Network hooks hanging the system - linux-kernel

Question

I was testing the network hook code given in https://en.wikipedia.org/wiki/Hooking . My kernel version is 3.11.
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/skbuff.h>
#include <net/ip.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/in.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
/* Port we want to drop packets on */
static const uint16_t port = 25;
/* This is the hook function itself */
static unsigned int hook_func(unsigned int hooknum,
struct sk_buff **pskb,
const struct net_device *in,
const struct net_device *out,
int (*okfn)(struct sk_buff *)){
struct iphdr *iph = ip_hdr(*pskb);
struct tcphdr *tcph, tcpbuf;
if (iph->protocol != IPPROTO_TCP)
return NF_ACCEPT;
tcph = skb_header_pointer(*pskb, ip_hdrlen(*pskb), sizeof(*tcph), &tcpbuf);
if (tcph == NULL)
return NF_ACCEPT;
return (tcph->dest == port) ? NF_DROP : NF_ACCEPT;
}
/* Used to register our hook function */
static struct nf_hook_ops nfho = {
.hook = hook_func,
.hooknum = NF_INET_PRE_ROUTING,
.pf = NFPROTO_IPV4,
.priority = NF_IP_PRI_FIRST,
};
static __init int my_init(void)
{
return nf_register_hook(&nfho);
}
static __exit void my_exit(void)
{
nf_unregister_hook(&nfho);
}
module_init(my_init);
module_exit(my_exit);
But after loading , the above code is freezing the system. Can anybody please tell, what is the reason for that?. I think that the above code might have already tested in some lower versions of kernel. So I doubt, some kernel parameters or features have been changed.