/* mod-syscall-hello.c 
 *   Linux Kernel Module Programmer's Guide for 2.4 Kernel.
 *   A simple system call, printk and return starting from the
 *   Ch 8.1 file access spy program.
 * 
 *   We'll used an existing syscall entry (futex) and replace it
 *
 *   Include file locations
 *      <linux> is /usr/src/linux/include/linux
 *      <asm>   is /usr/src/linux/include/asm
 *      <sys>   is /usr/include/sys
 *
 *   Source file locations
 *      sys_call_table[]    /usr/src/linux/arch/i386/kernel/entry.S
 */

/* kernel module programming */
#ifndef MODULE
#define MODULE
#endif

#ifndef LINUX
#define LINUX
#endif

#ifndef __KERNEL__
#define __KERNEL__
#endif

#define MAX_BUF 80

#include <linux/module.h>    /* needed by all modules */
#include <linux/kernel.h>    /* for KERN_ALERT */
#include <asm/unistd.h>      /* for __NR_futex system call number */

/* Not used yet */
/* #include <asm/uaccess.h>      user space memory access functions */


/* The kernel will fill in the call table at dynamic load (insmod) */
extern void *sys_call_table[];

/* prototypes */
asmlinkage void my_syscall_hello(void);
int init_module(void);
void cleanup_module(void);

/* My hello world system call */
asmlinkage void my_syscall_hello(void)
{ printk(KERN_ALERT "my_syscall_hello: entering.\n");
  printk(KERN_ALERT "my_syscall_hello: leaving.\n");
}

/* Original system call in sys.c of src/linux/kernel */
asmlinkage long (*original_call) (void);


/* Initialize the module at insmod - insert the new system call */
int init_module(void)
{ printk(KERN_ALERT "syscall module loaded\n");

  /* save the original syscall entry and insert our own entry */
  original_call = sys_call_table[__NR_futex];
  sys_call_table[__NR_futex] = my_syscall_hello;

  /* A non-zero return means init_module failed; module can't be loaded */
  return 0;
}

/* Restore the preious system call at rmmod */
void cleanup_module(void)
{ if (sys_call_table[__NR_futex] != my_syscall_hello)
  { printk(KERN_ALERT "syscall module entry has been corrupted\n");
  }

  /* restore the original syscall entry */
  sys_call_table[__NR_futex] = original_call;

  printk(KERN_ALERT "syscall module unloaded\n");
}

MODULE_LICENSE("GPL");