Network Programming in the Kernel

Prerequisites : Socket and Module programming

Advantage of implementing application within the kernel : 

• The overhead associated with user-space/kernel-space transition can be avoided by implementing the application completely in the Kernel Space .
• In specific research and high-performance computing environments, there is a need for achieving data transfers at great speeds. Kernel applications find use in such situations.

Disadvantages:

• Security is a primary concern within the kernel, since kernel process run with unlimited privileges.Consequently, special care needs to be taken while designing in-kernel applications.
• Large applications cannot be implemented in the kernel due to memory constraints.
• Every kernel needs a specific implementation route.
• Failure in the web server may cause the OS to crash.

/*  

 * Called when a process writes to dev file

 * Stores the string written to the device in

 * local buffer and sends the string using

 * SOCK_STREAM to 127.0.0.1:2001

 */

static ssize_t

device_write(struct file *filp, const char *buff, size_t len, loff_t * off)

{

int ret; //return value

int k,leng;

struct sockaddr_in server;

struct socket *soc=NULL;

struct iovec iov;

struct msghdr msgh;

mm_segment_t oldmm;

printk(KERN_INFO "[device_write]File opened for writing\n");

ret=copy_from_user(msg,buff,80);

//buff : buffer that accepts the string from 

// the program that issued a write.

// Points to user area memory

//msg  : kernel/module space that hold the 

// value written.

msgh.msg_name     = 0;

msgh.msg_namelen  = 0;

// Socket address members msg_name and msg_namelen.

// Required only when your socket is a datagram socket.

// msg_name member points to the socket address that 

// you are sending to or receiving from.

    msgh.msg_iov      = &iov;

    msgh.msg_iovlen   = 1;

// I/O vector references msg_iov and msg_iovlen.

// msg_iov member points to the struct iovec array.

// msg_iovlen indicates how many elements are in your 

// I/O vector array 

    msgh.msg_control  = NULL;

    msgh.msg_controllen = 0;

// Ancillary data buffer members msg_control    

// and msg_controllen. Used to pass protocol-specific 

// control messages. No control messages for TCP.

    msgh.msg_flags    = MSG_DONTWAIT;

// Received message flag bits msg_flags.

// MSG_DONTWAIT prevents the system call from blocking if, 

// for example, there are no data to be received.

    oldmm = get_fs();

    set_fs(KERNEL_DS);

// get_fs returns the current segment descriptor stored in FS.

// set_fs stores a descriptor into FS, so it will be used for 

//  data transfer instructions.

    msgh.msg_iov->iov_len = 80;

    msgh.msg_iov->iov_base = (char *) msg;

    printk(KERN_INFO "[chardev:device_write] msg : %s\n",msg);

    k=sock_create(PF_INET,SOCK_STREAM,IPPROTO_TCP,&soc);

    memset(&server,0,sizeof(server));

    server.sin_family=AF_INET;

    server.sin_port=2001;

    server.sin_addr.s_addr=inet_addr("127.0.0.1");

    k=soc->ops->connect(soc,(struct sockaddr *)&server,sizeof(server),O_RDWR);

    if(k!=-1) printk(KERN_INFO"[device_write] connect() OK. Next sock_sendmsg().\n");

    else printk(KERN_INFO"[device_write] connect() FAILED. Next sock_sendmsg().\n");

    leng = sock_sendmsg(soc, &msgh, 80);

//msg length 80

    if(leng) printk(KERN_INFO"[device_write] sock_sendmsg() OK. Next close().\n");

    else printk(KERN_INFO"[device_write] sock_sendmsg() FAILED. Next close().\n");

    sock_release(soc);

return ret;

}

Further Reading :

 In-Kernel Web Sever

      http://en.wikipedia.org/wiki/In-kernel_web_server

 kHTTPd

       http://www.linuxjournal.com/article/4132

       http://en.wikipedia.org/wiki/Khttpd