Find process and send signal to it in Kernel

工作上覺得還蠻實用的程式片段。

struct task_struct *p;  
struct siginfo info;
      
pid_t pid = -1;  
     
for_each_process(p) { 
   if (!strcmp(p->comm, "process name")){  
      pid = p->pid;      
      break;  
   }  
}


if(pid != -1){
   
   if(p != NULL) {
      info.si_signo = SIGUSR1;
      info.si_code = SI_QUEUE;
      info.si_int = 0x3;
      send_sig_info(SIGUSR1, &info, p);

      printk(KERN_EMERG "Send SIGUSR1 with value %d to %s(%d)\n", info.si_int, "process name", pid);
   }        

}

Netlink 使用方法

Kernel Korner - Why and How to Use Netlink Socket這篇寫得非常好，關於netlink的使用方法，範例也非常簡潔易懂。

這裡就貼上關於不使用group方式process送訊息至kernel。

User Space

#include <sys/socket.h>
#include <linux/netlink.h>

#define MAX_PAYLOAD 1024  /* maximum payload size*/
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(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 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);

 /* Read message from kernel */
 memset(nlh, 0, NLMSG_SPACE(MAX_PAYLOAD));
 recvmsg(fd, &msg, 0);
 printf(" Received message payload: %s\n",
    NLMSG_DATA(nlh));

 /* Close Netlink Socket */
 close(sock_fd);
}

Kernel Space

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;
 u32 pid;

 nl_sk = netlink_kernel_create(NETLINK_TEST,
                                   nl_data_ready);
 /* wait for message coming down from user-space */
 skb = skb_recv_datagram(nl_sk, 0, 0, &err);

 nlh = (struct nlmsghdr *)skb->data;
 printk("%s: received netlink message payload:%s\n",
        __FUNCTION__, NLMSG_DATA(nlh));

 pid = nlh->nlmsg_pid; /*pid of sending process */
 NETLINK_CB(skb).groups = 0; /* not in mcast group */
 NETLINK_CB(skb).pid = 0;      /* from kernel */
 NETLINK_CB(skb).dst_pid = pid;
 NETLINK_CB(skb).dst_groups = 0;  /* unicast */
 netlink_unicast(nl_sk, skb, pid, MSG_DONTWAIT);
 sock_release(nl_sk->socket);
}

ipv6 pton空字串

當inet_pton進一個空字串再反向inet_ntop回來時，值會變怎樣呢？
範例程式碼如下

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
int main()
{
    unsigned char a[16];
    char b[64] = "";
    char c[64] = "";
    
    
    inet_pton(PF_INET6, b, &a);
    inet_ntop(PF_INET6, b, c, sizeof(c));
    
    fprintf(stderr, "c is %s\n", c);
    
    return 0;
}

結果：

./ntop_test 
c is ::

2012.7.12更新

在這個範例中inet_pton轉換空字串其實是失敗的，並沒有寫入a中。

其實::的結果是c為空字串轉換回去的結果。