Generic netlink编程入门

通过generic netlink可以实现内核和用户空间的通信，genetlink是通过family来管理的（哈希表），genl_ctrl 是一个特殊的Family, 它是由Generic Netlink自己注册和实现，并用来查询Family列表、管理各个Family的添加、删除等事件的。用户空间先根据family name请求到相应的family ID，而后进行相互沟通。

 static struct genl_family genl_ctrl = {
          .id = GENL_ID_CTRL,
          .name = "nlctrl",
          .version = 0x2,
          .maxattr = CTRL_ATTR_MAX,
          .netnsok = true,
     };

     static struct genl_ops genl_ctrl_ops = {
          .cmd      = CTRL_CMD_GETFAMILY,
          .doit     = ctrl_getfamily,
          .dumpit   = ctrl_dumpfamily,
          .policy   = ctrl_policy,
     };

 static const struct nla_policy ctrl_policy[CTRL_ATTR_MAX+1] = {
          [CTRL_ATTR_FAMILY_ID] = { .type = NLA_U16 },
          [CTRL_ATTR_FAMILY_NAME] = { .type = NLA_NUL_STRING, .len = L_NAMSIZ - 1 },
     };

genl_kern.c

#include <net/genetlink.h>#include <linux/module.h>#include <linux/kernel.h>#include <linux/init.h>/*netlink attributes 可以通过枚举索引找到对应的类型*用户空间应用程序要传递这样的信息*/enum {DOC_EXMPL_A_UNSPEC,DOC_EXMPL_A_MSG,__DOC_EXMPL_A_MAX,};#define DOC_EXMPL_A_MAX (__DOC_EXMPL_A_MAX - 1)/*atribute policy就是定义各个属性的具体类型，参见net/netlink.h*/static struct nla_policy doc_exmpl_genl_policy[DOC_EXMPL_A_MAX + 1] = {        [DOC_EXMPL_A_MSG] = {.type = NLA_NUL_STRING},};#define VERSION_NR 1//generic netlink family 定义static struct genl_family doc_exmpl_genl_family = {        .id = GENL_ID_GENERATE,        .hdrsize = 0,        .name = "CONTROL_EXMPL",        .version = VERSION_NR,        .maxattr = DOC_EXMPL_A_MAX,};/*定义命令类型，用户空间以此来表明需要执行的命令*/enum{DOC_EXMPL_C_UNSPEC,DOC_EXMPL_C_ECHO,__DOC_EXMPL_C_MAX,};#define DOC_EXMPL_C_MAX (__DOC_EXMPL_C_MAX - 1)//echo command handler,接收一个msg并回复int doc_exmpl_echo(struct sk_buff *skb2, struct genl_info *info){        struct nlattr *na;        struct sk_buff *skb;        int rc;        void *msg_hdr;        char *data;        if(info == NULL)                goto error;        //对于每个属性，genl_info的域attrs可以索引到具体结构，里面有payload        na = info->attrs[DOC_EXMPL_A_MSG];        if(na){                data = (char *)nla_data(na);                if(!data) printk("Receive data error!\n");                else printk("Recv:%s\n",data);        }else{                printk("No info->attrs %d\n",DOC_EXMPL_A_MSG);        }        skb = genlmsg_new(NLMSG_GOODSIZE,GFP_KERNEL);        if(!skb) goto error;        /*构建消息头，函数原型是        genlmsgput(struct sk_buff *,int pid,int seq_number,                struct genl_family *,int flags,u8 command_index);        */        msg_hdr = genlmsg_put(skb,0,info->snd_seq+1,&doc_exmpl_genl_family,                                0,DOC_EXMPL_C_ECHO);        if(msg_hdr == NULL){                rc = -ENOMEM;                goto error;        }        //填充具体的netlink attribute:DOC_EXMPL_A_MSG，这是实际要传的数据        rc = nla_put_string(skb,DOC_EXMPL_A_MSG,"Hello World from kernel space!");        if(rc != 0) goto error;        genlmsg_end(skb,msg_hdr);//消息构建完成        //单播发送给用户空间的某个进程        rc = genlmsg_unicast(genl_info_net(info),skb,info->snd_pid);        if(rc != 0){                printk("Unicast to process:%d failed!\n",info->snd_pid);                goto error;        }        return 0;        error:        printk("Error occured in doc_echo!\n");        return 0;}//将命令command echo和具体的handler对应起来static struct genl_ops doc_exmpl_genl_ops_echo = {        .cmd = DOC_EXMPL_C_ECHO,        .flags = 0,        .policy = doc_exmpl_genl_policy,        .doit = doc_exmpl_echo,        .dumpit = NULL,};//内核入口，注册generic netlink family/operationsstatic int __init genKernel_init(void) {         int rc;         printk("Generic Netlink Example Module inserted.\n");         rc = genl_register_family(&doc_exmpl_genl_family);         if (rc != 0) {                  goto failure;         }         rc = genl_register_ops(&doc_exmpl_genl_family,&doc_exmpl_genl_ops_echo);         if (rc != 0) {         printk("Register ops: %i\n",rc);         genl_unregister_family(&doc_exmpl_genl_family);         goto failure;         }         return 0;        failure:         printk("Error occured while inserting generic netlink example module\n");         return -1;}static void __exit genKernel_exit(void) {         int ret;         printk("Generic Netlink Example Module unloaded.\n");         ret = genl_unregister_ops(&doc_exmpl_genl_family,&doc_exmpl_genl_ops_echo);         if(ret != 0) {                  printk("Unregister ops failed: %i\n",ret);                  return;         }         ret = genl_unregister_family(&doc_exmpl_genl_family);         if(ret !=0) {                  printk("Unregister family failed:%i\n",ret);         }}module_init(genKernel_init);module_exit(genKernel_exit);MODULE_LICENSE("GPL");

genl_user.c

#include <stdio.h>#include <stdlib.h>#include <errno.h>#include <unistd.h>#include <poll.h>#include <string.h>#include <fcntl.h>#include <sys/types.h>#include <sys/stat.h>#include <sys/socket.h>#include <sys/types.h>#include <signal.h>#include <linux/genetlink.h>//宏定义：根据generic netlink msg的具体构造定位#define GENLMSG_DATA(glh) ((void *)(NLMSG_DATA(glh) + GENL_HDRLEN))#define GENLMSG_PAYLOAD(glh) (NLMSG_PAYLOAD(glh, 0) - GENL_HDRLEN)#define NLA_DATA(na) ((void *)((char*)(na) + NLA_HDRLEN))#define MESSAGE_TO_KERNEL "Hello World from user space!"int nl_fd;struct sockaddr_nl nl_address;int nl_family_id;int len;struct nlattr *nl_na;struct { //    struct nlmsghdr n;    struct genlmsghdr g;    char buf[256];} nl_request_msg, nl_response_msg;int main(void) {        nl_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);        if (nl_fd < 0) {                perror("socket()");                return -1;        }        memset(&nl_address, 0, sizeof(nl_address));        nl_address.nl_family = AF_NETLINK;        nl_address.nl_groups = 0;        if (bind(nl_fd, (struct sockaddr *) &nl_address, sizeof(nl_address)) < 0) {                perror("bind()");                close(nl_fd);                return -1;        }        nl_request_msg.n.nlmsg_type = GENL_ID_CTRL;//这是内核中genl_ctl的id        nl_request_msg.n.nlmsg_flags = NLM_F_REQUEST;        nl_request_msg.n.nlmsg_seq = 0;        nl_request_msg.n.nlmsg_pid = getpid();        nl_request_msg.n.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);        //Populate the payload's "family header" : which in our case is genlmsghdr        nl_request_msg.g.cmd = CTRL_CMD_GETFAMILY;        nl_request_msg.g.version = 0x1;        //Populate the payload's "netlink attributes"        nl_na = (struct nlattr *) GENLMSG_DATA(&nl_request_msg);//其实就相当于在nl_request_msg 的buf域中构造一个nla        nl_na->nla_type = CTRL_ATTR_FAMILY_NAME;        nl_na->nla_len = strlen("CONTROL_EXMPL") + 1 + NLA_HDRLEN;        strcpy(NLA_DATA(nl_na), "CONTROL_EXMPL"); //Family name length can be upto 16 chars including \0        nl_request_msg.n.nlmsg_len += NLMSG_ALIGN(nl_na->nla_len);        memset(&nl_address, 0, sizeof(nl_address));        nl_address.nl_family = AF_NETLINK;        len= sendto(nl_fd, (char *) &nl_request_msg, nl_request_msg.n.nlmsg_len,                       0, (struct sockaddr *) &nl_address, sizeof(nl_address));        if (len != nl_request_msg.n.nlmsg_len) {                perror("sendto()");                close(nl_fd);                return -1;        }        len= recv(nl_fd, &nl_response_msg, sizeof(nl_response_msg), 0);        if (len < 0) {                perror("recv()");                return -1;        }        if (!NLMSG_OK((&nl_response_msg.n), len)) {                fprintf(stderr, "family ID request : invalid message\n");                return -1;        }        if (nl_response_msg.n.nlmsg_type == NLMSG_ERROR) { //error                fprintf(stderr, "family ID request : receive error\n");                return -1;        }        //解析出attribute中的family id        nl_na = (struct nlattr *) GENLMSG_DATA(&nl_response_msg);        nl_na = (struct nlattr *) ((char *) nl_na + NLA_ALIGN(nl_na->nla_len));        if (nl_na->nla_type == CTRL_ATTR_FAMILY_ID) {                nl_family_id = *(__u16 *) NLA_DATA(nl_na);//第一次通信就是为了得到需要的family ID        }        memset(&nl_request_msg, 0, sizeof(nl_request_msg));        memset(&nl_response_msg, 0, sizeof(nl_response_msg));        nl_request_msg.n.nlmsg_len = NLMSG_LENGTH(GENL_HDRLEN);        nl_request_msg.n.nlmsg_type = nl_family_id;        nl_request_msg.n.nlmsg_flags = NLM_F_REQUEST;        nl_request_msg.n.nlmsg_seq = 60;        nl_request_msg.n.nlmsg_pid = getpid();        nl_request_msg.g.cmd = 1; //corresponds to DOC_EXMPL_C_ECHO;        nl_na = (struct nlattr *) GENLMSG_DATA(&nl_request_msg);        nl_na->nla_type = 1; // corresponds to DOC_EXMPL_A_MSG        nl_na->nla_len = sizeof(MESSAGE_TO_KERNEL)+NLA_HDRLEN; //Message length        memcpy(NLA_DATA(nl_na), MESSAGE_TO_KERNEL, sizeof(MESSAGE_TO_KERNEL));        nl_request_msg.n.nlmsg_len += NLMSG_ALIGN(nl_na->nla_len);        memset(&nl_address, 0, sizeof(nl_address));        nl_address.nl_family = AF_NETLINK;        len = sendto(nl_fd, (char *) &nl_request_msg, nl_request_msg.n.nlmsg_len,                        0, (struct sockaddr *) &nl_address, sizeof(nl_address));        if (len != nl_request_msg.n.nlmsg_len) {                perror("sendto()");                close(nl_fd);                return -1;        }        printf("Sent to kernel: %s\n",MESSAGE_TO_KERNEL);        len = recv(nl_fd, &nl_response_msg, sizeof(nl_response_msg), 0);        if (len < 0) {                perror("recv()");                return -1;        }         //异常处理        if (nl_response_msg.n.nlmsg_type == NLMSG_ERROR) { //Error        printf("Error while receiving reply from kernel: NACK Received\n");                close(nl_fd);                return -1;        }        if (len < 0) {                printf("Error while receiving reply from kernel\n");                close(nl_fd);                return -1;        }        if (!NLMSG_OK((&nl_response_msg.n), len)) {                printf("Error while receiving reply from kernel: Invalid Message\n");                close(nl_fd);                return -1;        }        //解析收到的来自内核的reply        len = GENLMSG_PAYLOAD(&nl_response_msg.n);        nl_na = (struct nlattr *) GENLMSG_DATA(&nl_response_msg);        printf("Kernel replied: %s\n",(char *)NLA_DATA(nl_na));        close(nl_fd);        return 0;}

参考：     1.  http://people.ee.ethz.ch/~arkeller/linux/multi/kernel_user_space_howto-3.html