ipip,ip_tunnel,sit: Add FOU support for externally controlled ipip devices

Today ipip devices in collect-metadata mode don't allow for sending FOU
or GUE encapsulated packets. This patch lifts the restriction by adding
a struct ip_tunnel_encap to the tunnel metadata.

On the egress path, the members of this struct can be set by the
bpf_skb_set_fou_encap kfunc via a BPF tc-hook. Instead of dropping packets
wishing to use additional UDP encapsulation, ip_md_tunnel_xmit now
evaluates the contents of this struct and adds the corresponding FOU or
GUE header. Furthermore, it is making sure that additional header bytes
are taken into account for PMTU discovery.

On the ingress path, an ipip device in collect-metadata mode will fill this
struct and a BPF tc-hook can obtain the information via a call to the
bpf_skb_get_fou_encap kfunc.

The minor change to ip_tunnel_encap, which now takes a pointer to
struct ip_tunnel_encap instead of struct ip_tunnel, allows us to control
FOU encap type and parameters on a per packet-level.

Signed-off-by: Christian Ehrig <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/net/ip_tunnels.h

···
 	__u8			flow_flags;
 };
 
+struct ip_tunnel_encap {
+	u16			type;
+	u16			flags;
+	__be16			sport;
+	__be16			dport;
+};
+
 /* Flags for ip_tunnel_info mode. */
 #define IP_TUNNEL_INFO_TX	0x01	/* represents tx tunnel parameters */
 #define IP_TUNNEL_INFO_IPV6	0x02	/* key contains IPv6 addresses */
···
 #define IP_TUNNEL_OPTS_MAX					\
 	GENMASK((sizeof_field(struct ip_tunnel_info,		\
 			      options_len) * BITS_PER_BYTE) - 1, 0)
-
 struct ip_tunnel_info {
 	struct ip_tunnel_key	key;
+	struct ip_tunnel_encap	encap;
 #ifdef CONFIG_DST_CACHE
 	struct dst_cache	dst_cache;
 #endif
···
 	u16			relay_prefixlen;
 };
 #endif
-
-struct ip_tunnel_encap {
-	u16			type;
-	u16			flags;
-	__be16			sport;
-	__be16			dport;
-};
 
 struct ip_tunnel_prl_entry {
 	struct ip_tunnel_prl_entry __rcu *next;
···
 				   __be32 remote, __be32 local,
 				   __be32 key);
 
+void ip_tunnel_md_udp_encap(struct sk_buff *skb, struct ip_tunnel_info *info);
 int ip_tunnel_rcv(struct ip_tunnel *tunnel, struct sk_buff *skb,
 		  const struct tnl_ptk_info *tpi, struct metadata_dst *tun_dst,
 		  bool log_ecn_error);
···
 	return hlen;
 }
 
-static inline int ip_tunnel_encap(struct sk_buff *skb, struct ip_tunnel *t,
+static inline int ip_tunnel_encap(struct sk_buff *skb,
+				  struct ip_tunnel_encap *e,
 				  u8 *protocol, struct flowi4 *fl4)
 {
 	const struct ip_tunnel_encap_ops *ops;
 	int ret = -EINVAL;
 
-	if (t->encap.type == TUNNEL_ENCAP_NONE)
+	if (e->type == TUNNEL_ENCAP_NONE)
 		return 0;
 
-	if (t->encap.type >= MAX_IPTUN_ENCAP_OPS)
+	if (e->type >= MAX_IPTUN_ENCAP_OPS)
 		return -EINVAL;
 
 	rcu_read_lock();
-	ops = rcu_dereference(iptun_encaps[t->encap.type]);
+	ops = rcu_dereference(iptun_encaps[e->type]);
 	if (likely(ops && ops->build_header))
-		ret = ops->build_header(skb, &t->encap, protocol, fl4);
+		ret = ops->build_header(skb, e, protocol, fl4);
 	rcu_read_unlock();
 
 	return ret;

net/ipv4/ip_tunnel.c

···
 	return ERR_PTR(err);
 }
 
+void ip_tunnel_md_udp_encap(struct sk_buff *skb, struct ip_tunnel_info *info)
+{
+	const struct iphdr *iph = ip_hdr(skb);
+	const struct udphdr *udph;
+
+	if (iph->protocol != IPPROTO_UDP)
+		return;
+
+	udph = (struct udphdr *)((__u8 *)iph + (iph->ihl << 2));
+	info->encap.sport = udph->source;
+	info->encap.dport = udph->dest;
+}
+EXPORT_SYMBOL(ip_tunnel_md_udp_encap);
+
 int ip_tunnel_rcv(struct ip_tunnel *tunnel, struct sk_buff *skb,
 		  const struct tnl_ptk_info *tpi, struct metadata_dst *tun_dst,
 		  bool log_ecn_error)
···
 			    tunnel_id_to_key32(key->tun_id), RT_TOS(tos),
 			    dev_net(dev), 0, skb->mark, skb_get_hash(skb),
 			    key->flow_flags);
-	if (tunnel->encap.type != TUNNEL_ENCAP_NONE)
+
+	if (!tunnel_hlen)
+		tunnel_hlen = ip_encap_hlen(&tun_info->encap);
+
+	if (ip_tunnel_encap(skb, &tun_info->encap, &proto, &fl4) < 0)
 		goto tx_error;
 
 	use_cache = ip_tunnel_dst_cache_usable(skb, tun_info);
···
 			    dev_net(dev), tunnel->parms.link,
 			    tunnel->fwmark, skb_get_hash(skb), 0);
 
-	if (ip_tunnel_encap(skb, tunnel, &protocol, &fl4) < 0)
+	if (ip_tunnel_encap(skb, &tunnel->encap, &protocol, &fl4) < 0)
 		goto tx_error;
 
 	if (connected && md) {

net/ipv4/ipip.c

···
 			tun_dst = ip_tun_rx_dst(skb, 0, 0, 0);
 			if (!tun_dst)
 				return 0;
+			ip_tunnel_md_udp_encap(skb, &tun_dst->u.tun_info);
 		}
 		skb_reset_mac_header(skb);
 

net/ipv6/sit.c

···
 		ttl = iph6->hop_limit;
 	tos = INET_ECN_encapsulate(tos, ipv6_get_dsfield(iph6));
 
-	if (ip_tunnel_encap(skb, tunnel, &protocol, &fl4) < 0) {
+	if (ip_tunnel_encap(skb, &tunnel->encap, &protocol, &fl4) < 0) {
 		ip_rt_put(rt);
 		goto tx_error;
 	}