爱华网在 Linux 2.4 版以后版本的内核中，几乎全部的中断过程与用户态进程的通信都是使用 netlink套接字实现的，同时还使用 netlink 实现了 ip queue 工具，但 ip queue的使用有其局限性，不能自由地用于各种中断过程。内核的帮助文档和其他一些 Linux 相关文章都没有对 netlink套接字在中断过程和用户空间通信的应用上作详细的说明，使得很多用户对此只有一个模糊的概念。
Unicast Communication between Kernel and Application
在下面的例子中,一个用户空间进程发送一个netlink消息给内核模块,内核模块应答一个消息给发送进程,这里是用户空间的代码:
#include <sys/socket.h>
#include <linux/netlink.h>
#define MAX_PAYLOAD 1024
struct sockaddr_nl src_addr, dest_addr;
struct msghdr msg;
struct nlmsghdr *nlh = NULL;
struct iovec iov;
int sock_fd;
void main() ...{
sock_fd = socket(PF_NETLINK,SOCK_RAW,NETLINK_TEST);
memset(&src_addr, 0,sizeof(src_addr));————
src__addr.nl_family =AF_NETLINK;
src_addr.nl_pid =getpid();
src_addr.nl_groups = 0;
bind(sock_fd, (structsockaddr*)&src_addr,
sizeof(src_addr));
memset(&dest_addr, 0,sizeof(dest_addr));
dest_addr.nl_family = AF_NETLINK;
dest_addr.nl_pid =0;
dest_addr.nl_groups = 0;
nlh=(struct nlmsghdr *)malloc(
NLMSG_SPACE(MAX_PAYLOAD));
nlh->nlmsg_len =NLMSG_SPACE(MAX_PAYLOAD);
nlh->nlmsg_pid =getpid();
nlh->nlmsg_flags = 0;
strcpy(NLMSG_DATA(nlh), "Hello you!");
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(fd, &msg, 0);
memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));
recvmsg(fd, &msg, 0);
printf(" Received message payload: %s ",
NLMSG_DATA(nlh));
close(sock_fd);
}
这里是内核代码:
struct sock *nl_sk = NULL;
void nl_data_ready (struct sock *sk, int len)
...{
wake_up_interruptible(sk->sleep);
}
void netlink_test() ...{
struct sk_buff *skb = NULL;
struct nlmsghdr *nlh = NULL;
int err;
u32pid;
nl_sk = netlink_kernel_create(NETLINK_TEST,
nl_data_ready);
skb = skb_recv_datagram(nl_sk, 0, 0,&err);
nlh = (struct nlmsghdr*)skb->data;
printk("%s: received netlink message payload:%s",
__FUNCTION__, NLMSG_DATA(nlh));
pid = nlh->nlmsg_pid;
NETLINK_CB(skb).groups = 0;
NETLINK_CB(skb).pid =0;
NETLINK_CB(skb).dst_pid = pid;
NETLINK_CB(skb).dst_groups =0;
netlink_unicast(nl_sk, skb, pid,MSG_DONTWAIT);
sock_release(nl_sk->socket);
}
在内核模块被加载到内核,当我们运行用户程序,我们将看到下面的信息:
Received message payload: Hello you!
然后用dmesg我们可以看到内核输出:
netlink_test: received netlink message payload:
Hello you!
Multicast Communication between Kernel and Applications
这个例子中,两个应用程序在监听同一个netlink广播组.内核模块发送一个netlink消息给这个广播组,所用的应用程序都收到它,如下是用户程序代码:
#include <sys/socket.h>
#include <linux/netlink.h>
#define MAX_PAYLOAD 1024
struct sockaddr_nl src_addr, dest_addr;
struct nlmsghdr *nlh = NULL;
struct iovec iov;
int sock_fd;
void main() ...{
sock_fd=socket(PF_NETLINK, SOCK_RAW,NETLINK_TEST);
memset(&src_addr, 0,sizeof(local_addr));
src_addr.nl_family =AF_NETLINK;
src_addr.nl_pid =getpid();
src_addr.nl_groups = 1;
bind(sock_fd, (structsockaddr*)&src_addr,
sizeof(src_addr));
memset(&dest_addr, 0,sizeof(dest_addr));
nlh = (struct nlmsghdr *)malloc(
NLMSG_SPACE(MAX_PAYLOAD));
memset(nlh, 0,NLMSG_SPACE(MAX_PAYLOAD));
iov.iov_base = (void *)nlh;
iov.iov_len = NLMSG_SPACE(MAX_PAYLOAD);
msg.msg_name = (void*)&dest_addr;
msg.msg_namelen = sizeof(dest_addr);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
printf("Waiting for message from kernel ");
recvmsg(fd, &msg, 0);
printf(" Received message payload: %s ",
NLMSG_DATA(nlh));
close(sock_fd);
}
内核代码:
#define MAX_PAYLOAD 1024
struct sock *nl_sk = NULL;
void netlink_test() ...{
sturct sk_buff *skb = NULL;
struct nlmsghdr *nlh;
int err;
nl_sk = netlink_kernel_create(NETLINK_TEST,
nl_data_ready);
skb=alloc_skb(NLMSG_SPACE(MAX_PAYLOAD),GFP_KERNEL);
nlh = (struct nlmsghdr*)skb->data;
nlh->nlmsg_len =NLMSG_SPACE(MAX_PAYLOAD);
nlh->nlmsg_pid =0;
nlh->nlmsg_flags = 0;
strcpy(NLMSG_DATA(nlh), "Greeting fromkernel!");
NETLINK_CB(skb).groups = 1;
NETLINK_CB(skb).pid = 0;
NETLINK_CB(skb).dst_pid =0;
NETLINK_CB(skb).dst_groups = 1;
netlink_broadcast(nl_sk, skb, 0, 1,GFP_KERNEL);
sock_release(nl_sk->socket);
}
我们运行用户程序:
./nl_recv &
Waiting for message from kernel
./nl_recv &
Waiting for message from kernel
然后我们加载内核模块到内核空间,会看到如下信息::
Received message payload: Greeting from kernel!
Received message payload: Greeting from kernel!
以下是一个简单的测试内核事件的应用程序：
#defineMAX_PAYLOAD1024
structsockaddr_nlsrc_addr, dest_addr;
char *KernelMsg = NULL;
struct iovec iov;
int sock_fd;
struct msghdr msg;
int msglen;
#defineDEVICE_ADD"add"
#defineDEVICE_REMOVE"remove"
#defineDEVICE_NAME"event0"
#defineDEVICE_NAMELEN6
void * DeviceManagement(void *arg)
...{
sock_fd =socket(PF_NETLINK, SOCK_RAW, NETLINK_KOBJECT_UEVENT);
intmsgle;
memset(&src_addr, 0, sizeof(src_addr));
src_addr.nl_family = AF_NETLINK;
src_addr.nl_pid = pthread_self() <<16 | getpid();
src_addr.nl_groups = 1;
bind(sock_fd, (struct sockaddr *)&src_addr,sizeof(src_addr));
memset(&dest_addr, 0, sizeof(dest_addr));
KernelMsg =(struct nlmsghdr *)malloc(MAX_PAYLOAD);
memset(KernelMsg,0,MAX_PAYLOAD);
iov.iov_base= (void *)KernelMsg;
iov.iov_len= MAX_PAYLOAD;
msg.msg_name= (void *)&dest_addr;
msg.msg_namelen = sizeof(dest_addr);
msg.msg_iov= &iov;
msg.msg_iovlen = 1;
while(1)...{
//printf("Waiting for message from kernel ");
recvmsg(sock_fd, &msg, 0);
//printf("Receved message payload: %s ", KernelMsg);
msglen = strlen(KernelMsg);
//printf("Device: %s ", KernelMsg+msglen-DEVICE_NAMELEN);
if(!strncmp(DEVICE_NAME, KernelMsg+msglen-DEVICE_NAMELEN,DEVICE_NAMELEN)) ...{
if(!strncmp(DEVICE_ADD, KernelMsg, strlen(DEVICE_ADD)))
...{
printf("Add event0 device ");
USBKeyboardReady = 1;
}
else if(!strncmp(DEVICE_REMOVE, KernelMsg, strlen(DEVICE_REMOVE)))...{
printf("Remove event0 device ");
USBKeyboardReady = 0;
}
}
}
close(sock_fd);
}
示例：一个用户进程发送数据到内核，然后通过内核发送给另一个用户进程。
内核进程：netlink-exam-kern.c
//kernel module: netlink-exam-kern.c
#ifndef __KERNEL__
#define __KERNEL__
#endif
#ifndef MODULE
#define MODULE
#endif
#include <linux/config.h>
#include <linux/module.h>
#include <linux/netlink.h>
#include <linux/sched.h>
#include <net/sock.h>
#include <linux/proc_fs.h>
#define BUF_SIZE 16384
#define NL 30
static struct sock *netlink_exam_sock;
static unsigned char buffer[BUF_SIZE];
static unsigned int buffer_tail = 0;
static int exit_flag = 0;
static DECLARE_COMPLETION(exit_completion);
static void recv_handler(struct sock * sk, int length)
...{
wake_up(sk->sk_sleep);
}
static int process_message_thread(void * data)
...{
struct sk_buff * skb = NULL;
struct nlmsghdr * nlhdr = NULL;
int len;
DEFINE_WAIT(wait);
daemonize("mynetlink");
while (exit_flag == 0) ...{
prepare_to_wait(netlink_exam_sock->sk_sleep,&wait, TASK_INTERRUPTIBLE);
schedule();
finish_wait(netlink_exam_sock->sk_sleep,&wait);
while ((skb =skb_dequeue(&netlink_exam_sock->sk_receive_queue))
!= NULL) ...{
nlhdr = (struct nlmsghdr *)skb->data;
if (nlhdr->nlmsg_len < sizeof(structnlmsghdr)) ...{
printk("Corrupt netlink message. ");
continue;
}
len = nlhdr->nlmsg_len - NLMSG_LENGTH(0);
if (len + buffer_tail > BUF_SIZE) ...{
printk("netlink buffer is full. ");
}
else ...{
memcpy(buffer + buffer_tail, NLMSG_DATA(nlhdr), len);
buffer_tail += len;
}
nlhdr->nlmsg_pid = 0;
nlhdr->nlmsg_flags = 0;
NETLINK_CB(skb).pid = 0;
NETLINK_CB(skb).dst_pid = 0;
NETLINK_CB(skb).dst_group = 1;
netlink_broadcast(netlink_exam_sock, skb, 0, 1, GFP_KERNEL);
}
}
complete(&exit_completion);
return 0;
}
static int netlink_exam_readproc(char *page, char **start, off_toff,
int count, int *eof, void *data)
...{
int len;
if (off >= buffer_tail) ...{
* eof = 1;
return 0;
}
else ...{
len = count;
if (count > PAGE_SIZE) ...{
len = PAGE_SIZE;
}
if (len > buffer_tail - off) ...{
len = buffer_tail - off;
}
memcpy(page, buffer + off, len);
*start = page;
return len;
}
}
static int __init netlink_exam_init(void)
...{
netlink_exam_sock = netlink_kernel_create(NL, 0, recv_handler,THIS_MODULE);
if (!netlink_exam_sock) ...{
printk("Fail to create netlink socket. ");
return 1;
}
kernel_thread(process_message_thread, NULL, CLONE_KERNEL);
create_proc_read_entry("netlink_exam_buffer", 0444, NULL,netlink_exam_readproc, 0);
return 0;
}
static void __exit netlink_exam_exit(void)
...{
exit_flag = 1;
wake_up(netlink_exam_sock->sk_sleep);
wait_for_completion(&exit_completion);
sock_release(netlink_exam_sock->sk_socket);
}
module_init(netlink_exam_init);
module_exit(netlink_exam_exit);
MODULE_LICENSE("GPL");
编译成模块：
ifneq ($(KERNELRELEASE),)
debug-objs := netlink-exam-kern.o
obj-m := netlink-exam-kern1.o
CFLAGS += -w -Wimplicit-function-declaration
else
PWD := $(shell pwd)
KVER ?= $(shell uname -r)
KDIR := /lib/modules/$(KVER)/build
all:
$(MAKE) -C $(KDIR) M=$(PWD)
clean:
rm -rf .*.cmd *.o *.mod.c *.ko .tmp_versions
endif
用户发送进程：netlink-exam-user-send.c
//application sender: netlink-exam-user-send.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#define MAX_MSGSIZE 1024
int main(int argc, char * argv[])
...{
FILE * fp;
struct sockaddr_nl saddr, daddr;
struct nlmsghdr *nlhdr = NULL;
struct msghdr msg;
struct iovec iov;
int sd;
char text_line[MAX_MSGSIZE];
int ret = -1;
if (argc < 2) ...{
printf("Usage: %s atextfilename ", argv[0]);
exit(1);
}
if ((fp = fopen(argv[1], "r")) == NULL) ...{
printf("File %s dosen''t exist. ");
exit(1);
}
sd = socket(AF_NETLINK, SOCK_RAW, 30);
memset(&saddr, 0, sizeof(saddr));
memset(&daddr, 0, sizeof(daddr));
saddr.nl_family = AF_NETLINK;
saddr.nl_pid = getpid();
saddr.nl_groups = 0;
bind(sd, (struct sockaddr*)&saddr,sizeof(saddr));
daddr.nl_family = AF_NETLINK;
daddr.nl_pid = 0;
daddr.nl_groups = 0;
nlhdr = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_MSGSIZE));
while (fgets(text_line, MAX_MSGSIZE, fp)) ...{
memcpy(NLMSG_DATA(nlhdr), text_line, strlen(text_line));
memset(&msg, 0 ,sizeof(struct msghdr));
nlhdr->nlmsg_len =NLMSG_LENGTH(strlen(text_line));
nlhdr->nlmsg_pid = getpid();
nlhdr->nlmsg_flags = 0;
iov.iov_base = (void *)nlhdr;
iov.iov_len = nlhdr->nlmsg_len;
msg.msg_name = (void *)&daddr;
msg.msg_namelen = sizeof(daddr);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
ret = sendmsg(sd, &msg, 0);
if (ret == -1) ...{
perror("sendmsg error:");
}
}
close(sd);
return 0;
}
用户接收进程：netlink-exam-user-recv.c
//application sender: netlink-exam-user-send.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#define MAX_MSGSIZE 1024
int main(int argc, char * argv[])
...{
FILE * fp;
struct sockaddr_nl saddr, daddr;
struct nlmsghdr *nlhdr = NULL;
struct msghdr msg;
struct iovec iov;
int sd;
char text_line[MAX_MSGSIZE];
int ret = -1;
if (argc < 2) ...{
printf("Usage: %s atextfilename ", argv[0]);
exit(1);
}
if ((fp = fopen(argv[1], "r")) == NULL) ...{
printf("File %s dosen''t exist. ");
exit(1);
}
sd = socket(AF_NETLINK, SOCK_RAW, 30);
memset(&saddr, 0, sizeof(saddr));
memset(&daddr, 0, sizeof(daddr));
saddr.nl_family = AF_NETLINK;
saddr.nl_pid = getpid();
saddr.nl_groups = 0;
bind(sd, (struct sockaddr*)&saddr,sizeof(saddr));
daddr.nl_family = AF_NETLINK;
daddr.nl_pid = 0;
daddr.nl_groups = 0;
nlhdr = (struct nlmsghdr *)malloc(NLMSG_SPACE(MAX_MSGSIZE));
while (fgets(text_line, MAX_MSGSIZE, fp)) ...{
memcpy(NLMSG_DATA(nlhdr), text_line, strlen(text_line));
memset(&msg, 0 ,sizeof(struct msghdr));
nlhdr->nlmsg_len =NLMSG_LENGTH(strlen(text_line));
nlhdr->nlmsg_pid = getpid();
nlhdr->nlmsg_flags = 0;
iov.iov_base = (void *)nlhdr;
iov.iov_len = nlhdr->nlmsg_len;
msg.msg_name = (void *)&daddr;
msg.msg_namelen = sizeof(daddr);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
ret = sendmsg(sd, &msg, 0);
if (ret == -1) ...{
perror("sendmsg error:");
}
}
close(sd);
return 0;
}
文章出处：http://www.diybl.com/course/6_system/linux/Linuxjs/20071226/94120.html
