Kernel与用户进程通信
测试IPv6 ready logo rfc 3315的时候，遇到一个问题，要求在收到ICMPv6 RA的时候，DHCPv6 Client要发Solicit消息。在平常的应用中，都是启动DHCPv6 Client后，client每隔一段时间就会发送Solicit，而并不是要在收到RA的时候发送。于是就修改了下kernel和client的代码，使得在kernel收到RA消息时，发送消息给DHCPv6 Client，使client发送Solicit。
思路为：使用netlink_kernel_create建立消息机制。在DHCPv6 Client初始化的时候，新建类型为NETLINK_RTK_ICMP6_RA 的netlink socket，监听kernel的消息，并且把自己的pid发送给kernel， kernel 收到消息后，存储pid为全局变量。当kernel收到RA包后，将发送消息给pid的用户进程。用户进程收到消息，发送solicit。
Kernel中接收ICMPv6 RA的代码在linux-3.10/net/ipv6/ndisc.c。
Kernel：
- 初始化IPC
在ndisc_init()中加上进程通信的socket。
//create netlink socket
struct netlink_kernel_cfg cfg = {
.input = nl_icmp_input,
};
printk("%s: before netlink_kernel_create\n", __func__);
icmp6_ra_sock = netlink_kernel_create(&init_net, NETLINK_RTK_ICMP6_RA, &cfg);
if (icmp6_ra_sock == NULL) {
printk(KERN_ERR "Netlink[Kernel] Cannot create netlink socket for ipv6 ra.\n");
return -EAFNOSUPPORT;
}
printk("Netlink[Kernel] create netlink socket for ipv6 ra ok.\n");
其中NETLINK_RTK_ICMP6_RA 为在netlink.h中定义的一个类型。nl_icmp_input用于接收用户层发来的信息。
2. 接收DHCPv6 Client发来的pid 信息
static void nl_icmp_input(struct sk_buff *__skb)
{
struct sk_buff *skb;
struct nlmsghdr *nlh;
char str[100];
struct completion cmpl;
int i=10;
skb = skb_get (__skb);
if(skb->len >= NLMSG_SPACE(0))
{
nlh = nlmsg_hdr(skb);
memcpy(str, NLMSG_DATA(nlh), sizeof(str));
printk("Message received:%s\n",str) ;
DHCP6C_PID = nlh->nlmsg_pid;
printk("DHCP PID = %d:%s\n", DHCP6C_PID) ;
kfree_skb(skb);
}
return;
}
3. 收到RA后，发消息给用户进程
在ndisc_router_discovery中，确认RA包ok后，发送消息。
if (icmp6_ra_sock) {
printk("%s: ra socket is not null\n", __func__);
payloadLen = NLMSG_SPACE(payloadLen);
/*Alloc skb ,this check helps to call the fucntion from interrupt context */
if(in_atomic())
{
skb_ra = alloc_skb(payloadLen, GFP_ATOMIC);
}
else
{
skb_ra = alloc_skb(payloadLen, GFP_KERNEL);
}
if(!skb_ra)
{
printk(KERN_ERR "failed to alloc skb in %s",__FUNCTION__);
return;
}
nl_msgHdr = (struct nlmsghdr *)skb_ra->data;
nl_msgHdr->nlmsg_type = MESSAGE_ICMP6_RA;
nl_msgHdr->nlmsg_pid=0;/*from kernel */
nl_msgHdr->nlmsg_len = payloadLen;
nl_msgHdr->nlmsg_flags =0;
NETLINK_CB(skb_ra).portid = 0; /*from kernel */
skb_ra->len = payloadLen;
err = netlink_unicast(icmp6_ra_sock, skb_ra, DHCP6C_PID, MSG_DONTWAIT); //send to dhcp6c
if (err < 0)
printk("%s: nfnetlink_unicast icmpv6 ra socket fail\n", __func__);
else
printk("%s: nfnetlink_unicast icmpv6 ra socket ok\n", __func__);
}
用户层：
- Dhcp6c 初始化，并发送pid给kernel
int init_ra_monitor_fd()
{
int ret = 0;
struct sockaddr_nl src_addr, dest_addr;
struct nlmsghdr *nlh = NULL;
struct iovec iov;
struct msghdr msg;
int state_smg = 0;
int pid = getpid();
if ((rasock = socket(AF_NETLINK, SOCK_RAW, NETLINK_RTK_ICMP6_RA)) < 0)
{
printf("Could not open netlink socket for kernel monitor\n");
return 1;
}
else
{
printf("kernelMonitorFd=%d\n", rasock);
}
memset(&src_addr, 0, sizeof(src_addr));
src_addr.nl_family = AF_NETLINK;
src_addr.nl_pid = pid;
src_addr.nl_groups = 0; // multicast
if (bind(rasock, (struct sockaddr *)&src_addr, sizeof(src_addr)) < 0)
{
printf("Could not bind netlink socket for kernel monitor\n");
close(rasock);
rasock = -1;
return 1;
}
/* Tell kernel dhcp6c pid, kernel will send messages to dhcp6c when it receive ICMPv6 RA */
/* Fix IPv6 Ready logo rfc3315 */
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));
if(!nlh){
printf("malloc nlmsghdr error!\n");
close(rasock);
return -1;
}
nlh->nlmsg_len = NLMSG_SPACE(MAX_PAYLOAD);
nlh->nlmsg_pid = pid;
nlh->nlmsg_flags = 0;
sprintf(NLMSG_DATA(nlh), "%d", pid);
iov.iov_base = (void *)nlh;
iov.iov_len = NLMSG_SPACE(MAX_PAYLOAD);
memset(&msg, 0, sizeof(msg));
msg.msg_name = (void *)&dest_addr;
msg.msg_namelen = sizeof(dest_addr);
msg.msg_iov = &iov;
msg.msg_iovlen = 1;
printf("state_smg\n");
state_smg = sendmsg(rasock, &msg, 0);
if(state_smg == -1)
{
printf("get error sendmsg = %s\n",strerror(errno));
}
}
2. 等待接收消息
// add to read sockets
if (rasock >= 0) {
FD_SET(rasock, &r);
maxsock = (rasock > maxsock) ? rasock : maxsock;
}
//wait
ret = select(maxsock + 1, &r, NULL, NULL, w);
if (FD_ISSET(rasock, &r)) {
printf("%s: have receive the ra message\n", __func__);
dhcp6_ra_recv(); //send solicit
}
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