/* mod-syscall-retparm.c 
 *   System calls with printk, but this time replace three
 *   system calls, no parms and results, no parms with results,
 *     and parms with results.
 *
 *   Derived from the no-parameter or result hello-1 example in the
 *   Linux Kernel Module Programmer's Guide for 2.4 Kernel.
 * 
 *   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 & other system call numbers */


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

/* prototypes */
asmlinkage void my_syscall(void);
asmlinkage int my_syscall_ret(void);
asmlinkage int my_syscall_parm(char *msg);
int init_module(void);
void cleanup_module(void);

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

/* My module system call to return a value
 * The value must be on the stack. If the stack
 *   value is a reference to memory, then it
 *   won't work.
 */
asmlinkage int my_syscall_ret(void)
{ printk(KERN_ALERT "my_syscall_ret: entering.\n");
  printk(KERN_ALERT "my_syscall_ret: leaving.\n");
  return 7;
}

/* My module system call to pass a parameter and return a value */
asmlinkage int my_syscall_parm(char *msg)
{ printk(KERN_ALERT "my_syscall_parm: entering.\n");
  printk(KERN_ALERT "message from user: %s\n", msg);
  printk(KERN_ALERT "my_syscall_parm: leaving.\n");
  return 11;
}

/* Original system call for NR_futex (240) in sys.c of src/linux/kernel/sys.c
 * The original call is sys_ni_syscall and returns an error number.
 * Same for the other two unused system calls 241 & 242 here.
 */
asmlinkage long (*original_call_240) (void);

/* Original system call for NR_sched_setaffinity sys.c of src/linux/kernel */
asmlinkage long (*original_call_241) (void);

/* Original system call for NR_sched_getaffinity sys.c of src/linux/kernel */
asmlinkage long (*original_call_242) (void);


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

  /* save the original syscall entry and insert our own entry */
  original_call_240 = sys_call_table[__NR_futex];
  original_call_241 = sys_call_table[__NR_sched_setaffinity];
  original_call_242 = sys_call_table[__NR_sched_getaffinity];
  sys_call_table[__NR_futex] = my_syscall;
  sys_call_table[__NR_sched_setaffinity] = my_syscall_ret;
  sys_call_table[__NR_sched_getaffinity] = my_syscall_parm;

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

/* Restore the preious system call at rmmod.
 * Check to see if someone else tried to replace our
 * system calls while we were loaded.
 */
void cleanup_module(void)
{ if (sys_call_table[__NR_futex] != my_syscall ||
      sys_call_table[__NR_sched_setaffinity] != my_syscall_ret ||
      sys_call_table[__NR_sched_getaffinity] != my_syscall_parm)
  { printk(KERN_ALERT "syscall module entry has been corrupted\n");
  }

  /* restore the original syscall entry */
  sys_call_table[__NR_futex] = original_call_240;
  sys_call_table[__NR_sched_setaffinity] = original_call_241;
  sys_call_table[__NR_sched_getaffinity] = original_call_242;

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

MODULE_LICENSE("GPL");