netfilter: synproxy: extract SYNPROXY infrastructure from {ipt, ip6t}_SYNPROXY

Add common functions into nf_synproxy_core.c to prepare for nftables support.
The prototypes of the functions used by {ipt, ip6t}_SYNPROXY are in the new
file nf_synproxy.h
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_conntrack_synproxy.h

@@ -72,21 +72,12 @@ struct synproxy_options {
 };
 
 struct tcphdr;
-struct xt_synproxy_info;
+struct nf_synproxy_info;
 bool synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
 			    const struct tcphdr *th,
 			    struct synproxy_options *opts);
-unsigned int synproxy_options_size(const struct synproxy_options *opts);
-void synproxy_build_options(struct tcphdr *th,
-			    const struct synproxy_options *opts);
 
-void synproxy_init_timestamp_cookie(const struct xt_synproxy_info *info,
+void synproxy_init_timestamp_cookie(const struct nf_synproxy_info *info,
 				    struct synproxy_options *opts);
-void synproxy_check_timestamp_cookie(struct synproxy_options *opts);
-
-unsigned int synproxy_tstamp_adjust(struct sk_buff *skb, unsigned int protoff,
-				    struct tcphdr *th, struct nf_conn *ct,
-				    enum ip_conntrack_info ctinfo,
-				    const struct nf_conn_synproxy *synproxy);
 
 #endif /* _NF_CONNTRACK_SYNPROXY_H */

include/net/netfilter/nf_synproxy.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _NF_SYNPROXY_SHARED_H
+#define _NF_SYNPROXY_SHARED_H
+
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <net/ip6_checksum.h>
+#include <net/ip6_route.h>
+#include <net/tcp.h>
+
+#include <net/netfilter/nf_conntrack_seqadj.h>
+#include <net/netfilter/nf_conntrack_synproxy.h>
+
+void synproxy_send_client_synack(struct net *net, const struct sk_buff *skb,
+				 const struct tcphdr *th,
+				 const struct synproxy_options *opts);
+
+bool synproxy_recv_client_ack(struct net *net,
+			      const struct sk_buff *skb,
+			      const struct tcphdr *th,
+			      struct synproxy_options *opts, u32 recv_seq);
+
+unsigned int ipv4_synproxy_hook(void *priv, struct sk_buff *skb,
+				const struct nf_hook_state *nhs);
+int nf_synproxy_ipv4_init(struct synproxy_net *snet, struct net *net);
+void nf_synproxy_ipv4_fini(struct synproxy_net *snet, struct net *net);
+
+#if IS_ENABLED(CONFIG_IPV6)
+void synproxy_send_client_synack_ipv6(struct net *net,
+				      const struct sk_buff *skb,
+				      const struct tcphdr *th,
+				      const struct synproxy_options *opts);
+
+bool synproxy_recv_client_ack_ipv6(struct net *net, const struct sk_buff *skb,
+				   const struct tcphdr *th,
+				   struct synproxy_options *opts, u32 recv_seq);
+
+unsigned int ipv6_synproxy_hook(void *priv, struct sk_buff *skb,
+				const struct nf_hook_state *nhs);
+int nf_synproxy_ipv6_init(struct synproxy_net *snet, struct net *net);
+void nf_synproxy_ipv6_fini(struct synproxy_net *snet, struct net *net);
+#endif /* CONFIG_IPV6 */
+
+#endif /* _NF_SYNPROXY_SHARED_H */