netfilter: nf_nat: support mangling a single TCP packet multiple times

nf_nat_mangle_tcp_packet() can currently only handle a single mangling
per window because it only maintains two sequence adjustment positions:
the one before the last adjustment and the one after.

This patch makes sequence number adjustment tracking in
nf_nat_mangle_tcp_packet() optional and allows a helper to manually
update the offsets after the packet has been fully handled.

Signed-off-by: Patrick McHardy <[e-mail]>

include/net/netfilter/nf_nat_helper.h

···
 struct sk_buff;
 
 /* These return true or false. */
-extern int nf_nat_mangle_tcp_packet(struct sk_buff *skb,
-				    struct nf_conn *ct,
-				    enum ip_conntrack_info ctinfo,
-				    unsigned int match_offset,
-				    unsigned int match_len,
-				    const char *rep_buffer,
-				    unsigned int rep_len);
+extern int __nf_nat_mangle_tcp_packet(struct sk_buff *skb,
+				      struct nf_conn *ct,
+				      enum ip_conntrack_info ctinfo,
+				      unsigned int match_offset,
+				      unsigned int match_len,
+				      const char *rep_buffer,
+				      unsigned int rep_len, bool adjust);
+
+static inline int nf_nat_mangle_tcp_packet(struct sk_buff *skb,
+					   struct nf_conn *ct,
+					   enum ip_conntrack_info ctinfo,
+					   unsigned int match_offset,
+					   unsigned int match_len,
+					   const char *rep_buffer,
+					   unsigned int rep_len)
+{
+	return __nf_nat_mangle_tcp_packet(skb, ct, ctinfo,
+					  match_offset, match_len,
+					  rep_buffer, rep_len, true);
+}
+
 extern int nf_nat_mangle_udp_packet(struct sk_buff *skb,
 				    struct nf_conn *ct,
 				    enum ip_conntrack_info ctinfo,
···
 				    unsigned int match_len,
 				    const char *rep_buffer,
 				    unsigned int rep_len);
+
+extern void nf_nat_set_seq_adjust(struct nf_conn *ct,
+				  enum ip_conntrack_info ctinfo,
+				  __be32 seq, s16 off);
 extern int nf_nat_seq_adjust(struct sk_buff *skb,
 			     struct nf_conn *ct,
 			     enum ip_conntrack_info ctinfo);

net/ipv4/netfilter/nf_nat_helper.c

···
 	return 1;
 }
 
+void nf_nat_set_seq_adjust(struct nf_conn *ct, enum ip_conntrack_info ctinfo,
+			   __be32 seq, s16 off)
+{
+	if (!off)
+		return;
+	set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
+	adjust_tcp_sequence(ntohl(seq), off, ct, ctinfo);
+	nf_conntrack_event_cache(IPCT_NATSEQADJ, ct);
+}
+EXPORT_SYMBOL_GPL(nf_nat_set_seq_adjust);
+
 /* Generic function for mangling variable-length address changes inside
  * NATed TCP connections (like the PORT XXX,XXX,XXX,XXX,XXX,XXX
  * command in FTP).
···
  * skb enlargement, ...
  *
  * */
-int
-nf_nat_mangle_tcp_packet(struct sk_buff *skb,
-			 struct nf_conn *ct,
-			 enum ip_conntrack_info ctinfo,
-			 unsigned int match_offset,
-			 unsigned int match_len,
-			 const char *rep_buffer,
-			 unsigned int rep_len)
+int __nf_nat_mangle_tcp_packet(struct sk_buff *skb,
+			       struct nf_conn *ct,
+			       enum ip_conntrack_info ctinfo,
+			       unsigned int match_offset,
+			       unsigned int match_len,
+			       const char *rep_buffer,
+			       unsigned int rep_len, bool adjust)
 {
 	struct rtable *rt = skb_rtable(skb);
 	struct iphdr *iph;
···
 		inet_proto_csum_replace2(&tcph->check, skb,
 					 htons(oldlen), htons(datalen), 1);
 
-	if (rep_len != match_len) {
-		set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
-		adjust_tcp_sequence(ntohl(tcph->seq),
-				    (int)rep_len - (int)match_len,
-				    ct, ctinfo);
-		nf_conntrack_event_cache(IPCT_NATSEQADJ, ct);
-	}
+	if (adjust && rep_len != match_len)
+		nf_nat_set_seq_adjust(ct, ctinfo, tcph->seq,
+				      (int)rep_len - (int)match_len);
+
 	return 1;
 }
-EXPORT_SYMBOL(nf_nat_mangle_tcp_packet);
+EXPORT_SYMBOL(__nf_nat_mangle_tcp_packet);
 
 /* Generic function for mangling variable-length address changes inside
  * NATed UDP connections (like the CONNECT DATA XXXXX MESG XXXXX INDEX XXXXX