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

[Daniel Borkmann]

On 10/15/20 5:46 PM, Toke Høiland-Jørgensen wrote:

From: Toke Høiland-Jørgensen <t...@redhat.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-a627c7c63...@iogearbox.net/

Signed-off-by: Toke Høiland-Jørgensen <t...@redhat.com>

Overall looks good from what I can tell, just small nits below on top of
David's feedback:

[...]

-static int __bpf_redirect_neigh_v4(struct sk_buff *skb, struct net_device *dev)
+static int __bpf_redirect_neigh_v4(struct sk_buff *skb, struct net_device *dev,
+                                  struct bpf_nh_params *nh)
  {
        const struct iphdr *ip4h = ip_hdr(skb);
        struct net *net = dev_net(dev);
        int err, ret = NET_XMIT_DROP;
-       struct rtable *rt;
-       struct flowi4 fl4 = {
-               .flowi4_flags   = FLOWI_FLAG_ANYSRC,
-               .flowi4_mark    = skb->mark,
-               .flowi4_tos     = RT_TOS(ip4h->tos),
-               .flowi4_oif     = dev->ifindex,
-               .flowi4_proto   = ip4h->protocol,
-               .daddr          = ip4h->daddr,
-               .saddr          = ip4h->saddr,
-       };

- rt = ip_route_output_flow(net, &fl4, NULL);

-       if (IS_ERR(rt))
-               goto out_drop;
-       if (rt->rt_type != RTN_UNICAST && rt->rt_type != RTN_LOCAL) {
-               ip_rt_put(rt);
-               goto out_drop;
-       }
+       if (!nh->nh_family) {
+               struct rtable *rt;
+               struct flowi4 fl4 = {
+                       .flowi4_flags = FLOWI_FLAG_ANYSRC,
+                       .flowi4_mark = skb->mark,
+                       .flowi4_tos = RT_TOS(ip4h->tos),
+                       .flowi4_oif = dev->ifindex,
+                       .flowi4_proto = ip4h->protocol,
+                       .daddr = ip4h->daddr,
+                       .saddr = ip4h->saddr,
+               };
+
+               rt = ip_route_output_flow(net, &fl4, NULL);
+               if (IS_ERR(rt))
+                       goto out_drop;
+               if (rt->rt_type != RTN_UNICAST && rt->rt_type != RTN_LOCAL) {
+                       ip_rt_put(rt);
+                       goto out_drop;
+               }

- skb_dst_set(skb, &rt->dst);

+               skb_dst_set(skb, &rt->dst);
+               nh = NULL;
+       }

- err = bpf_out_neigh_v4(net, skb);

+       err = bpf_out_neigh_v4(net, skb, dev, nh);
        if (unlikely(net_xmit_eval(err)))
                dev->stats.tx_errors++;
        else
@@ -2355,7 +2383,8 @@ static int __bpf_redirect_neigh_v4(struct sk_buff *skb, 
struct net_device *dev)
  }
  #endif /* CONFIG_INET */

-static int __bpf_redirect_neigh(struct sk_buff *skb, struct net_device *dev)

+static int __bpf_redirect_neigh(struct sk_buff *skb, struct net_device *dev,
+                               struct bpf_nh_params *nh)
  {
        struct ethhdr *ethh = eth_hdr(skb);

@@ -2370,9 +2399,9 @@ static int __bpf_redirect_neigh(struct sk_buff *skb, struct net_device *dev)

        skb_reset_network_header(skb);

if (skb->protocol == htons(ETH_P_IP))

-               return __bpf_redirect_neigh_v4(skb, dev);
+               return __bpf_redirect_neigh_v4(skb, dev, nh);
        else if (skb->protocol == htons(ETH_P_IPV6))
-               return __bpf_redirect_neigh_v6(skb, dev);
+               return __bpf_redirect_neigh_v6(skb, dev, nh);
  out:
        kfree_skb(skb);
        return -ENOTSUPP;
@@ -2455,8 +2484,8 @@ int skb_do_redirect(struct sk_buff *skb)
                return -EAGAIN;
        }
        return flags & BPF_F_NEIGH ?
-              __bpf_redirect_neigh(skb, dev) :
-              __bpf_redirect(skb, dev, flags);
+               __bpf_redirect_neigh(skb, dev, &ri->nh) :
+               __bpf_redirect(skb, dev, flags);
  out_drop:
        kfree_skb(skb);
        return -EINVAL;
@@ -2504,16 +2533,23 @@ static const struct bpf_func_proto 
bpf_redirect_peer_proto = {
        .arg2_type      = ARG_ANYTHING,
  };

-BPF_CALL_2(bpf_redirect_neigh, u32, ifindex, u64, flags)

+BPF_CALL_4(bpf_redirect_neigh, u32, ifindex, struct bpf_redir_neigh *, params,
+          int, plen, u64, flags)
  {
        struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);

- if (unlikely(flags))

+       if (unlikely((plen && plen < sizeof(*params)) || flags))
                return TC_ACT_SHOT;

ri->flags = BPF_F_NEIGH;

        ri->tgt_index = ifindex;

+ BUILD_BUG_ON(sizeof(struct bpf_redir_neigh) != sizeof(struct bpf_nh_params));

+       if (plen)
+               memcpy(&ri->nh, params, sizeof(ri->nh));
+       else
+               ri->nh.nh_family = 0; /* clear previous value */

I'd probably just add an internal flag and do ...

  ri->flags = BPF_F_NEIGH | (plen ? BPF_F_NEXTHOP : 0);

... instead of above clearing, and skb_do_redirect() then becomes:

  __bpf_redirect_neigh(skb, dev, flags & BPF_F_NEXTHOP ? &ri->nh : NULL)

... which would then also avoid this !nh->nh_family check where you later on
set nh = NULL to pass it onwards.

        return TC_ACT_REDIRECT;
  }