Adding new System Call to Linux Kernel 3.13 on 64 bit system

Question

I'm trying to add a simple helloworld System Call to kernel 3.13.0-37-generic on 64 bit system.

I'll try to show what I did so far step by step :

1- I've downloaded kernel source by :

sudo apt-get source linux-image-3.13.0-37-generic

After that, kernel source files extracted to /usr/src/

2- Define a new system call sys_hello() :

I've created a directory with hello name in the kernel source directory in /usr/src/linux-3.13/

And I created a hello.c file in hello directory with below content :

#include <linux/kernel.h>

asmlinkage long sys_hello(void)
{
        printk(“Hello world\n”);
        return 0;
}

Then I created a Makefile in the hello directory with following content :

obj-y := hello.o

3- Add the hello directory to the kernel’s Makefile

I changed following line in /usr/src/linux-3.13/Makefile:

core-y += kernel/ mm/ fs/ ipc/ security/ crypto/ block/

to :

core-y += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ hello/

4- Add the new system call sys_hello() into the system call table (syscall_64.tbl file)

Because I'm using 64 bit system, I need to alter the syscall_64.tbl file in :

/usr/src/linux-3.13/arch/x86/syscalls/syscall_64.tbl

Added the following line in the end of the file :

-Last line number was 313

314     common    hello    sys_hello

5- Add the new system call sys_hello() in the system call header file

vim /usr/src/linux-3.13/include/linux/syscalls.h

I've added the following line to the end of the file just before the #endif statement at the very bottom :

asmlinkage long sys_hello(void);

6- Compiling this kernel on my system

To configure the kernel I tried the following command :

sudo make menuconfig

After above command a pop up window came up and I made sure that ext4 was selected and then save.

Then :

# cd /usr/src/linux-3.13/
# make

It tooks 2~3 hours.

After that :

# make modules_install install

After that, I rebooted my system.

7- Test the system call (Problem Is Here)

After reboot, I created a file with name hello.c in home folder with following content :

#include <stdio.h>
#include <linux/kernel.h>
#include <sys/syscall.h>
#include <unistd.h>
int main()
{
         long int amma = syscall(314); // 314 is the number of sys_hello line in `syscall_64.tbl`
         printf(“System call sys_hello returned %ld\n”, amma);
         return 0;
}

Then :

# gcc hello.c
# ./a.out

The output is :

System call sys_hello returned -1

Problem exactly is -1. It must return 0 not -1.

Seems like sys_hello doesn't added to Kernel System Call.

What am I doing wrong?

Answer 1

The problem was from step 6 to last step (Compile Kernel).

After step 5, we have to do following steps :

6- Compiling this kernel on my system

To configure the kernel I tried the following command :

# make menuconfig

After above command a pop up window came up and I made sure that ext4 was selected and then save.

Then to create DEB file from new kernel we have to :

# make -j 5 KDEB_PKGVERSION=1.arbitrary-name deb-pkg

It will create some deb files in /usr/src/.

After that we need to install them :

# dpkg -i linux*.deb

It will install new kernel on your system.

Now, reboot your system. After system rebooted you can find out whether new kernel is installed or not :

$ uname -r

And if you want to know your new System Call added to kernel or not just type :

$ cat /proc/kallsyms | grep <system call name>

In my case :

$ cat /proc/kallsyms | grep hello

Following output indicates that your System Call successfully added to the Kernel :

0000000000000000 T sys_hello