Kernel Korner - Network Programming in the Kernel

Now, let's take a look at a user-space program that invokes our system call to transfer a file. We explain the relevant details for calling a new system call:

...
#define __NR_my_sys_call 223
_syscall1(long long int, my_sys_call,
          struct params *, p);

int main(int argc, char **argv)
{
  struct params pm;
  /* fill pm with appropriate values */
  ...
  r =  my_sys_call(&pm);
  ...
}

#define __NR_my_sys_call 223 assigns a number to our system call. _syscall1() is a macro that creates a stub for the system call. It shows the type and number of arguments that our system call expects. With this in place, my_sys_call can be invoked just like any other system call. Upon running the program, with correct values for the source and destination files, a file from a remote FTP server is downloaded onto the client machine. Here is a transcript of a sample run:

# make
make -C /lib/modules/2.6.9/build SUBDIRS=/home/ppadala/ftp modules
make[1]: Entering directory `/home/ppadala/linux-2.6.9'
  CC [M]  /home/ppadala/ftp/ftp.o
  Building modules, stage 2.
  MODPOST
  CC      /home/ppadala/ftp/ftp.mod.o
  LD [M]  /home/ppadala/ftp/ftp.ko
make[1]: Leaving directory `/home/ppadala/linux-2.6.9'
# gcc do_ftp.c
# ./a.out <local host's IP address> 152.2.210.80 /README /tmp/README anonymous anon@cs.edu
Connection from 152.2.210.80
return = 215 (length of file copied)

We have seen a basic implementation of an FTP client within the kernel. This article explains various issues of socket programming in the kernel. Interested readers can follow these ideas to write various network applications, such as an HTTP client or even a Web server in the kernel. Kernel applications, such as the TUX Web server are used for high-performance content serving and are well suited for environments that demand data transfer at high rates. Careful attention has to be paid to the design, implementation and security issues of such applications.

Resources for this article: /article/8453.