// SPDX-License-Identifier: GPL-2.0-only

  #include <linux/types.h>
  #include <linux/netfilter.h>
  #include <net/tcp.h>
  
  #include <net/netfilter/nf_conntrack.h>
  #include <net/netfilter/nf_conntrack_extend.h>
  #include <net/netfilter/nf_conntrack_seqadj.h>

  int nf_ct_seqadj_init(struct nf_conn *ct, enum ip_conntrack_info ctinfo,
  		      s32 off)
  {
  	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
  	struct nf_conn_seqadj *seqadj;
  	struct nf_ct_seqadj *this_way;
  
  	if (off == 0)
  		return 0;
  
  	set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
  
  	seqadj = nfct_seqadj(ct);
  	this_way = &seqadj->seq[dir];
  	this_way->offset_before	 = off;
  	this_way->offset_after	 = off;
  	return 0;
  }
  EXPORT_SYMBOL_GPL(nf_ct_seqadj_init);

  int nf_ct_seqadj_set(struct nf_conn *ct, enum ip_conntrack_info ctinfo,
  		     __be32 seq, s32 off)
  {
  	struct nf_conn_seqadj *seqadj = nfct_seqadj(ct);
  	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
  	struct nf_ct_seqadj *this_way;
  
  	if (off == 0)
  		return 0;

  	if (unlikely(!seqadj)) {

  		WARN_ONCE(1, "Missing nfct_seqadj_ext_add() setup call
  ");

  		return 0;
  	}

  	set_bit(IPS_SEQ_ADJUST_BIT, &ct->status);
  
  	spin_lock_bh(&ct->lock);
  	this_way = &seqadj->seq[dir];
  	if (this_way->offset_before == this_way->offset_after ||

  	    before(this_way->correction_pos, ntohl(seq))) {
  		this_way->correction_pos = ntohl(seq);

  		this_way->offset_before	 = this_way->offset_after;
  		this_way->offset_after	+= off;
  	}
  	spin_unlock_bh(&ct->lock);
  	return 0;
  }
  EXPORT_SYMBOL_GPL(nf_ct_seqadj_set);
  
  void nf_ct_tcp_seqadj_set(struct sk_buff *skb,
  			  struct nf_conn *ct, enum ip_conntrack_info ctinfo,
  			  s32 off)
  {
  	const struct tcphdr *th;
  
  	if (nf_ct_protonum(ct) != IPPROTO_TCP)
  		return;
  
  	th = (struct tcphdr *)(skb_network_header(skb) + ip_hdrlen(skb));
  	nf_ct_seqadj_set(ct, ctinfo, th->seq, off);
  }
  EXPORT_SYMBOL_GPL(nf_ct_tcp_seqadj_set);
  
  /* Adjust one found SACK option including checksum correction */
  static void nf_ct_sack_block_adjust(struct sk_buff *skb,
  				    struct tcphdr *tcph,
  				    unsigned int sackoff,
  				    unsigned int sackend,
  				    struct nf_ct_seqadj *seq)
  {
  	while (sackoff < sackend) {
  		struct tcp_sack_block_wire *sack;
  		__be32 new_start_seq, new_end_seq;
  
  		sack = (void *)skb->data + sackoff;
  		if (after(ntohl(sack->start_seq) - seq->offset_before,
  			  seq->correction_pos))
  			new_start_seq = htonl(ntohl(sack->start_seq) -
  					seq->offset_after);
  		else
  			new_start_seq = htonl(ntohl(sack->start_seq) -
  					seq->offset_before);
  
  		if (after(ntohl(sack->end_seq) - seq->offset_before,
  			  seq->correction_pos))
  			new_end_seq = htonl(ntohl(sack->end_seq) -
  				      seq->offset_after);
  		else
  			new_end_seq = htonl(ntohl(sack->end_seq) -
  				      seq->offset_before);

  		pr_debug("sack_adjust: start_seq: %u->%u, end_seq: %u->%u
  ",
  			 ntohl(sack->start_seq), ntohl(new_start_seq),
  			 ntohl(sack->end_seq), ntohl(new_end_seq));

  
  		inet_proto_csum_replace4(&tcph->check, skb,

  					 sack->start_seq, new_start_seq, false);

  		inet_proto_csum_replace4(&tcph->check, skb,

  					 sack->end_seq, new_end_seq, false);

  		sack->start_seq = new_start_seq;
  		sack->end_seq = new_end_seq;
  		sackoff += sizeof(*sack);
  	}
  }
  
  /* TCP SACK sequence number adjustment */
  static unsigned int nf_ct_sack_adjust(struct sk_buff *skb,
  				      unsigned int protoff,

  				      struct nf_conn *ct,
  				      enum ip_conntrack_info ctinfo)
  {

  	struct tcphdr *tcph = (void *)skb->data + protoff;

  	struct nf_conn_seqadj *seqadj = nfct_seqadj(ct);

  	unsigned int dir, optoff, optend;

  
  	optoff = protoff + sizeof(struct tcphdr);
  	optend = protoff + tcph->doff * 4;

  	if (skb_ensure_writable(skb, optend))

  		return 0;

  	tcph = (void *)skb->data + protoff;

  	dir = CTINFO2DIR(ctinfo);
  
  	while (optoff < optend) {
  		/* Usually: option, length. */
  		unsigned char *op = skb->data + optoff;
  
  		switch (op[0]) {
  		case TCPOPT_EOL:
  			return 1;
  		case TCPOPT_NOP:
  			optoff++;
  			continue;
  		default:
  			/* no partial options */
  			if (optoff + 1 == optend ||
  			    optoff + op[1] > optend ||
  			    op[1] < 2)
  				return 0;
  			if (op[0] == TCPOPT_SACK &&
  			    op[1] >= 2+TCPOLEN_SACK_PERBLOCK &&
  			    ((op[1] - 2) % TCPOLEN_SACK_PERBLOCK) == 0)
  				nf_ct_sack_block_adjust(skb, tcph, optoff + 2,
  							optoff+op[1],
  							&seqadj->seq[!dir]);
  			optoff += op[1];
  		}
  	}
  	return 1;
  }
  
  /* TCP sequence number adjustment.  Returns 1 on success, 0 on failure */
  int nf_ct_seq_adjust(struct sk_buff *skb,
  		     struct nf_conn *ct, enum ip_conntrack_info ctinfo,
  		     unsigned int protoff)
  {
  	enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
  	struct tcphdr *tcph;
  	__be32 newseq, newack;
  	s32 seqoff, ackoff;
  	struct nf_conn_seqadj *seqadj = nfct_seqadj(ct);
  	struct nf_ct_seqadj *this_way, *other_way;

  	int res = 1;

  
  	this_way  = &seqadj->seq[dir];
  	other_way = &seqadj->seq[!dir];

  	if (skb_ensure_writable(skb, protoff + sizeof(*tcph)))

  		return 0;
  
  	tcph = (void *)skb->data + protoff;
  	spin_lock_bh(&ct->lock);
  	if (after(ntohl(tcph->seq), this_way->correction_pos))
  		seqoff = this_way->offset_after;
  	else
  		seqoff = this_way->offset_before;

  	newseq = htonl(ntohl(tcph->seq) + seqoff);
  	inet_proto_csum_replace4(&tcph->check, skb, tcph->seq, newseq, false);
  	pr_debug("Adjusting sequence number from %u->%u
  ",
  		 ntohl(tcph->seq), ntohl(newseq));
  	tcph->seq = newseq;
  
  	if (!tcph->ack)
  		goto out;

  	if (after(ntohl(tcph->ack_seq) - other_way->offset_before,
  		  other_way->correction_pos))
  		ackoff = other_way->offset_after;
  	else
  		ackoff = other_way->offset_before;

  	newack = htonl(ntohl(tcph->ack_seq) - ackoff);

  	inet_proto_csum_replace4(&tcph->check, skb, tcph->ack_seq, newack,
  				 false);

  	pr_debug("Adjusting ack number from %u->%u, ack from %u->%u
  ",

  		 ntohl(tcph->seq), ntohl(newseq), ntohl(tcph->ack_seq),
  		 ntohl(newack));

  	tcph->ack_seq = newack;

  	res = nf_ct_sack_adjust(skb, protoff, ct, ctinfo);

  out:

  	spin_unlock_bh(&ct->lock);
  
  	return res;
  }
  EXPORT_SYMBOL_GPL(nf_ct_seq_adjust);
  
  s32 nf_ct_seq_offset(const struct nf_conn *ct,
  		     enum ip_conntrack_dir dir,
  		     u32 seq)
  {
  	struct nf_conn_seqadj *seqadj = nfct_seqadj(ct);
  	struct nf_ct_seqadj *this_way;
  
  	if (!seqadj)
  		return 0;
  
  	this_way = &seqadj->seq[dir];
  	return after(seq, this_way->correction_pos) ?
  		 this_way->offset_after : this_way->offset_before;
  }
  EXPORT_SYMBOL_GPL(nf_ct_seq_offset);

  static const struct nf_ct_ext_type nf_ct_seqadj_extend = {

  	.len	= sizeof(struct nf_conn_seqadj),
  	.align	= __alignof__(struct nf_conn_seqadj),
  	.id	= NF_CT_EXT_SEQADJ,
  };
  
  int nf_conntrack_seqadj_init(void)
  {
  	return nf_ct_extend_register(&nf_ct_seqadj_extend);
  }
  
  void nf_conntrack_seqadj_fini(void)
  {
  	nf_ct_extend_unregister(&nf_ct_seqadj_extend);
  }