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System Call

                                 SYSTEM CALL IMPLEMENTATION

A system call is how a program requests a service from an operating system's kernel that  it does not normally have permission to run. System calls provide the interface between a  process  and the operating  system. Most operations interacting  with the system require permissions not available to a user level process, e.g. I/O performed with a device present  on the system, or any form of communication with other processes requires the use of  system calls.

Generally, systems provide a library that sits between normal programs and the operating  system, usually an implementation of the C library(libc), such as glibc. This library exists  between the OS and the application, and increases portability. On Unix, Unix­like and other  POSIX­compatible   Operating   Systems,   popular   system   calls  are open, read, write, close, wait, exec, fork, exit,   and kill.   Many   of   today's   operating  systems have hundreds of system calls. For example, Linuxhas 319 different system calls.  Similarly, FreeBSD has almost 500.

Under   Linux   the   execution   of   a   system   call   is   invoked   by   a   maskable   interrupt  or exception class   transfer,   caused   by  the   instruction int   0x80.   We   use   vector   0x80   to  transfer control to the kernel. This interrupt vector is initialized during system startup, along  with other important vectors like the system clock vector. After the switch to kernel mode,  the processor must save all of its registers and dispatch execution to the proper kernel  function.

Implementation

Note:   This   is   implemented   in   Debian   Lenny   with   2.6.26   kernel.   The   method   differs   in  different distributions/kernel. The system call is named mycall and it takes two integers as input and return their sum.

                  1)Add “.long sys_mycall” at the end of the list in the file syscall_table.S.

Full   path   for   the   file   syscall_table.S                         is   /usr/src//linux­source-2.6.26/arch/x86/kernel/syscall_table_32.S.

                 2) Add system call along with the number in unistd_32.h

Add “ #define __NR_mycall 327 ” at the end of the list. 327 is the last system call number + 
1. [#define __NR_timerfd_settime 325 #define __NR_timerfd_gettime 326 ].

Full   path   for   the   file   unistd_32.h   is   /usr/src//linux­source­2.6.26/include/asm­
x86/unistd_32.h

               3) Add the following line at the end of the file syscalls.h:

asmlinkage long sys_mycall(int i, int j);
All  system   calls  are   marked   with   the   asmlinkage   tag,   so   they   all   look   to   the   stack   for 
arguments.

Full path of syscalls.h is /usr/src//linux­source­2.6.26/include/linux/syscalls.h.

              4) Add mycall/ to core­y += in Makefile.

The line in the end shall look like:
core­y += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ mycall/

Full path for Makefile is /usr/src//linux­source­2.6.26/Makefile.

           5) Create a new directory in /usr/src//linux­source­2.6.26/ and name it mycall

          6) Create a new file called mycall.c in /usr/src//linux­source­2.6.26/mycall. Contents of the file shall be as follows:

#include <linux/linkage.h>
asmlinkage long sys_mycall(int i, int j)
{
return(i+j);
}

           7) Create Makefile in /usr/src//linux­source­2.6.26/mycall. Makefile shall be like:

obj­y := mycall.o

          8) Compile the kernel througg the normal procedure

         9) Create   the   following   userspace   program   to   test   your   system   call   and   name   it  testmycall.c. The contents of this file shall be:

#include <unistd.h>
#include <stdio.h>
#define __NR_mycall 327
long mycall(int i, int j)
{
return syscall(__NR_mycall, i, j);
}
int main()
{
printf("%d\n", mycall(12,10));
return 0;
}

         10) Compile it to test whether the system call works

root@jestinjoy­desktop:/# gcc mycall.c
root@jestinjoy­desktop:/# ./a.out
root@jestinjoy­desktop:/#22

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