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程式設計

NETLINK-與kernel溝通的另一種方法

作者: Joey 日期: 2009-01-22 18:58

字體大小: 小中大

新年快到了,先祝大家新年快樂,牛年發大財,快到了年關,也該好好輕鬆一下,回頭看看自己這一年是不是做了些足以令你自傲的豐功偉業,也希望我能繼續保持對coding的熱誠在新的一年繼續奮鬥

話說回來,有了ioctl,和proc,我們是不是還有其它可以與kernel交換資料的界面呢?沒錯,就是netlink,那什麼時候會用到netlink呢?當你設routing table或防火牆時就會用到了,kernel就是用netlink界面與route指令溝通,scsi驅動也可以看到netlink的影子,netlink有什麼好處呢?

優點:
1.它沒有ioctl等ret回來的kernel round trip time
2.可發多個訊息給不同的user process(如果user processes都在同一group)
缺點:
1.因為只有UDP封包,所以你送資料後也不曉得該資料到底是不是去到對的地方
2.用了一堆socket API,程式碼寫起來並不簡潔

簡單描述我現在做的測試,以下面的kernel udp echo server為例,它會接收使用者的訊息並echo回來,所以整個程式分為兩部份,udp echo server在kernel space,使用者程式在user space

udp echo server重點列如下
1. netlink_kernel_create: create kernel socket並且註冊你的callback函式處理netlink封包,請特別注意第二個參數,我用了NETLINK_USERSOCK這個linux保留給使用者測試的netlink type.
2. sock_release:移除module時,一定要release kernel socket
3. udp_receive:這個函式就是我們處理netlink傳入封包的地方,在這裡你可以取得user process id和data
4. udp_reply:做一個新的封包並呼叫函式netlink_unicast echo回去

[kudp echo server]

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/init.h>

#include <net/sock.h>

#include <net/netlink.h>

#include <linux/skbuff.h>

static struct sock *netlink_sock;

static void udp_reply(int pid,int seq,void *payload)

{

struct sk_buff *skb;

struct nlmsghdr *nlh;

int size=strlen(payload)+1;

int len = NLMSG_SPACE(size);

void *data;

int ret;

skb = alloc_skb(len, GFP_ATOMIC);

if (!skb)

return;

nlh= NLMSG_PUT(skb, pid, seq, 0, size);

nlh->nlmsg_flags = 0;

data=NLMSG_DATA(nlh);

memcpy(data, payload, size);

NETLINK_CB(skb).pid = 0; /* from kernel */

NETLINK_CB(skb).dst_group = 0; /* unicast */

ret=netlink_unicast(netlink_sock, skb, pid, MSG_DONTWAIT);

if (ret <0)

{

printk("send failed\n");

return;

}

return;

nlmsg_failure: /* Used by NLMSG_PUT */

if (skb)

kfree_skb(skb);

}

/* Receive messages from netlink socket. */

static void udp_receive(struct sk_buff *skb)

{

u_int uid, pid, seq, sid;

void *data;

struct nlmsghdr *nlh;

nlh = (struct nlmsghdr *)skb->data;

pid = NETLINK_CREDS(skb)->pid;

uid = NETLINK_CREDS(skb)->uid;

sid = NETLINK_CB(skb).sid;

seq = nlh->nlmsg_seq;

data = NLMSG_DATA(nlh);

printk("recv skb from user space uid:%d pid:%d seq:%d,sid:%d\n",uid,pid,seq,sid);

printk("data is :%s\n",(char *)data);

udp_reply(pid,seq,data);

return ;

}

static int __init kudp_init(void)

{

netlink_sock = netlink_kernel_create(&init_net, NETLINK_USERSOCK, 0,udp_receive, NULL, THIS_MODULE);

return 0;

}

static void __exit kudp_exit(void)

{

sock_release(netlink_sock->sk_socket);

printk("netlink driver remove successfully\n");

}

module_init(kudp_init);

module_exit(kudp_exit);

MODULE_DESCRIPTION("kudp echo server module");

MODULE_AUTHOR("Joey Cheng<jemicheng@gmail.com>");

MODULE_LICENSE("GPL");

MODULE_ALIAS("QT2410:kudp echo server module");

user space client重點列如下
1.宣告socket fd請用sock_fd = socket(PF_NETLINK, SOCK_RAW,NETLINK_USERSOCK),第三個傳入參數為netlink type
2.注意pid與group的使用方式,如果只是unicast,要把group設為0
3.小心struct msghdr msg,要先把裡面的iov,nlmsghdr等欄位宣告並初始化
4.簡單的用sendmsg和recvmsg就大功告成啦
[user space client]

#include <stdio.h>

#include <string.h>

#include <malloc.h>

#include <sys/socket.h>

#include <linux/netlink.h>

#define MAX_PAYLOAD 1024

int main(int argc,char *argv[])

{

struct sockaddr_nl src_addr, dest_addr;

struct nlmsghdr *nlh = NULL;

struct iovec iov;

int sock_fd;

struct msghdr msg;

sock_fd = socket(PF_NETLINK, SOCK_RAW,NETLINK_USERSOCK);

memset(&src_addr, 0, sizeof(src_addr));

src_addr.nl_family = AF_NETLINK;

src_addr.nl_pid = getpid(); /* self pid */

src_addr.nl_groups = 0; /* not in mcast groups */

bind(sock_fd, (struct sockaddr*)&src_addr,sizeof(src_addr));

memset(&dest_addr, 0, sizeof(dest_addr));

dest_addr.nl_family = AF_NETLINK;

dest_addr.nl_pid = 0; /* For Linux Kernel */

dest_addr.nl_groups = 0; /* unicast */

nlh=(struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_PAYLOAD));

/* Fill the netlink message header */

nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD);

nlh->nlmsg_pid = getpid(); /* self pid */

nlh->nlmsg_flags = 0;

/* Fill in the netlink message payload */

strcpy(NLMSG_DATA(nlh), "Hello World!");

iov.iov_base = (void *)nlh;

iov.iov_len = nlh->nlmsg_len;

msg.msg_name = (void *)&dest_addr;

msg.msg_namelen = sizeof(dest_addr);

msg.msg_iov = &iov;

msg.msg_iovlen = 1;

sendmsg(sock_fd, &msg, 0);

printf("Waiting for message from kernel\n");

/* Read message from kernel */

recvmsg(sock_fd, &msg, 0);

printf("Received message payload: %s\n",NLMSG_DATA(msg.msg_iov->iov_base));

close(sock_fd);

return 0;

}

全部的程式請在這邊下載,如果要嘗試multicast server,那麼請參考kdupb.c這支程式,和它的user space client mc1.c和mc2.c,只要group設定正確mc1和mc2都可以收到kudpb的multicast訊息,kudpb利用netlink_broadcast廣播給mc1和mc2,執行畫面如下