Mdthbs

I am trying to write a network device driver that can duplicate the functionality of the loopback device. I modified the code available at the link. I am testing on Linux 4.8. I removed code related to stats and loopback registration with network namespace. For this variable, I couldn't find the definition, so for now its removed.

/* testlo.c */



#include <linux/kernel.h>

#include <linux/jiffies.h>

#include <linux/module.h>

#include <linux/interrupt.h>

#include <linux/fs.h>

#include <linux/types.h>

#include <linux/string.h>

#include <linux/socket.h>

#include <linux/errno.h>

#include <linux/fcntl.h>

#include <linux/in.h>



#include <asm/uaccess.h>

#include <asm/io.h>



#include <linux/inet.h>

#include <linux/netdevice.h>

#include <linux/etherdevice.h>

#include <linux/skbuff.h>

#include <linux/ethtool.h>

#include <net/sock.h>

#include <net/checksum.h>

#include <linux/if_ether.h> /* For the statistics structure. */

#include <linux/if_arp.h>   /* For ARPHRD_ETHER */

#include <linux/ip.h>

#include <linux/tcp.h>

#include <linux/percpu.h>

#include <net/net_namespace.h>

#include <linux/u64_stats_sync.h>



/*

 * The higher levels take care of making this non-reentrant (it's

 * called with bh's disabled).

 */

netdev_tx_t loopback_xmit(struct sk_buff *skb,

             struct net_device *dev)

{

    //struct pcpu_lstats *lb_stats;

    int len;



    skb_orphan(skb);



    /* Before queueing this packet to netif_rx(),

     * make sure dst is refcounted.

     */

    skb_dst_force(skb);



    skb->protocol = eth_type_trans(skb, dev);



    len = skb->len;

    if (likely(netif_rx(skb) == NET_RX_SUCCESS)) { }



    printk("Received packet at device driver n");



    return NETDEV_TX_OK;

}



u32 always_on(struct net_device *dev)

{

    return 1;

}



const struct ethtool_ops loopback_ethtool_ops = {

    .get_link       = always_on,

};



int loopback_dev_init(struct net_device *dev)

{

    return 0;

}



void loopback_dev_free(struct net_device *dev)

{

    free_netdev(dev);

}



const struct net_device_ops loopback_ops = {

    .ndo_init      = loopback_dev_init,

    .ndo_start_xmit= loopback_xmit,

    .ndo_set_mac_address = eth_mac_addr,

};



/*

 * The loopback device is special. There is only one instance

 * per network namespace.

 */

void loopback_setup(struct net_device *dev)

{

    dev->mtu        = 64 * 1024;

    dev->hard_header_len    = ETH_HLEN; /* 14   */

    dev->addr_len       = ETH_ALEN; /* 6    */

    dev->type       = ARPHRD_LOOPBACK;  /* 0x0001*/

    dev->flags      = IFF_LOOPBACK;

    dev->priv_flags     |= IFF_LIVE_ADDR_CHANGE | IFF_NO_QUEUE;

    netif_keep_dst(dev);

    dev->hw_features    = NETIF_F_GSO_SOFTWARE;

    dev->features       = NETIF_F_SG | NETIF_F_FRAGLIST

        | NETIF_F_GSO_SOFTWARE

        | NETIF_F_HW_CSUM

        | NETIF_F_RXCSUM

        | NETIF_F_SCTP_CRC

        | NETIF_F_HIGHDMA

        | NETIF_F_LLTX

        | NETIF_F_NETNS_LOCAL

        | NETIF_F_VLAN_CHALLENGED

        | NETIF_F_LOOPBACK;

    dev->ethtool_ops    = &loopback_ethtool_ops;

    dev->netdev_ops     = &loopback_ops;

    dev->destructor     = loopback_dev_free;

}



struct net_device *global_dev;



/* Setup and register the loopback device. */

int init_dev (void)

{

    struct net_device *dev;

    int err;



    printk ("Init Modulen");



    err = -ENOMEM;

    dev = alloc_netdev(0, "testlo", NET_NAME_UNKNOWN, loopback_setup);

    if (!dev)

        goto out;



    err = register_netdev(dev);

    if (err)

        goto out_free_netdev;



    BUG_ON(dev->ifindex != LOOPBACK_IFINDEX);   

    global_dev = dev; 



    return 0;



out_free_netdev:

    free_netdev(dev);

out:

    return err;

}



void cleanup (void)

{

    printk ("Cleaning Up Modulen");

    unregister_netdev (global_dev);

    return;

}



module_init (init_dev);

module_exit (cleanup);

Makefile:

obj-m += testlo.o



all:

      make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules



clean:

      make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Once I compile the code and insert the kernel module, I can see my new device (testlo) in the list from ip a. I can set the device state to up and assign it an IP address. However, the code's function loopback_xmit() doesn't get triggered if I try to ping that IP (I do see ping responses). If I check in wireshark, there is no trace if I ping the new device's IP. In the default loopback device (lo), pinging 127.0.0.1 shows a trace in wireshark.

Can someone tell what is missing in the code to make it behave like a loopback device ?