ipv4_dst);
> } else {
> struct in6_addr *dst = (struct in6_addr *)params->ipv6_dst;
>
> params->family = AF_INET6;
> *dst = nhc->nhc_gw.ipv6;
> - neigh = __ipv6_neigh_lookup_noref_stub(dev, dst);
> }
>
> + if (flags & BPF_FIB_LOOKUP_SKIP_NEIGH)
> + goto set_fwd_params;
> +
> + if (params->family == AF_INET6)

Nit, would have probably more intuitive to keep the same test also here
(nhc->nhc_gw_family != AF_INET6), but either way, lgtm.

Are you still required to fill the params->smac in bpf_fib_set_fwd_params()
in that case, meaning, shouldn't bpf_redirect_neigh() take care of it as well
from neigh_output()? Looks unnecessary and could be moved out too.

(Took in first 2 in the meantime which look good.)

> + neigh = __ipv6_neigh_lookup_noref_stub(dev, params->ipv6_dst);
> + else
> + neigh = __ipv4_neigh_lookup_noref(dev,
> + (__force u32)params->ipv4_dst);
> +
> if (!neigh || !(neigh->nud_state & NUD_VALID))
> return BPF_FIB_LKUP_RET_NO_NEIGH;
> + memcpy(params->dmac, neigh->ha, ETH_ALEN);
>
> - return bpf_fib_set_fwd_params(params, neigh, dev, mtu);
> +set_fwd_params:
> + return bpf_fib_set_fwd_params(params, dev, mtu);