netlink 内核与用户层互通信

skb常用操作函数，比较容易弄混

skb_put : skb->tail += len, skb->len += len

skb_pull: skb->data += len, skb->len -= len
skb_push: skb->data -= len, skb->len += len

内核版本Linux2.6.38，编译环境gcc 4.4.4，centos6.0

内核模块代码：

/*
 * author: [email protected]
 * date  : 2011-10-29
 */
#include <linux/module.h>
#include <linux/netlink.h>
#include <linux/sched.h>
#include <net/sock.h>
#include <linux/proc_fs.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>
#include <linux/ip.h>
#include <linux/tcp.h>
#include <linux/icmp.h>
#include <linux/udp.h>

#define NETLINK_TEST 30

/* 调试信息 */
#define LOGMSG(fmt, arg...) \
do{ \
    printk("[func:%s,line:%d]: "fmt, __FUNCTION__, __LINE__, ##arg); \
}while(0)
/* 错误信息 */
#define LOGERR(fmt, arg...) \
do{ \
    printk("[func:%s,line:%d]: "fmt, __FUNCTION__, __LINE__, ##arg); \
}while(0)
/* 断言 */
#define ASSERT(expr) \
if (unlikely(!(expr))) { \
    printk("Assertion failed! %s,%s,%s,line=%d\n", \
    #expr, __FILE__, __func__, __LINE__); \
}
/* 消息最大值 */
#define MAX_MSG_LEN 1024
enum{
    NLMSG_TYPE_NONE = 0,
    NLMSG_TYPE_SETPID,  /* 设置PID */
    NLMSG_TYPE_KERNEL,  /* 消息来自内核 */
    NLMSG_TYPE_APP,     /* 消息来自应用层 */
};
struct nlmsg{
    int type;               /* 消息类型 */
    int len;                /* 消息长度，包括头部 */
    char msg[MAX_MSG_LEN];  /* 消息正文 */
};
/* netlink socket */
static struct sock *g_nl_sk = NULL;
static int g_nlpid = -1;    /* 应用层接收程序PID */

/*
 * 发送整个从ip头开始的skb数据到应用层
 *
 * param[in]: sk, skb发送目的socket
 * param[in]: skb, 待发送的skb
 * return -1, 失败; 0, 成功
 * */
int nl_sendskb(struct sock *sk, struct sk_buff *skb)
{
    struct iphdr *iph = NULL;
    struct nlmsghdr *nlh = NULL;
    struct sk_buff *nl_skb = NULL;

    int skb_len = 0;

    ASSERT(skb != NULL);
    ASSERT(sk != NULL);
    if(g_nlpid < 0)
        return 0;

    iph = ip_hdr(skb);
    skb_len = iph->tot_len;
    /* NLMSG_SPACE: sizeof(struct nlmsghdr) + len按4字节对齐 */
    nl_skb = alloc_skb(NLMSG_SPACE(skb_len), GFP_ATOMIC);
    if(!nl_skb)
    {
        LOGERR("nl_skb == NULL, failed!\n");
        return -1;
    }
    /*
     * static inline struct nlmsghdr *nlmsg_put(struct sk_buff *skb, u32 pid, u32 seq,
     *               int type, int payload, int flags);
     * 设置skb->tail指针指向skb->data + sizeof(struct nlmsghdr) + payload
     * skb->len = sizeof(struct nlmsghdr) + payload
     */
    nlh = nlmsg_put(nl_skb, 0, 0, 0, NLMSG_SPACE(skb_len) - sizeof(struct nlmsghdr), 0);
    NETLINK_CB(nl_skb).pid = 0; /* 0代表数据来自内核 */
    memcpy(NLMSG_DATA(nlh), (char *)iph, htons(iph->tot_len));
    /* 将数据发送给进程号22345的进程 */
    return netlink_unicast(sk, nl_skb, g_nlpid , MSG_DONTWAIT);
}
/*
 * 发送字符串到应用层
 *
 * param[in]: sk, 数据发往的socket
 * param[in]: pmsg, 待发送字符串
 * param[in]: msglen, 待发送字符串长度
 *
 * return: -1, 失败; 0, 成功
 * */
int nl_sendmsg(struct sock *sk, struct nlmsg *pmsg)
{
    struct nlmsghdr *nlh = NULL;
    struct sk_buff *nl_skb = NULL;
    int msglen = pmsg->len;

    ASSERT(pmsg != NULL);
    ASSERT(sk != NULL);

    if(g_nlpid < 0)
        return 0;
    nl_skb = alloc_skb(NLMSG_SPACE(msglen), GFP_ATOMIC);
    if(!nl_skb)
    {
        LOGERR("nl_skb == NULL, msglen = %d, failed!\n", msglen);
        return -1;
    }

    nlh = nlmsg_put(nl_skb, 0, 0, 0,
                    NLMSG_SPACE(msglen) - NLMSG_HDRLEN, 0);
    NETLINK_CB(nl_skb).pid = 0;
    memcpy(NLMSG_DATA(nlh), pmsg, msglen);

    return netlink_unicast(sk, nl_skb, g_nlpid , MSG_DONTWAIT);
}
/*
 * 从应用层接收数据, netlink_kernel_create注册的回调
 * param[in]: skb, 包含netlink数据的skb
 *
 * skb常用操作函数
 * skb_put : skb->tail += len, skb->len += len
 * skb_pull: skb->data += len, skb->len -= len
 * skb_push: skb->data -= len, skb->len += len
 */
static void nl_recvmsg(struct sk_buff *skb)
{
    struct nlmsg *pmsg = NULL;
    struct nlmsghdr *nlh = NULL;
    uint32_t rlen = 0;

    while(skb->len >= NLMSG_SPACE(0))
    {
        nlh = nlmsg_hdr(skb);
        if(nlh->nlmsg_len < sizeof(*nlh) || skb->len < nlh->nlmsg_len)
            return;
        rlen = NLMSG_ALIGN(nlh->nlmsg_len);
        if(rlen > skb->len)
            rlen = skb->len;
        pmsg = (struct nlmsg*)NLMSG_DATA(nlh);
        switch(pmsg->type)
        {
        case NLMSG_TYPE_SETPID:
            g_nlpid = nlh->nlmsg_pid;
            LOGMSG("pid: %d\n", g_nlpid);
            LOGMSG("msg: %s\n", pmsg->msg);
            break;
        case NLMSG_TYPE_KERNEL:
            break;
        case NLMSG_TYPE_APP:
            break;
        }
        /* 获取下一条netlink消息 */
        skb_pull(skb, rlen);
    }
}

/*
 * netfilter PRE_ROUTING钩子
 * */
unsigned int pre_routing_hook(unsigned int hooknum,
                           struct sk_buff *skb,
                           const struct net_device *in,
                           const struct net_device *out,
                           int (*okfn)(struct sk_buff *))
{
    char *psend = "msg for kernel";
    struct nlmsg msg;
    int ret = 0;

    msg.type = NLMSG_TYPE_KERNEL;
    msg.len = strlen(psend) + offsetof(struct nlmsg, msg) + 1;
    memcpy(msg.msg, psend, msg.len);
    //ret = nl_sendskb(g_nl_sk, skb);
    ret = nl_sendmsg(g_nl_sk, &msg);
    //LOGMSG("ok\n");
    return NF_ACCEPT;
}

static struct nf_hook_ops local_in_ops __read_mostly = {
    .hook = pre_routing_hook,
    .owner = THIS_MODULE,
    .pf = PF_INET,
    .hooknum = NF_INET_PRE_ROUTING,
    .priority = NF_IP_PRI_FIRST
};

static int __init nl_init(void)
{
    int ret = 0;
    /*
     * struct sock *netlink_kernel_create(struct net *net, int unit, unsigned int groups,
     *                                    void (*input)(struct sk_buff *skb),
     *                                    struct mutex *cb_mutex, struct module *module)
     */
    g_nl_sk = netlink_kernel_create(&init_net, NETLINK_TEST, 0, nl_recvmsg, NULL, THIS_MODULE);
    if (!g_nl_sk) {
        LOGERR("Fail to create netlink socket.\n");
        return -1;
    }

    ret = nf_register_hook(&local_in_ops);
    if(ret < 0)
    {
        LOGMSG("nf_register_hook failed!\n");
        goto sock_release;
    }
    LOGMSG("ok!\n");
    return 0;

sock_release:
    if(g_nl_sk)
        sock_release(g_nl_sk->sk_socket);
    return -1;
}

static void __exit nl_exit(void)
{
    synchronize_sched();
    if(g_nl_sk)
        sock_release(g_nl_sk->sk_socket);
    nf_unregister_hook(&local_in_ops);
    LOGMSG("ok!\n");
}

module_init(nl_init);
module_exit(nl_exit);
MODULE_LICENSE("GPL");
MODULE_AUTHOR("[email protected]");