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Some Help Needed With Programming My Kernel Module
Subject: Some Help Needed With Programming My Kernel Module
Author: kannerke    Posted: 2005-04-19 15:24:37    Length: 6,624 byte(s)
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I'm programming a kernel module to intercept certain UDP-packets and add some new headers to them.  The new headers

must come before the original IP-header.  So, after the operation, I want to send a packet like this:

new IP header|new UPD header|overlay header|original IP header|original UDP header|original payload

When I try my code, I get a kernel panic.  I can't find what I'm doing wrong here.

Here's a fragment of my code:


   if ((sk->nh.iph->protocol == IPPROTO_UDP) && (enter_network)) {
      print_string("UDP datagram entering overlay network");
      /* allocate space for temporary headers */
      tmp_iph = kmalloc(sizeof(struct iphdr),GFP_NFS);
      tmp_udph = kmalloc(sizeof(struct udphdr),GFP_NFS);
      /* copy original IP header */
      __memcpy(tmp_iph,sk->nh.iph,sizeof(struct iphdr));
      /* copy original UDP header */
      __memcpy(tmp_udph,(struct udphdr *)(sk->data +(sk->nh.iph->ihl * 4)),sizeof(struct udphdr));

      overlayh = (struct overlayhdr *)skb_push(sk,sizeof(struct overlayhdr));
      //complete overlay-header
      overlayh->qos0 = qos0;
      overlayh->qos1 = qos1;
      overlayh->dst = tmp_iph->daddr;         
      overlayh->dstport = tmp_udph->dest;
      overlayh->lsthp = *(unsigned int*)lasthop_ip;
               /*expand for new udp header */
               udph = (struct udphdr *)skb_push(sk,sizeof(struct udphdr));
      //complete udp-header
      udph->source = tmp_udph->source;
      udph->dest = htons(overlay_port);
      udph->len = htons(htons(tmp_iph->tot_len) + sizeof(struct udphdr) + sizeof(struct overlayhdr));
      udph->check = 0;
      print_string("SFSG 7\n");

      /* calculate checksum */
      udph->check = csum ((unsigned short *)udph, htons(udph->len) >> 1);
      /* expand for new ip header */
      iph = (struct iphdr *)skb_push(sk,sizeof(struct iphdr));
      //complete new ip-header
      iph->ihl = 5;
      iph->version = 4;
      iph->tos = 0;
      iph->tot_len = htons(htons(tmp_iph->tot_len) + sizeof(struct iphdr) + sizeof(struct udphdr) +

sizeof(struct overlayhdr));
      iph->id = tmp_iph->id;
      iph->frag_off = tmp_iph->frag_off;
      iph->ttl = tmp_iph->ttl;
      iph->protocol = 17;      /* UDP */
      iph->check = 0;   /* set it to 0 before computing the actual checksum later */
      iph->saddr = tmp_iph->saddr;
      iph->daddr = *(unsigned int*)overlay_ip;
      /* calculate checksum */
      iph->check = csum ((unsigned short *)iph, sizeof(struct iphdr) >> 1);
      //free used memory
      return NF_ACCEPT;


/* Initialisation routine */
static int hooks_init() {
   /* Fill in our hook structure */
   nfho.hook      = hook_func;         /* Handler function */
   nfho.hooknum   = NF_IP_PRE_ROUTING; /* First hook for IPv4 */
   nfho.pf        = PF_INET;
   nfho.priority  = NF_IP_PRI_FIRST;   /* Make our function first */
   return 0;

/* Cleanup routine */
static void hooks_exit() {


enter_network = 1 when the packet needs to be expanded.
sk is a (struct sk_buff *)

I guess I'm doing some things wrong because of the kernel panic I get, but I don't know what I did wrong.  I think

it has something to do with the skb_push operation.  All help is welcome or when someone has a good example for

doing this kind of operations, please let me know

[Original] [Print] [Top]
Subject: Some Help Needed With Programming My Kernel Module
Author: x86processor    Posted: 2005-04-20 04:21:03    Length: 168 byte(s)
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You can put printk statements after every statement and see where you hit the kernel panic. Not the best debugging solution, but, something you can try,

Linux is the kernel. The entire system is called GNU/Linux.

My domain: shakthimaan.com (Offline)
orkut ID: shakthimaan
IRC nick: mbuf
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Subject: Some Help Needed With Programming My Kernel Module
Author: kannerke    Posted: 2005-04-24 15:48:38    Length: 806 byte(s)
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I could solve the problem, but now I allready have another one.

first, I'll tell how I solved it: instead of a push, I used the put-operation.  Then I copied the whole packet to memory and copied it back to de sk_buff, but with an offset, as big as the size of the header I want to put in front of the packet.  I'll try to change this with an operation that can copy memory that overlaps.

The problem I have now is that the modified packet doesn't arrive.  There are two possibilities I can find: 1) the checksum is't correct, 2) there's a problem with the way I copy the original headers (because I use this data to fill in the new headers), but I checked some fields (daddr and ttl from iphdr and dest from udphdr and they were ok.
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