This is just a brief description, so read at your own risk.
I have tested & used it. So it should be working. Please install a fresh install of Ubuntu.
This tutorial is for both 32 and 64 bit x86 processors and operating system. I have assumed that you are working in Ubuntu 10.10 and using kernel version 2.6.37.3 . If you are using any other kernel version just replace 2.6.37.3 with your version. I am also assuming you have extracted the source code.
Now let the new system call’s name be “add2”.
1. Now you will find a “arch” folder in the source code folder. Open the file arch/x86/kernel/syscall_table_32.S in a text editor. Go to the end of the document and add this line –
.long sys_add2 /* my code */
2. Now open arch/x86/include/asm/unistd_32.h and find out
#define __NR_prlimit64 340
Add a new line after this:
#define __NR_add2 341
Don’t just close yet. After 3-4 lines, you will find a line like
#define NR_syscalls 341
Change it to
#define NR_syscalls 342
4. Now edit arch/x86/include/asm/unistd_64.h
Find out:
#define __NR_prlimit64 302
__SYSCALL(__NR_prlimit64, sys_prlimit64)
Now after these two lines, add these two lines
#define __NR_add2 303
__SYSCALL(__NR_add2, sys_add2)
5. Now again in the source folder you will find a folder named include. Open the file include/linux/syscalls.h and go to the end of the file. Before the line
#endif
write this prototype definition line:
asmlinkage long sys_add2(int i,int j);
6. Now find out the kernel folder in the source directory. Create a new empty file in the kernel folder with the name “mysysteamcalls.c” . Add the following codes in the file:
#include<linux/linkage.h>
asmlinkage long sys_add2(int i,int j)
{
return i+j;
}
7. Now open the Makefile in this folder(/kernel/Makefile) and find out
obj-y += groups.o
Add a new line before this line :
obj-y += mysysteamcalls.o
Ok, this is the edit you need to do to add a new system call. Now compile or recompile the source code and enjoy your new system call.
Here is a sample code to call the system call :
#include <stdio.h>
#include <linux/unistd.h>
#include <sys/syscall.h>
//comment the following line if you are using 64 bit, this number is the same used previously
#define sys_add2 341
//comment the following line if you are using 32 bit, this number is the same used previously
#define sys_add2 303
int main(void)
{
int a,b,c;
printf("Adding Two Numbers in Kernel Space\n");
printf("Input a: ");
scanf("%d",&a);
printf("Input b: ");
scanf("%d", &b);
c = syscall(sys_add2, a, b);
printf("System call returned %d\n", c);
return 0;
}
Important note: To add a new system call, you don’t need to create a new file, you can just add a new function in the same “mysysteamcalls.c” file. And if you don’t create a new file you don’t have to do the step 7.
-Enzam
