Question

I am trying to write a module which is similar to ping. The problem is coming in receiving the packet. The code is shown below:

#include<asm/uaccess.h>
#include<linux/sched.h>
#include<linux/wait.h>
#include<linux/netfilter.h>
#include<linux/netfilter_ipv4.h>

struct net_device *dev;
struct sk_buff *skb;
struct napi_struct *rnapi;

int rrpoll(struct napi_struct *napi,int budget)
{
   printk(KERN_ALERT"we are HEARRR%s,%d\n",__func__,__LINE__);
   skb = (struct sk_buff *)kmalloc(sizeof(struct sk_buff *),GFP_KERNEL);
   netif_receive_skb(skb);
   kfree(skb);
   return 0;
  }

static int rec_init(void)
{
  dev =  __dev_get_by_name(&init_net,"wlan0");
  rnapi = (struct napi_struct *)kmalloc(sizeof(struct napi_struct *),GFP_KERNEL);
  netif_napi_add(dev,rnapi,&rrpoll,100);
  napi_schedule(rnapi);
  return 0;
  }

static void rec_exit(void)
{
  //napi_complete(rnapi);
    netif_napi_del(rnapi);
}

The modules compiles and runs without any error, but the whole system hangs after some time.

Answer 1

I see a couple of problems with your sample code:

1. the skb should be allocated by alloc_skb() not kmalloc
2. after you call netif_receive_skb, you should not free the skb. Your code is no longer the owner of that skb since you’ve given it to the network stack.
3. I don’t see you filling in the skb in any way. Maybe you are just simplifying the code. But as it is now, you are sending up an uninitialized (or if you use alloc_skb, initialized but empty) skb to the networking stack.

Michael