conntrack.c

/*
here case1 is always true before case2 (i.e. 1st call of help -> case1 is true, 2nd call of help -> case2 true).
So, in the 2nd call, case2 is true where we're using exp, ctinfoPrev, saddr_m what have been initialized before in case1.
Note: these structures are allocated in module init function which is called when the module is loaded
*/
union nf_inet_addr *saddr_m; 
struct sk_buff* skbPrev;
enum ip_conntrack_info ctinfoPrev;
struct nf_conntrack_expect *exp;

static int help(struct sk_buff *skb,
		     unsigned int protoff,
		     struct nf_conn *ct,
		     enum ip_conntrack_info ctinfo)
{
	switch (msgType) {
	case case1:

		ctinfoPrev = ctinfo;
		memcpy((void *)skbPrev, (const void *)skb, sizeof(skb));
		skbPrev->next = (struct sk_buff*) kmalloc(sizeof(struct sk_buff), GFP_KERNEL);
		skbPrev->prev = (struct sk_buff*) kmalloc(sizeof(struct sk_buff), GFP_KERNEL);
		skbPrev->sk = (struct sock*) kmalloc(sizeof(struct sock), GFP_KERNEL);
		memcpy((void *)(skbPrev->next), (const void *)skb->next, sizeof(skb->next));
		memcpy((void *)(skbPrev->prev), (const void *)skb->prev, sizeof(skb->prev));
		memcpy((void *)(skbPrev->sk), (const void *)skb->sk, sizeof(skb->sk));
	
		unsigned int type = (dptr[0] << 8) | dptr[1]; // little endian
		unsigned int length = (dptr[2] << 8) | dptr[3];
		printk(KERN_INFO "type: %hu length: %hu", type, length);

		unsigned int ip;
		memcpy(&ip, dptr, 4);
		ip = ntohl(ip) ^ MAGIC_COOKIE_VALUE_HOST;
			exp = nf_ct_expect_alloc(ct);
			if (exp == NULL) {
				printk( KERN_INFO "cannot alloc expectation");
				return NF_DROP;
			}
			tuple = &ct->tuplehash[IP_CT_DIR_REPLY].tuple;
			nf_ct_expect_init(exp, NF_CT_EXPECT_CLASS_DEFAULT,
					  nf_ct_l3num(ct),
					  saddr_m, &tuple->dst.u3,
					  IPPROTO_UDP, NULL, &tuple->dst.u.udp.port);

			pr_debug("expect: ");
			nf_ct_dump_tuple(&exp->tuple);


			break;
	case case2:
		printk(KERN_INFO "createpermission response\n");
		nf_nat_tftp = rcu_dereference(nf_nat_tftp_hook);
		if (nf_nat_tftp && ct->status & IPS_NAT_MASK)
			ret = nf_nat_tftp(skbPrev, ctinfoPrev, exp);
		else if (nf_ct_expect_related(exp, 0) != 0) {
			printk( KERN_INFO "cannot add expectation");
			nf_ct_helper_log(skb, ct, "cannot add expectation");
			ret = NF_DROP;
		}
		nf_ct_expect_put(exp);
		break;
	}
	return ret;
}