Make sure you have linux-headers

If you have been compiled kernel from source code,and you will have linux-headers-x.x.x (Personality i compile kernel with make deb-pkg command).

Next we will use add ours custom defined system call into kernel.

The aricle based on kernel-v5.0.0-rc6

The first: please know what bits belongs to your system.

uname -m #uname -m => i686

So, my machine system is 32 bits

Add systemcall source code in kernel dir

Yes, you can git clone kernel or download kernel tar file.

You extact the source code and then cd root directory and new create a dir:

mkdir eudyptula && cd eudyptula

Open the file arch/x86/kernel/syscall_table_32.S and add the following line. (64 bits should open arch/x86/kernel/syscall_table_64.S)

387	i386	eudyptula		sys_eudyptula			__ia32_sys_eudyptula

This is the last one entry in the file above.

Source: Documentation/process/adding-syscalls.rst