在 linux kernel 中发送数据包

Question

I wrote a module to send a packet in kernel-space. but after insmod it gives a segmentation fault error. I have tried to change some parts of it but I still get errors.
the code:

//libraries

#define IP_Header_RM 20
#define UDP_Header_RM 8

static int __init init_Module(void){
    unsigned char *Data = "Test_Packet";
    int i = strlen(Data);
    struct sk_buff* skb = alloc_skb(ETH_HLEN + IP_Header_RM + UDP_Header_RM + i, GFP_ATOMIC);
    struct iphdr* iph = (struct iphdr*)skb_push(skb, IP_Header_RM);
    struct ethhdr* eth = (struct ethhdr*)skb_push(skb, sizeof (struct ethhdr));
    struct udphdr* uh = (struct udphdr*)skb_push(skb, UDP_Header_RM);
    struct net_device *Device;
    uint16_t proto;
    uint8_t Mac_Addr[ETH_ALEN] = {0x38, 0xd5, 0x47, 0xa1, 0x07, 0x41};
    Data = skb_put(skb, i);
    skb_reserve(skb, ETH_HLEN);
    Device = dev_get_by_name(&init_net,"enp0s3");
    proto = ETH_P_IP;
    uh->len = htons(i); 
    uh->source = htons(2121);
    uh->dest = htons(2121);

    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len= htons(IP_Header_RM + i); 
    iph->frag_off = 0; 
    iph->ttl = 64;
    iph->protocol = IPPROTO_UDP;
    iph->check = 0; 
    iph->saddr = 19216805;
    iph->daddr = 19216804;
    skb->protocol = eth->h_proto = htons(proto);
    skb->no_fcs = 1;
    memcpy(eth->h_source, Device->dev_addr, ETH_ALEN);
    memcpy(eth->h_dest, Mac_Addr, ETH_ALEN);
    

    skb->pkt_type = PACKET_OUTGOING;
    dev_queue_xmit(skb);
    return 1;
    }

static void __exit exit_Module(void){
    printk(KERN_INFO "Done");
    }
 
module_init(init_Module);
module_exit(exit_Module);

which parts I made mistake?
Thanks in Advance

Answer 1

You need to do skb_reserve() up front on the allocated buffer, before doing any skb_put() calls for user data or skb_push() for eth/IP headers. You were segfaulting trying to skb_push() first, on a buffer without proper reservations. I also had a few other suggestions for you:

1. Include your complete source next time!
2. Rearrange the order of your pushed headers to make a legit UDP/IP packet
3. dev_get_by_name() may fail; should check before trying to memcpy to its buffer
4. Push the user data before any eth/IP header(s)
5. Return 0 rather than 1 from a module's init() to indicate success

A tutorial page like this may help to put it all together: http://vger.kernel.org/~davem/skb_data.html The code below does not segfault on my Debian 10 system with a 4.19 Linux kernel.

If this answer helps you, please mark as the accepted one - thanks.

//libraries

#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/ethtool.h>
#include <linux/module.h>
#include <linux/virtio.h>
#include <linux/virtio_net.h>
#include <linux/bpf.h>
#include <linux/bpf_trace.h>
#include <linux/scatterlist.h>
#include <linux/if_vlan.h>
#include <linux/slab.h>
#include <linux/cpu.h>
#include <linux/average.h>
#include <linux/filter.h>
#include <linux/kernel.h>
#include <linux/pci.h>
#include <net/route.h>
#include <net/xdp.h>
#include <net/net_failover.h>

#define IP_Header_RM 20
#define UDP_Header_RM 8

static int __init init_Module(void){
    unsigned char *Data = "Test_Packet";
    int i = strlen(Data);
    struct sk_buff* skb = alloc_skb(ETH_HLEN + IP_Header_RM + UDP_Header_RM + i, GFP_ATOMIC);
    struct net_device *Device;
    uint16_t proto;
    struct iphdr* iph;
    struct ethhdr* eth;
    struct udphdr* uh;
    uint8_t Mac_Addr[ETH_ALEN] = {0x38, 0xd5, 0x47, 0xa1, 0x07, 0x41};

    skb_reserve(skb, ETH_HLEN + IP_Header_RM + UDP_Header_RM + i);
    Data = skb_put(skb, i);
    iph = (struct iphdr*)skb_push(skb, IP_Header_RM);
    uh = (struct udphdr*)skb_push(skb, UDP_Header_RM);
    eth = (struct ethhdr*)skb_push(skb, sizeof (struct ethhdr));

    Device = dev_get_by_name(&init_net,"enp0s3");
    if (Device == NULL) {
        printk(KERN_INFO "init_Module: no such device enp0s3\n");
        return 1;
    }
    proto = ETH_P_IP;
    uh->len = htons(i); 
    uh->source = htons(2121);
    uh->dest = htons(2121);

    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len= htons(IP_Header_RM + i); 
    iph->frag_off = 0; 
    iph->ttl = 64;
    iph->protocol = IPPROTO_UDP;
    iph->check = 0; 
    iph->saddr = 19216805;
    iph->daddr = 19216804;
    skb->protocol = eth->h_proto = htons(proto);
    skb->no_fcs = 1;
    memcpy(eth->h_source, Device->dev_addr, ETH_ALEN);
    memcpy(eth->h_dest, Mac_Addr, ETH_ALEN);


    skb->pkt_type = PACKET_OUTGOING;
    dev_queue_xmit(skb);
    return 0;
    }

static void __exit exit_Module(void){
    printk(KERN_INFO "Done");
    }
 
module_init(init_Module);
module_exit(exit_Module);

MODULE_DESCRIPTION("Stack Overflow 66846959");
MODULE_LICENSE("GPL");