bpf_fib_lookup: optionally skip neighbour lookup

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, at the cost of a bit of duplicated work.

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. So let's just
always return the ifindex, and also add a flag that lets the caller turn
off the neighbour lookup entirely in bpf_fib_lookup().

v2:
- Add flag (Daniel)
- Remove misleading code example from commit message (David)

Cc: David Ahern <dsahern@gmail.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

Author: tohojo

include/uapi/linux/bpf.h

@@ -4813,12 +4813,14 @@ struct bpf_raw_tracepoint_args {
 	__u64 args[0];
 };
 
-/* DIRECT:  Skip the FIB rules and go to FIB table associated with device
- * OUTPUT:  Do lookup from egress perspective; default is ingress
+/* DIRECT:      Skip the FIB rules and go to FIB table associated with device
+ * OUTPUT:      Do lookup from egress perspective; default is ingress
+ * SKIP_NEIGH:  Skip neighbour lookup and return BPF_FIB_LKUP_RET_NO_NEIGH on success
  */
 enum {
-	BPF_FIB_LOOKUP_DIRECT  = (1U << 0),
-	BPF_FIB_LOOKUP_OUTPUT  = (1U << 1),
+	BPF_FIB_LOOKUP_DIRECT      = (1U << 0),
+	BPF_FIB_LOOKUP_OUTPUT      = (1U << 1),
+	BPF_FIB_LOOKUP_SKIP_NEIGH  = (1U << 2),
 };
 
 enum {

net/core/filter.c

@@ -5196,7 +5196,6 @@ static int bpf_fib_set_fwd_params(struct bpf_fib_lookup *params,
 	memcpy(params->smac, dev->dev_addr, ETH_ALEN);
 	params->h_vlan_TCI = 0;
 	params->h_vlan_proto = 0;
-	params->ifindex = dev->ifindex;
 
 	return 0;
 }
@@ -5293,6 +5292,10 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 	dev = nhc->nhc_dev;
 
 	params->rt_metric = res.fi->fib_priority;
+	params->ifindex = dev->ifindex;
+
+	if (flags & BPF_FIB_LOOKUP_SKIP_NEIGH)
+		return BPF_FIB_LKUP_RET_NO_NEIGH;
 
 	/* xdp and cls_bpf programs are run in RCU-bh so
 	 * rcu_read_lock_bh is not needed here
@@ -5418,6 +5421,10 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 
 	dev = res.nh->fib_nh_dev;
 	params->rt_metric = res.f6i->fib6_metric;
+	params->ifindex = dev->ifindex;
+
+	if (flags & BPF_FIB_LOOKUP_SKIP_NEIGH)
+		return BPF_FIB_LKUP_RET_NO_NEIGH;
 
 	/* xdp and cls_bpf programs are run in RCU-bh so rcu_read_lock_bh is
 	 * not needed here.
@@ -5436,7 +5443,8 @@ BPF_CALL_4(bpf_xdp_fib_lookup, struct xdp_buff *, ctx,
 	if (plen < sizeof(*params))
 		return -EINVAL;
 
-	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
+	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT |
+		      BPF_FIB_LOOKUP_SKIP_NEIGH))
 		return -EINVAL;
 
 	switch (params->family) {
@@ -5473,7 +5481,8 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 	if (plen < sizeof(*params))
 		return -EINVAL;
 
-	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
+	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT |
+		      BPF_FIB_LOOKUP_SKIP_NEIGH))
 		return -EINVAL;
 
 	switch (params->family) {

tools/include/uapi/linux/bpf.h

@@ -4813,12 +4813,14 @@ struct bpf_raw_tracepoint_args {
 	__u64 args[0];
 };
 
-/* DIRECT:  Skip the FIB rules and go to FIB table associated with device
- * OUTPUT:  Do lookup from egress perspective; default is ingress
+/* DIRECT:      Skip the FIB rules and go to FIB table associated with device
+ * OUTPUT:      Do lookup from egress perspective; default is ingress
+ * SKIP_NEIGH:  Skip neighbour lookup and return BPF_FIB_LKUP_RET_NO_NEIGH on success
  */
 enum {
-	BPF_FIB_LOOKUP_DIRECT  = (1U << 0),
-	BPF_FIB_LOOKUP_OUTPUT  = (1U << 1),
+	BPF_FIB_LOOKUP_DIRECT      = (1U << 0),
+	BPF_FIB_LOOKUP_OUTPUT      = (1U << 1),
+	BPF_FIB_LOOKUP_SKIP_NEIGH  = (1U << 2),
 };
 
 enum {