strcmp inside kernel module crash - linux-kernel

Question

I am trying to to detect the a outgoing packets in my kernel(Netfilter) module. I am using a strcmp function to achieve it. The kernel always crashes after loading my kernel module with strcmp function. I tried removing the strcmp function - loaded without any problem. I hope the problem is with all string function, I also tried strstr() - my system crashed
The logic behind this, Incoming packet will have eth[0-9]+ assigned to "in->name" and "out->name" will be and vice-versa for outgoing packet.
Any insight to detect a outgoing packet? I knew another option is to use output_hook instead of prerouting and postrouting hook. But here I want to mangle both incoming and outgoing packet in different way. Does the kernel version I am using doesn't support string function inside modules?
$ uname -a
Linux vmdsk01 2.6.32-21-generic #32-Ubuntu SMP Fri Apr 16 08:09:38 UTC 2010 x86_64 GNU/Linux
Include Part
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/init.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/skbuff.h>
#include <linux/inet.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <net/ip.h>
#include <linux/string.h>
Main Hook
31 unsigned int main_hook(unsigned int hooknum,
32 struct sk_buff *skb,
33 const struct net_device *in,
34 const struct net_device *out,
35 int (*okfn)(struct sk_buff*))
36 {
37 if( strcmp(out->name, "<NULL>") == NULL ) // Outgoing packet must not have <NULL>
38 {
39 printk( KERN_INFO "OUTGOING PACKET");
40 }
41 ....
I also tried replacing line 37 with following, my system hangs
37 if( strstr(out->name, "eth") != NULL ) // Outgoing packet must have eth[0-9]+

Answer 1

You might have NULL pointer in out struct pointer. You may add some sanity checks in main_hook like:
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*))
{
if (!out)
return -EINVAL;
if( strncmp(out->name, "<NULL>", IFNAMSIZ) == 0 ) // Outgoing packet must not have <NULL>
{
printk( KERN_INFO "OUTGOING PACKET");
}
....
So I've added check for out pointer and using strncmp instead of strcmp where IFNAMSIZ is size of out->name as defined in include/linux/netdevice.h. Also, str(n)cmp does not return NULL, it returns 0.
Check it and please provide any crash messages.

Answer 2

I understood the issue, the hook function is sequence of iterations like while(1) checking for packet. A iteration may or may not received a packet. If an iteration received a packet, the struct "out" would be available and its members could be accessible; I made a mistake by trying to access a member without checking availability of struct.
The following code fixed the purpose and working fine.
if(out)
{
if( strcmp(out->name, "<NULL>") ) // Outgoing packet must not have <NULL>
{
printk( KERN_INFO "Outgoing Packet");
}
}