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下面是一个C语言代码示例，可以在xdp程序内核端使用netlink监听来自用户空间的消息，并针对收到的消息调用回调函数：

#include <linux/bpf.h>
#include <linux/if_ether.h>
#include <linux/in.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/pkt_cls.h>
#include <linux/socket.h>
#include <linux/netlink.h>

#define NETLINK_USER 31
#define MAX_PAYLOAD 1024

struct nlmsghdr *nlh = NULL;
struct sk_buff *skb_out;
int user_pid;

static int receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
{
    int pid, res;
    struct sk_buff *skb_out;
    struct nlmsghdr *nlh_out;
    char msg[MAX_PAYLOAD];

    memset(msg, 0, MAX_PAYLOAD);
    nlh_out = nlmsg_put(skb_out, 0, 0, NLMSG_DONE, MAX_PAYLOAD, 0);
    NETLINK_CB(skb_out).dst_group = 0;
    strncpy(msg, "Message from kernel!", sizeof("Message from kernel!"));
    res = nlmsg_unicast(nl_sk, skb_out, pid);
    printk(KERN_INFO "Sent message to user!\n");

    return 0;
}

static void nl_callback(struct sk_buff *skb)
{
    int pid, res;
    struct nlmsghdr *nlh;
    char msg[MAX_PAYLOAD];
    int err;

    nlh = (struct nlmsghdr *)skb->data;
    printk(KERN_INFO "Received netlink message payload: %s\n", (char*)nlmsg_data(nlh));

    pid = nlh->nlmsg_pid; /*pid of sending process */

    user_pid = pid;

    skb_out = nlmsg_new(MAX_PAYLOAD, 0);
    if (!skb_out)
    {
        printk(KERN_ERR "Failed to allocate new skb\n");
        return;
    }

    receive_msg(skb, nlh);

    return;
}

int init_module()
{
    struct netlink_kernel_cfg cfg;
    cfg.groups = 0;
    cfg.input = nl_callback;

    nl_sk = netlink_kernel_create(&init_net, NETLINK_USER, &cfg);
    if (!nl_sk)
    {
        printk(KERN_ERR "Failed to create netlink socket\n");
        return -10;
    }

    return 0;
}

void cleanup_module()
{
    nlmsg_free(skb_out);
    netlink_kernel_release(nl_sk);
}