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：

1. 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：

1. 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

}