Modifying necessary kernel files to integrate our system call

Modify Makefile

First, we need to add the new ‘info’ directory to the kernel’s Makefile.

Change back into the linux-4.13.11 folder and open Makefile

Goto line number 944 which says:

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

and change it to

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

This tells the compiler that the source files of our new system call can be found in the info/ directory.

Modify syscall_32.tbl

Now, we’ll have to alter the syscall_32.tbl. A neat way to figure out where this file is present is to use the find command on the terminal from the linux-4.13.11 directory.

find -name syscall_32.tbl   ### Should show the file's location

(This is equivalent to using the find option(ctrl+F) in nautilus to look for where a specific file is located)

In kernel 4.13.11, it is present in /arch/x86/entry/syscalls/syscall_32.tbl.

Now, edit the file as shown to include the new system call number and its entry point. Just note the system call number for reference. (Ideally, we should implement a wrapper for our system call so that users can invoke this new sys call by a function name. But, in this example, we skip this and users will need to invoke this new sys call through the sys call number.)

The number of the system call should be one plus the number of the last system call (it was 385 in our case).

380    i386    pkey_mprotect        sys_pkey_mprotect
381    i386    pkey_alloc        sys_pkey_alloc
382    i386    pkey_free        sys_pkey_free
383    i386    statx            sys_statx
384    i386    arch_prctl        sys_arch_prctl            compat_sys_arch_prctl
##############< Make your edit here >###################

Modify syscalls.h

Finally, we’ll have to alter the syscalls.h file. Again, use find look for where the syscalls.h file is present.

find -name syscalls.h

In kernel 4.13.11, it is found in /include/linux/.

Add the following line to the end of the file (before the #endif) as shown:

asmlinkage long sys_listProcessInfo(void);