用户态与内核态交互通信的方法不止一种，sockopt是比较方便的一个，写法也简单.
缺点就是使用 copy_from_user()/copy_to_user()完成内核和用户的通信， 效率其实不高， 多用在传递控制 选项 信息，不适合做大量的数据传输

用户态函数：
发送：int setsockopt ( int sockfd, int proto, int cmd, void *data, int datelen);
接收：int getsockopt(int sockfd, int proto, int cmd, void *data, int datalen)
第一个参数是socket描述符；
第二个参数proto是sock协议，IP RAW的就用SOL_SOCKET/SOL_IP等，TCP/UDP socket的可用SOL_SOCKET/SOL_IP/SOL_TCP/SOL_UDP等，即高层的socket是都可以使用低层socket的命令字 的，IPPROTO_IP；
第三个参数cmd是操作命令字，由自己定义；
第四个参数是数据缓冲区起始位置指针，set操作时是将缓冲区数据写入内核，get的时候是将内核中的数 据读入该缓冲区；
第五个参数数据长度

内核态函数
注册：nf_register_sockopt(struct nf_sockopt_ops *sockops)
解除：nf_unregister_sockopt(struct nf_sockopt_ops *sockops)

结构体 nf_sockopt_ops test_sockops

struct nf_sockopt_ops {
struct list_head list;

u\_int8\_t pf; // 协议族

/\* Non-inclusive ranges: use 0/0/NULL to never get called. \*/  
int set\_optmin;// 定义最小set命令字  
int set\_optmax;// 定义最大set命令字  
int (\*set)(struct sock \*sk, int optval, void \_\_user \*user, unsigned int len);// 定义set处理函数  

#ifdef CONFIG_COMPAT
int (*compat_set)(struct sock *sk, int optval,
void __user *user, unsigned int len);
#endif
int get_optmin;// 定义最小get命令字
int get_optmax;// 定义最大get命令字
int (*get)(struct sock *sk, int optval, void __user *user, int *len);// 定义get处理函数
#ifdef CONFIG_COMPAT
int (*compat_get)(struct sock *sk, int optval,
void __user *user, int *len);
#endif
/* Use the module struct to lock set/get code in place */
struct module *owner;
};

其中命令字不能和内核已有的重复，宜大不宜小。命令字很重要，是用来做标识符的。而且用户态和内核态要定义的相同，

系统调用如下：sockt==》sock_common_setsockopt==》sk->sk_prot->setsockopt==》ip_setsockopt（tcp为例）

　　　　==>do_ip_setsockopt

　　　　==> nf_setsockopt==>nf_sockopt

int ip_setsockopt(struct sock *sk, int level,
int optname, char __user *optval, unsigned int optlen)
{
int err;

if (level != SOL\_IP)  
    return -ENOPROTOOPT;

err = do\_ip\_setsockopt(sk, level, optname, optval, optlen);  

#ifdef CONFIG_NETFILTER
/* we need to exclude all possible ENOPROTOOPTs except default case */
if (err == -ENOPROTOOPT && optname != IP_HDRINCL &&
optname != IP_IPSEC_POLICY &&
optname != IP_XFRM_POLICY &&
!ip_mroute_opt(optname)) {
lock_sock(sk);
err = nf_setsockopt(sk, PF_INET, optname, optval, optlen);
release_sock(sk);
}
#endif
return

err;
}

/* Call get/setsockopt() */
static int nf_sockopt(struct sock *sk, u_int8_t pf, int val,
char __user *opt, int *len, int get)
{
struct nf_sockopt_ops *ops;
int ret;

ops = nf\_sockopt\_find(sk, pf, val, get);  
if (IS\_ERR(ops))  
    return PTR\_ERR(ops);

if (get)  
    ret = ops->get(sk, val, opt, len);  
else  
    ret = ops->set(sk, val, opt, \*len);

module\_put(ops->owner);  
return ret;  

}

int nf_setsockopt(struct sock *sk, u_int8_t pf, int val, char __user *opt,
unsigned int len)
{
return nf_sockopt(sk, pf, val, opt, &len, 0);
}

#include
#include
#include
#include
#include
#include
#include

#define SOCKET_OPS_BASE 128
#define SOCKET_OPS_SET (SOCKET_OPS_BASE)
#define SOCKET_OPS_GET (SOCKET_OPS_BASE)
#define SOCKET_OPS_MAX (SOCKET_OPS_BASE + 1)

#define KMSG "--------kernel---------"
#define KMSG_LEN sizeof("--------kernel---------")

MODULE_LICENSE("GPL");
MODULE_AUTHOR("SiasJack");/*作者*/
MODULE_DESCRIPTION("sockopt module,simple module");//描述
MODULE_VERSION("1.0");//版本号

static int recv_msg(struct sock *sk, int cmd, void __user *user, unsigned int len)
{
int ret = 0;
printk(KERN_INFO "sockopt: recv_msg()\n");

if (cmd == SOCKET\_OPS\_SET)  
{  
    char umsg\[64\];  
    int len = sizeof(char)\*64;  
    memset(umsg, 0, len);  
    ret = copy\_from\_user(umsg, user, len);  
    printk("recv\_msg: umsg = %s. ret = %d\\n", umsg, ret);  
}  
return 0;  

}

static int send_msg(struct sock *sk, int cmd, void __user *user, int *len)
{
int ret = 0;
printk(KERN_INFO "sockopt: send_msg()\n");
if (cmd == SOCKET_OPS_GET)
{
ret = copy_to_user(user, KMSG, KMSG_LEN);
printk("send_msg: umsg = %s. ret = %d. success\n", KMSG, ret);
}
return 0;

}

static struct nf_sockopt_ops test_sockops =
{
.pf = PF_INET,
.set_optmin = SOCKET_OPS_SET,
.set_optmax = SOCKET_OPS_MAX,
.set = recv_msg,
.get_optmin = SOCKET_OPS_GET,
.get_optmax = SOCKET_OPS_MAX,
.get = send_msg,
.owner = THIS_MODULE,
};

static int __init init_sockopt(void)
{
printk(KERN_INFO "sockopt: init_sockopt()\n");
return nf_register_sockopt(&test_sockops);
}

static void __exit exit_sockopt(void)
{
printk(KERN_INFO "sockopt: fini_sockopt()\n");
nf_unregister_sockopt(&test_sockops);
}

module_init(init_sockopt);

#include
#include
#include
#include
#include
#include

#define SOCKET_OPS_BASE 128
#define SOCKET_OPS_SET (SOCKET_OPS_BASE)
#define SOCKET_OPS_GET (SOCKET_OPS_BASE)
#define SOCKET_OPS_MAX (SOCKET_OPS_BASE + 1)

#define UMSG "----------user------------"
#define UMSG_LEN sizeof("----------user------------")

char kmsg[64];

int main(void)
{
int sockfd;
int len;
int ret;

sockfd = socket(AF\_INET, SOCK\_RAW, IPPROTO\_RAW);  
if(sockfd < 0)  
{  
    printf("can not create a socket\\n");  
    return -1;  
}   

/\*call function recv\_msg()\*/  
ret = setsockopt(sockfd, IPPROTO\_IP, SOCKET\_OPS\_SET, UMSG, UMSG\_LEN);  
printf("setsockopt: ret = %d. msg = %s\\n", ret, UMSG);  
len = sizeof(char)\*64; 

/\*call function send\_msg()\*/  
ret = getsockopt(sockfd, IPPROTO\_IP, SOCKET\_OPS\_GET, kmsg, &len);  
printf("getsockopt: ret = %d. msg = %s\\n", ret, kmsg);  
if (ret != 0)  
{  
    printf("getsockopt error: errno = %d, errstr = %s\\n", errno, strerror(errno));  
}   

close(sockfd);  
return 0;  

}

https://blog.csdn.net/jk110333/article/details/8642261  来自转载