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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> Kesava Srinivas [mailto:vunnavafuture@gmail.com] <br><b>Sent:</b> Monday, November 21, 2011 11:32 AM<br><b>To:</b> Jeff Haran<br><b>Cc:</b> kernelnewbies@kernelnewbies.org<br><b>Subject:</b> Re: sock_buff is NULL !<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='font-family:"Courier New"'>Thanks Jeff for the reply. Yes; after changing the double pointer to a Normal Pointer., I can see that ., "if" block was skipped. <br><br>-Thnx,<br>VKS</span><o:p></o:p></p><div><p class=MsoNormal>On Tue, Nov 22, 2011 at 12:50 AM, Jeff Haran <<a href="mailto:jharan@bytemobile.com">jharan@bytemobile.com</a>> wrote:<o:p></o:p></p><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> </span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> </span><o:p></o:p></p><div style='border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt'><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> <a href="mailto:kernelnewbies-bounces@kernelnewbies.org" target="_blank">kernelnewbies-bounces@kernelnewbies.org</a> [mailto:<a href="mailto:kernelnewbies-bounces@kernelnewbies.org" target="_blank">kernelnewbies-bounces@kernelnewbies.org</a>] <b>On Behalf Of </b>Kesava Srinivas<br><b>Sent:</b> Monday, November 21, 2011 11:03 AM<br><b>To:</b> <a href="mailto:kernelnewbies@kernelnewbies.org" target="_blank">kernelnewbies@kernelnewbies.org</a><br><b>Subject:</b> sock_buff is NULL !</span><o:p></o:p></p></div></div><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'> <o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-family:"Courier New"'>HI Team,</span><br><br><span style='font-family:"Courier New"'>Trying to make this code work; but some how it is entering the if condition mentioned below.sock_buff is still a NULL Pointer.I can see that Function was registered with the Ipv4 Hooks. Any ideas/scenarios where the sock_buff remains as NULL Pointer??? Testing this code by trying to do ssh with this machine.<br><br>Any Help is really appreciated !!<br><br>#########<br>static struct nf_hook_ops netfilter_ops;<br>struct sk_buff *sock_buff;<br><br>unsigned int main_hook(unsigned int hooknum,struct sk_buff **skb,const struct net_device *in,const struct net_device *out,int (*okfn)(struct sk_buff*)) {<br> sock_buff = *skb;<br> if(!sock_buff){<br> printk(KERN_ERR "Entered unexpected loop \n");<br> return NF_ACCEPT;<br> }/* Check for sk_buff NULL */<br>}<br>int init_module()<br>{<br> netfilter_ops.hook = (nf_hookfn *) main_hook;<br> netfilter_ops.hooknum = NF_INET_PRE_ROUTING;<br> <a href="http://netfilter_ops.pf" target="_blank">netfilter_ops.pf</a> = PF_INET;<br> netfilter_ops.priority = NF_IP_PRI_FIRST;<br> nf_register_hook(&netfilter_ops);<br> <br>}<br>######################<br><br><br>-Thnx,<br>VKS</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> </span><o:p></o:p></p></div></div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Which kernel version are you using? The function prototype of a netfilter hook does vary from kernel version to kernel version, but in the kernels I’ve seen the skb parameter is a typed sk_buff *skb, not sk_buff **skb. In other words, you get passed a pointer to the sk_buff, not the address of a pointer to an sk_buff.</span><o:p></o:p></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> </span><span style='color:#888888'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Jeff Haran</span><span style='color:#888888'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> </span><span style='color:#888888'><o:p></o:p></span></p></div></div></div></div><p class=MsoNormal style='margin-bottom:12.0pt'><br><br clear=all><br>-- <br><span style='font-size:10.0pt;font-family:"Courier New"'>-VKS</span><span style='font-family:"Courier New"'><br>Masters,Computer Science&Engineering.<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I am a bit surprised that you didn’t get a compiler warning when you built this. Type checking is your friend. 8^)<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Jeff Haran<o:p></o:p></span></p><p class=MsoNormal style='margin-bottom:12.0pt'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p></div></div><font face="monospace"></font></body></html>