CS 300 : Operating Systems

Adding a new system call to the Linux kernel

• cd linux-source
• Change your current working directory to the kernel directory. From now on we assume you are in the kernel directory
  
  
• mkdir cs300
• Make a new directory to host your new system call. We will call this directory cs300/
  
  
• vi cs300/cs300_test.c
• Open a new file and add your system call there. As described, ours will be called cs300_test, it will take a single argument (int) and print it along with a simple greeting:
  
  #include <linux/kernel.h>
  
  asmlinkage long sys_cs300_test(int arg0)
  {
    printk(" Hello World !");
    printk("--syscall arg %d", arg0);
  
    return((long) arg0);
  }
  
  Save your file under cs300/
  
• Function printk is the kernel's analogue of printf
• Due to space limitations we have jmacs in the virtual disk image but not emacs - it is Joe's editor with emacs key-bindings and should be familiar for emacs users.
  
  
• vi cs300/Makefile
• Open a new Makefile to inform the kernel how to compile your new file(s). You only need to define how the single file you gave is turned into an object file, so in this simple case
  
  #Makefile start
  
  obj-y := cs300_test.o
  
  #Makefile end
  
  Save your file under cs300/
  
  
• vi Makefile
• Open the topmost Makefile and inform it of the new directory. The Makefile will descend into it and recursively build whatever it can. To do so, find the line where core-y is defined (~610) and add your new directory to it:
  
  core-y        += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ cs300/
  
  Save the Makefile where it was.
  

• vi arch/x86/kernel/syscall_table_32.S
• Edit (with vi or any editor of your preference) syscall_table_32.S to add a new system call function pointer. Go to the last line and add the line
  
  .long sys_cs300_test           /* 327 */
  
  By this line you add the pointer to your function in the system-calls table; this is system call number 327 for i386 processors.
  
  
• vi  include/asm-x86/unistd_32.h
  
  • Edit unistd_32.h to add your  new system call number. Find where the rest of the system call numbers are, go to the last one (326) and add the line
    
    #define __NR_cs300_test        327
    
    Now you defined how your system call function pointer can be referenced from the system calls table.
    

• mkdir cs300
• Make a new directory to host your new library call. We will call this directory cs300/
  
  
• cd cs300
• Move to the new directory
  
  
• vi cs300_test.c
• Open a new file and add your library call declaration there.
  
  #include <stdio.h>
  #include <linux/unistd.h>
  #include <sys/syscall.h>
  
  #define _CS2456_TEST_ 327
  
  int main(int argc, char *argv[])
  {
    printf("\nDiving to kernel level\n\n");
    syscall(_CS2456_TEST_ , 2456);
    printf("\nRising to user level\n\n");
      
     return 0;
  }
  
  Save your file.
  
  
• gcc cs300_test.c - o cs300_test
• Compile your user level syscall interface
  

• Boot your new kernel
• qemu-system-x86_64 -m 64M -hda debian_squeeze_amd64_standard.qcow2 -append "root=/dev/hda1 console=ttyS0,115200n8 console=tty0" -kernel linux-source/arch/i386/boot/bzImage
• You should be able to verify that this is a new kernel by checking its revision ( ex. uname -a )
  
• Go the directory where you stored your library interface to the syscall and execute the test you wrote
• Your system call should now work if you try it :
• The example above printed the following output to our virtual machine:
  
  Diving to kernel level                                                          
                                                                                   
   [   76.703723]  Hello World !--syscall arg 2456                                 
   Rising to user level
• If you try the same system call in the kernel that does not support it (like the default Debian kernel), you would just see the user level printouts:
• Diving to kernel level                                                          
                                                                                   
   Rising to user level