How to access data/payload from tcphdr (sk_buff) struct on debian 64 bits?

Question

I'm working on a small firewall, i had to retrieve the datas from each tcp packet from port 80 (http) for parsing them. This code works well on a debian 32 bits virtual machine, i'm able to print the headers of each web page, but when i try to load my kernel module and to transfer some datas through the http port, it prints no datas.

When i compile, it shows those warnings only on my 64bits computer :

/home/dev3/C/FIREWALL/firewall.c: In function ‘hook_func’:
/home/dev3/C/FIREWALL/firewall.c:179: warning: cast from pointer to integer of different size
/home/dev3/C/FIREWALL/firewall.c:179: warning: cast to pointer from integer of different size

Anyone got any ideas, please ?

Thanks.

Code :

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/netfilter.h>

#undef __KERNEL__
#include <linux/netfilter_ipv4.h>
#define __KERNEL__

#include <linux/ip.h>
#include <linux/tcp.h>

static struct nf_hook_ops nfho;

unsigned int hook_func( unsigned int hooknum,
                    struct sk_buff * skb,
                    const struct net_device * in,
                    const struct net_device * out,
                    int (*okfn)(struct sk_buff *))
{
    struct iphdr    * iph;
    struct tcphdr   * tcph;
    unsigned char   * http_port = "\x00\x50";
    char            * data;

    if (skb)
    {
        iph = ip_hdr(skb);

        if (iph && iph->protocol && (iph->protocol == IPPROTO_TCP))
        {
            tcph = (struct tcphdr *)((__u32 *)iph + iph->ihl);

            if ((tcph->source) == *(unsigned short *)http_port)
            {
                data    = (char *)((int)tcph + (int)(tcph->doff * 4));

                printk(KERN_DEBUG "TCP source : %hu, TCP  dest : %hu\n", ntohs(tcph->source), ntohs(tcph->dest));
                printk(KERN_DEBUG "TCP seq : %u, TCP ack_seq : %u\n", ntohl(tcph->seq), ntohl(tcph->ack_seq));
                printk(KERN_DEBUG "TCP doff : %hu, TCP window : %hu\n", ntohs(tcph->doff), ntohs(tcph->window));
                printk(KERN_DEBUG "TCP check : 0x%hx, TCP urg_ptr : %hu\n", ntohs(tcph->check), ntohs(tcph->urg_ptr));
                printk(KERN_DEBUG "FLAGS=%c%c%c%c%c%c\n\n",
                        tcph->urg ? 'U' : '-',
                        tcph->ack ? 'A' : '-',
                        tcph->psh ? 'P' : '-',
                        tcph->rst ? 'R' : '-',
                        tcph->syn ? 'S' : '-',
                        tcph->fin ? 'F' : '-');
                //printk(KERN_DEBUG "sending packet to : %pI4\n", &iph->daddr);
                printk(KERN_DEBUG "data len : %d\ndata : \n", (int) strlen(data));
                printk(KERN_DEBUG "%s\n", data);
            }
        }
    }

    return NF_ACCEPT;
}

int init_module()
{
    int result;

    nfho.hook   = (nf_hookfn *) hook_func;
    nfho.hooknum    = NF_IP_POST_ROUTING;
    nfho.pf     = PF_INET;
    nfho.priority   = NF_IP_PRI_FIRST;

    result = nf_register_hook(&nfho);

    if(result)
    {
        printk(KERN_DEBUG "firewall : erreur nf_register_hook !\n");
        return 1;
    }

    printk(KERN_DEBUG "firewall : module charge.\n");

    return 0;
}

void cleanup_module()
{
    nf_unregister_hook(&nfho);
    printk(KERN_DEBUG "firewall : module decharge.\n");
}

Solution

(char *)((int)tcph + (int)(tcph->doff * 4)); is wrong, it should be

(char *)((unsigned char *)tcph + (tcph->doff * 4));

Note that printk(KERN_DEBUG "data len : %d\ndata : \n", (int) strlen(data)); is not at all a safe thing to do. You don't know if the data contains text, and if it does, it might not contain a 0 terminated string which strlen(data) would need to work as you expect.

You probably need to care about endianess as well, when comparing the ports and possibly other fields.

Comments

[Zzz0_o]

Someone please explain me Why we are adding tcph with tcph->doff*4. Since doff is already telling us about the tcp header length.

[Zzz0_o]

How it is different from [IP Total Length] - ( ([IP IHL] + [TCP Data offset]) * 4 )??