How To Add System Call In Ubuntu OS

How To Add System Call In
UBUNTU Operating System
Presented
by:
Pratik
Tambekar
Guided
by:
Mr. T.R
Ravi

CONTENTS
WHY UBUNTU?
INTRODUCTION
WHAT IS KERNEL?
WHAT IS SYSTEM CALL?
HOW TO ADD SYSTEM CALL
HOW TO REPLACES NEW KERNEL
FILE WITH OLD KERNEL FILE
REFRENCES

WHY UBUNTU?
Free Ubuntu is and will always be free in the
future. Ubuntu is developed by people all over the
world embracing the principle of Free libre open-source
software (FLOSS). This enables new
software and updates to be available free of cost
since they are written by volunteers and also the
employees of Canonical, the parent company of
Ubuntu.
No Viruses While using Ubuntu, you do not need
to worry about installing any anti-virus programs
since Ubuntu is completely free of viruses.

CONT….
Community Support When you need help using
your system, the community is available
everywhere around you to support you at all times.
All this are done voluntarily by people passionate
about Ubuntu.
Up to date software Ubuntu will always be up to
date with updates released regularly to ensure that
your system is secure and bug free. These regular
updates will be always be available for free.
Beautiful, Polished, Stable These are the goals of
every Ubuntu release. Your Ubuntu is designed
with the help of the community and experts after
extensive discussion. Ubuntu is regularly user
tested to ensure that it is easy and simple to use
while preserving its elegance and polish.

INTRODUCTION
 Ubuntu was founded by Mark Shuttleworth, a South
African entrepreneur coming up with their first release
Ubuntu 4.10 codenamed Warty Warthog in October
2004.
 Ubuntu is an African concept meaning “humanity
toward others”
 Sponsored by Canonical Ltd. Owned by South African
billionaire Mark Shuttleworth.
 Ubuntu is a Linux-based Operating System that is open
sourced (free)
 Pronounced (oo-BOON-too)
 Strong focus on usability and ease of installation

Advantages and Disadvantages
over windows.
- Advantages:
1) Not vulnerable to Windows malware/viruses
2) Free
3) Tends to run more efficiently than Windows
4) Not subject to Microsoft's cripple ware.
- Disadvantages:
1) Won't run most Windows applications (especially
games)
2) Requires a higher than average nerd factor (not as
user friendly)
3) Doesn't work with all hardware (but neither does
Windows!)
4) Can be somewhat buggy

What is Kernel?
The central module of an operating system. It is
the part of the operating system that loads first,
and it remains in main memory. Because it stays
in memory, it is important for the kernel to be as
small as possible while still providing all the
essential services required by other parts of the
operating system and applications.
It acts as an interface between the user
applications and the hardware.
Typically, the kernel is responsible for memory
management, process management, task
management, I/O communication and disk
management.

CONT....
Types of Kernels:
Kernels may be classified mainly in two
categories
1) Monolithic
2) Micro Kernel
Linux follows the monolithic modular
approach

CONT....
Monolithic kernel:
In this type of kernel
architecture, all the basic system services
like process and memory management,
interrupt handling etc were packaged into a
single module in kernel space. This type of
architecture led to some serious drawbacks
like
1) Size of kernel, which was huge.
2) Poor maintainability, which means bug
fixing or addition of new features resulted in
recompilation of the whole kernel which could
consume hours

What is System Call?
System calls are low level functions the
operating system makes available to
applications via a defined API (Application
Programming Interface)
System calls represent the interface the kernel
presents to user applications.
In Linux all low-level I/O is done by reading and
writing file handles, regardless of what
particular peripheral device is being
accessed—a tape, a socket, even your
terminal, they are all files.
Low level I/O is performed by making system
calls.

Anatomy of a System Call
A System Call is an explicit request to the kernel
made via a software interrupt.
The interrupt call ‘0x80’ call to a system call
handler (sometimes called the “call gate”).
The system call handler in turns calls the system
call interrupt service routine (ISR).
To perform Linux system calls we have to do
following:
- Put the system call number in EAX register.
- Set up the arguments to the system call in
EBX,ECX, etc.
- call the relevant interrupt (for DOS, 21h; for
Linux, 80h).
- The result is usually returned in EAX.

CONT….
There are six registers that are used for the
arguments that the system call takes. The first
argument goes in EBX, the second in ECX, then
EDX, ESI, EDI, and finally EBP. If more then 6
arguments needed (not likely), the EBX register
must contain the memory location where the list
of arguments is stored.
basic system calls:
◦ sys_open
◦ sys_close
◦ sys_read
◦ sys_write
◦ sys_Fork
◦ sys_exit

CONT….
Sys_open - open a file
system call number (in EAX): 5
arguments:
◦ EBX: The pathname of the file to open/create
◦ ECX: set file access bits (can be OR’d togather):
O_RDONLY open for reading only
O_WRONLY open for writing only
O_RDRW open for both reading and writing
O_APPEND open for appending to the end of
file
O_TRUNC truncate to 0 length if file exists
O_CREAT create the file if it doesn’t exist
◦ EDX: set file permissions.
Returns in EAX: file descriptor.

Error handling
System calls set a global integer called
errno on error.
The constants that errno may be set to are
(partial list):
◦ EPERM operation not permitted.
◦ ENOENT no such file or directory (not
there).
◦ EIO I/O error – EEXIST file already exists.
◦ ENODEV no such device exists.
◦ EINVAL invalid argument passed.

How to add a System Call
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”.

CONT….
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 */

CONT….
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

CONT….
#define NR_syscalls 341
 Change it to
#define NR_syscalls 342.
3. Now edit
arch/x86/include/asm/unistd_64.h
 Find out:
#define __NR_prlimit64 302
__SYSCALL(__NR_prlimit64,sys_prli
mit64)

CONT….
 Now after these two lines, add these
two lines
#define __NR_add2 303
__SYSCALL(__NR_add2, sys_add2)
4. 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

CONT…
#endif
write this prototype definition line:
asmlinkage long sys_add2(int i,int j);
5. 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:

CONT…
#include<linux/linkage.h>
asmlinkage long sys_add2(int i,int j)
{
return i+j;
}
6. Now open the Makefile in this
folder(/kernel/Makefile) and find out
obj-y += groups.o

CONT…
 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.

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

CONT…
int main(void)
{
int a,b,c;
printf("Adding Two Numbers in Kernel Spacen");
printf("Input a: ");
scanf("%d",&a);
printf("Input b: ");
scanf("%d", &b);
c = syscall(sys_add2, a, b);
printf("System call returned %dn", c);
return 0;
}

How to replaces new kernel file
with old Kernel File:
download the kernel from kernel.org
http://www.kernel.org/pub/linux/kernel/
v3.0/linux-3.X.tar.bz2
Extract the tarball to /usr/src/linux-3.X/
tar -xvf linux-3.X.tar.bz2 -C
/usr/src/linux-3.X/
Change the directory to /usr/src/linux-
3.x
$ cd /usr/src/linux-3.x

CONT…
Configure
Lets configure our new Linux Kernel with the
configuration of old kernel (which is already
installed in our machine)
$ sudo make oldconfig
The new kernel is installed, however we need to
say to the Ubuntu to use this new one. Just type in
the shell:
sudo update-initramfs -k 2.6.38.3 -u
sudo update-grub
Restart your computer. After loaded, type in the
shell:
uname -a

.deb Files
linux-headers-2.6.34-
02063415_2.6.34-
02063415.201402101835_all.deb
linux-headers-2.6.34-02063415-
server_2.6.34-
02063415.201402101835_amd64.deb
linux-image-2.6.34-02063415-
generic_2.6.34-
02063415.201402101835_amd64.deb
sudo dpkg -i linux*2.6.34-7.37*.deb
sudo reboot

Refrences:
http://www.kernel.org
http://www.Ubuntu.com
kernel.ubuntu.com/~kernel-ppa/
mainline/
https://wiki.ubuntu.com/Kernel/Mainlin
eBuilds
ubuntuforums.org
www.ubuntu.com/support/community
http://www.linuxquestions.org

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