bpf: Fix bpf_fib_lookup helper MTU check for SKB ctx

BPF end-user on Cilium slack-channel (Carlo Carraro) wants to use
bpf_fib_lookup for doing MTU-check, but *prior* to extending packet size,
by adjusting fib_params 'tot_len' with the packet length plus the expected
encap size. (Just like the bpf_check_mtu helper supports). He discovered
that for SKB ctx the param->tot_len was not used, instead skb->len was used
(via MTU check in is_skb_forwardable() that checks against netdev MTU).

Fix this by using fib_params 'tot_len' for MTU check. If not provided (e.g.
zero) then keep existing TC behaviour intact. Notice that 'tot_len' for MTU
check is done like XDP code-path, which checks against FIB-dst MTU.

V16:
- Revert V13 optimization, 2nd lookup is against egress/resulting netdev

V13:
- Only do ifindex lookup one time, calling dev_get_by_index_rcu().

V10:
- Use same method as XDP for 'tot_len' MTU check

Fixes: 4c79579b ("bpf: Change bpf_fib_lookup to return lookup status")
Reported-by: Carlo Carraro <colrack@gmail.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/161287789444.790810.15247494756551413508.stgit@firesoul

net/core/filter.c

@@ -5581,6 +5581,7 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 {
 	struct net *net = dev_net(skb->dev);
 	int rc = -EAFNOSUPPORT;
+	bool check_mtu = false;
 
 	if (plen < sizeof(*params))
 		return -EINVAL;
@@ -5588,22 +5589,28 @@ BPF_CALL_4(bpf_skb_fib_lookup, struct sk_buff *, skb,
 	if (flags & ~(BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT))
 		return -EINVAL;
 
+	if (params->tot_len)
+		check_mtu = true;
+
 	switch (params->family) {
 #if IS_ENABLED(CONFIG_INET)
 	case AF_INET:
-		rc = bpf_ipv4_fib_lookup(net, params, flags, false);
+		rc = bpf_ipv4_fib_lookup(net, params, flags, check_mtu);
 		break;
 #endif
 #if IS_ENABLED(CONFIG_IPV6)
 	case AF_INET6:
-		rc = bpf_ipv6_fib_lookup(net, params, flags, false);
+		rc = bpf_ipv6_fib_lookup(net, params, flags, check_mtu);
 		break;
 #endif
 	}
 
-	if (!rc) {
+	if (rc == BPF_FIB_LKUP_RET_SUCCESS && !check_mtu) {
 		struct net_device *dev;
 
+		/* When tot_len isn't provided by user, check skb
+		 * against MTU of FIB lookup resulting net_device
+		 */
 		dev = dev_get_by_index_rcu(net, params->ifindex);
 		if (!is_skb_forwardable(dev, skb))
 			rc = BPF_FIB_LKUP_RET_FRAG_NEEDED;