/*  hello-1.c - The simplest kernel module.
 */
#include <linux/module.h>  /* Needed by all modules */
#include <linux/kernel.h>  /* Needed for KERN_ALERT */


int init_module(void)
{
   printk("<1>Hello world 1.\n");
	
   // A non 0 return means init_module failed; module can't be loaded.
   return 0;
}


void cleanup_module(void)
{
  printk(KERN_ALERT "Goodbye world 1.\n");
}  

TARGET  := hello-1
WARN    := -W -Wall -Wstrict-prototypes -Wmissing-prototypes
INCLUDE := -isystem /lib/modules/`uname -r`/build/include
CFLAGS  := -O2 -DMODULE -D__KERNEL__ ${WARN} ${INCLUDE}
CC      := gcc-3.0
	
${TARGET}.o: ${TARGET}.c

.PHONY: clean

clean:
    rm -rf ${TARGET}.o

/*  hello-2.c - Demonstrating the module_init() and module_exit() macros.  This is the 
 *     preferred over using init_module() and cleanup_module().
 */
#include <linux/module.h>   // Needed by all modules
#include <linux/kernel.h>   // Needed for KERN_ALERT
#include <linux/init.h>     // Needed for the macros


static int hello_2_init(void)
{
   printk(KERN_ALERT "Hello, world 2\n");
   return 0;
}


static void hello_2_exit(void)
{
   printk(KERN_ALERT "Goodbye, world 2\n");
}


module_init(hello_2_init);
module_exit(hello_2_exit);

WARN    := -W -Wall -Wstrict-prototypes -Wmissing-prototypes
INCLUDE := -isystem /lib/modules/`uname -r`/build/include
CFLAGS  := -O2 -DMODULE -D__KERNEL__ ${WARN} ${INCLUDE}
CC      := gcc-3.0
OBJS    := ${patsubst %.c, %.o, ${wildcard *.c}}

all: ${OBJS}

.PHONY: clean

clean:
    rm -rf *.o

/*  hello-3.c - Illustrating the __init, __initdata and __exit macros.
 */
#include <linux/module.h>      /* Needed by all modules */
#include <linux/kernel.h>      /* Needed for KERN_ALERT */
#include <linux/init.h>        /* Needed for the macros */

static int hello3_data __initdata = 3;


static int __init hello_3_init(void)
{
   printk(KERN_ALERT "Hello, world %d\n", hello3_data);
   return 0;
}


static void __exit hello_3_exit(void)
{
   printk(KERN_ALERT "Goodbye, world 3\n");
}


module_init(hello_3_init);
module_exit(hello_3_exit);

/*  hello-4.c - Demonstrates module documentation.
 */
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#define DRIVER_AUTHOR "Peiter Jay Salzman <p@dirac.org>"
#define DRIVER_DESC   "A sample driver"

int init_hello_3(void);
void cleanup_hello_3(void);


static int init_hello_4(void)
{
   printk(KERN_ALERT "Hello, world 4\n");
   return 0;
}


static void cleanup_hello_4(void)
{
   printk(KERN_ALERT "Goodbye, world 4\n");
}


module_init(init_hello_4);
module_exit(cleanup_hello_4);


/*  You can use strings, like this:
 */
MODULE_LICENSE("GPL");           // Get rid of taint message by declaring code as GPL.

/*  Or with defines, like this:
 */
MODULE_AUTHOR(DRIVER_AUTHOR);    // Who wrote this module?
MODULE_DESCRIPTION(DRIVER_DESC); // What does this module do?

/*  This module uses /dev/testdevice.  The MODULE_SUPPORTED_DEVICE macro might be used in
 *  the future to help automatic configuration of modules, but is currently unused other
 *  than for documentation purposes.
 */
MODULE_SUPPORTED_DEVICE("testdevice");

/*  hello-5.c - Demonstrates command line argument passing to a module.
 */
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Peiter Jay Salzman");

// These global variables can be set with command line arguments when you insmod
// the module in. 
//
static u8             mybyte = 'A';
static unsigned short myshort = 1;
static int            myint = 20;
static long           mylong = 9999;
static char           *mystring = "blah";
static int            myintArray[2] = { 0, 420 };

/*  Now we're actually setting the mechanism up -- making the variables command
 *  line arguments rather than just a bunch of global variables.
 */
MODULE_PARM(mybyte, "b");
MODULE_PARM(myshort, "h");
MODULE_PARM(myint, "i");
MODULE_PARM(mylong, "l");
MODULE_PARM(mystring, "s");
MODULE_PARM(myintArray, "1-2i");

MODULE_PARM_DESC(mybyte, "This byte really does nothing at all.");
MODULE_PARM_DESC(myshort, "This short is *extremely* important.");
// You get the picture.  Always use a MODULE_PARM_DESC() for each MODULE_PARM().


static int __init hello_5_init(void)
{
   printk(KERN_ALERT "mybyte is an 8 bit integer: %i\n", mybyte);
   printk(KERN_ALERT "myshort is a short integer: %hi\n", myshort);
   printk(KERN_ALERT "myint is an integer: %i\n", myint);
   printk(KERN_ALERT "mylong is a long integer: %li\n", mylong);
   printk(KERN_ALERT "mystring is a string: %s\n", mystring);
	 printk(KERN_ALERT "myintArray is %i and %i\n", myintArray[0], myintArray[1]);
   return 0;
}


static void __exit hello_5_exit(void)
{
   printk(KERN_ALERT "Goodbye, world 5\n");
}


module_init(hello_5_init);
module_exit(hello_5_exit);

/*  start.c - Illustration of multi filed modules
 */

#include <linux/kernel.h>       /* We're doing kernel work */
#include <linux/module.h>       /* Specifically, a module */

int init_module(void)
{
  printk("Hello, world - this is the kernel speaking\n");
  return 0;
}

/*  stop.c - Illustration of multi filed modules
 */

#if defined(CONFIG_MODVERSIONS) && ! defined(MODVERSIONS)
   #include <linux/modversions.h> /* Will be explained later */
   #define MODVERSIONS
#endif        
#include <linux/kernel.h>  /* We're doing kernel work */
#include <linux/module.h>  /* Specifically, a module  */
#define __NO_VERSION__     /* It's not THE file of the kernel module */
#include <linux/version.h> /* Not included by module.h because of
	                                      __NO_VERSION__ */
	
void cleanup_module()
{
   printk("<1>Short is the life of a kernel module\n");
}  

CC=gcc
MODCFLAGS := -O -Wall -DMODULE -D__KERNEL__
   	
hello.o:	hello2_start.o hello2_stop.o
   ld -m elf_i386 -r -o hello2.o hello2_start.o hello2_stop.o
   	
start.o: hello2_start.c
   ${CC} ${MODCFLAGS} -c hello2_start.c
   	
stop.o: hello2_stop.c
   ${CC} ${MODCFLAGS} -c hello2_stop.c

/*  procfs.c -  create a "file" in /proc 
 */

#include <linux/kernel.h>   /* We're doing kernel work */
#include <linux/module.h>   /* Specifically, a module */

/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif        


/* Necessary because we use the proc fs */
#include <linux/proc_fs.h>



/* In 2.2.3 /usr/include/linux/version.h includes a 
 * macro for this, but 2.0.35 doesn't - so I add it 
 * here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif



/* Put data into the proc fs file.

   Arguments
   =========
   1. The buffer where the data is to be inserted, if 
      you decide to use it.
   2. A pointer to a pointer to characters. This is 
      useful if you don't want to use the buffer 
      allocated by the kernel.
   3. The current position in the file. 
   4. The size of the buffer in the first argument.  
   5. Zero (for future use?).


   Usage and Return Value
   ======================
   If you use your own buffer, like I do, put its 
   location in the second argument and return the 
   number of bytes used in the buffer.

   A return value of zero means you have no further 
   information at this time (end of file). A negative 
   return value is an error condition.
   

   For More Information
   ==================== 
   The way I discovered what to do with this function 
   wasn't by reading documentation, but by reading the 
   code which used it. I just looked to see what uses 
   the get_info field of proc_dir_entry struct (I used a 
   combination of find and grep, if you're interested), 
   and I saw that  it is used in <kernel source 
   directory>/fs/proc/array.c.

   If something is unknown about the kernel, this is 
   usually the way to go. In Linux we have the great 
   advantage of having the kernel source code for 
   free - use it.
 */
int procfile_read(char *buffer, 
		  char **buffer_location, 
		  off_t offset, 
		  int buffer_length, 
		  int zero)
{
  int len;  /* The number of bytes actually used */

  /* This is static so it will still be in memory 
   * when we leave this function */
  static char my_buffer[80];  

  static int count = 1;

  /* We give all of our information in one go, so if the 
   * user asks us if we have more information the 
   * answer should always be no. 
   *
   * This is important because the standard read 
   * function from the library would continue to issue 
   * the read system call until the kernel replies
   * that it has no more information, or until its 
   * buffer is filled.
   */
  if (offset > 0)
    return 0;

  /* Fill the buffer and get its length */
  len = sprintf(my_buffer, 
    "For the %d%s time, go away!\n", count,
    (count % 100 > 10 && count % 100 < 14) ? "th" : 
      (count % 10 == 1) ? "st" :
        (count % 10 == 2) ? "nd" :
          (count % 10 == 3) ? "rd" : "th" );
  count++;

  /* Tell the function which called us where the 
   * buffer is */
  *buffer_location = my_buffer;

  /* Return the length */
  return len;
}


struct proc_dir_entry Our_Proc_File = 
  {
    0, /* Inode number - ignore, it will be filled by 
        * proc_register[_dynamic] */
    4, /* Length of the file name */
    "test", /* The file name */
    S_IFREG | S_IRUGO, /* File mode - this is a regular 
                        * file which can be read by its 
                        * owner, its group, and everybody
                        * else */
    1,	/* Number of links (directories where the 
         * file is referenced) */
    0, 0,  /* The uid and gid for the file - we give it 
            * to root */
    80, /* The size of the file reported by ls. */
    NULL, /* functions which can be done on the inode 
           * (linking, removing, etc.) - we don't 
           * support any. */
    procfile_read, /* The read function for this file, 
                    * the function called when somebody 
                    * tries to read something from it. */
    NULL /* We could have here a function to fill the 
          * file's inode, to enable us to play with 
          * permissions, ownership, etc. */
  }; 





/* Initialize the module - register the proc file */
int init_module()
{
  /* Success if proc_register[_dynamic] is a success, 
   * failure otherwise. */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
  /* In version 2.2, proc_register assign a dynamic 
   * inode number automatically if it is zero in the 
   * structure , so there's no more need for 
   * proc_register_dynamic
   */
  return proc_register(&proc_root, &Our_Proc_File);
#else
  return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
 
  /* proc_root is the root directory for the proc 
   * fs (/proc). This is where we want our file to be 
   * located. 
   */
}


/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
  proc_unregister(&proc_root, Our_Proc_File.low_ino);
}  

/*  procfs.c -  create a "file" in /proc, which allows both input and output.
 */

#include <linux/kernel.h>   /* We're doing kernel work */
#include <linux/module.h>   /* Specifically, a module */

/* Necessary because we use proc fs */
#include <linux/proc_fs.h>


/* In 2.2.3 /usr/include/linux/version.h includes a 
 * macro for this, but 2.0.35 doesn't - so I add it 
 * here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif



#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h>  /* for get_user and put_user */
#endif

/* The module's file functions ********************** */


/* Here we keep the last message received, to prove 
 * that we can process our input */
#define MESSAGE_LENGTH 80
static char Message[MESSAGE_LENGTH];


/* Since we use the file operations struct, we can't 
 * use the special proc output provisions - we have to 
 * use a standard read function, which is this function */
#if LINUX_VERSION_CODE &gt;= KERNEL_VERSION(2,2,0)
static ssize_t module_output(
    struct file *file,   /* The file read */
    char *buf, /* The buffer to put data to (in the
                * user segment) */
    size_t len,  /* The length of the buffer */
    loff_t *offset) /* Offset in the file - ignore */
#else
static int module_output(
    struct inode *inode, /* The inode read */
    struct file *file,   /* The file read */
    char *buf, /* The buffer to put data to (in the
                * user segment) */
    int len)  /* The length of the buffer */
#endif
{
  static int finished = 0;
  int i;
  char message[MESSAGE_LENGTH+30];

  /* We return 0 to indicate end of file, that we have 
   * no more information. Otherwise, processes will 
   * continue to read from us in an endless loop. */
  if (finished) {
    finished = 0;
    return 0;
  }

  /* We use put_user to copy the string from the kernel's 
   * memory segment to the memory segment of the process 
   * that called us. get_user, BTW, is
   * used for the reverse. */
  sprintf(message, "Last input:%s", Message);
  for(i=0; i&lt;len && message[i]; i++) 
    put_user(message[i], buf+i);


  /* Notice, we assume here that the size of the message 
   * is below len, or it will be received cut. In a real 
   * life situation, if the size of the message is less 
   * than len then we'd return len and on the second call 
   * start filling the buffer with the len+1'th byte of 
   * the message. */
  finished = 1; 

  return i;  /* Return the number of bytes "read" */
}


/* This function receives input from the user when the 
 * user writes to the /proc file. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t module_input(
    struct file *file,   /* The file itself */
    const char *buf,     /* The buffer with input */
    size_t length,       /* The buffer's length */
    loff_t *offset)      /* offset to file - ignore */
#else
static int module_input(
    struct inode *inode, /* The file's inode */
    struct file *file,   /* The file itself */
    const char *buf,     /* The buffer with the input */
    int length)          /* The buffer's length */
#endif
{
  int i;

  /* Put the input into Message, where module_output 
   * will later be able to use it */
  for(i=0; i<MESSAGE_LENGTH-1 && i<length; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
    get_user(Message[i], buf+i);
  /* In version 2.2 the semantics of get_user changed, 
   * it not longer returns a character, but expects a 
   * variable to fill up as its first argument and a 
   * user segment pointer to fill it from as the its 
   * second.
   *
   * The reason for this change is that the version 2.2 
   * get_user can also read an short or an int. The way 
   * it knows the type of the variable it should read 
   * is by using sizeof, and for that it needs the 
   * variable itself.
   */ 
#else 
    Message[i] = get_user(buf+i);
#endif
  Message[i] = '\0';  /* we want a standard, zero 
                       * terminated string */
  
  /* We need to return the number of input characters 
   * used */
  return i;
}



/* This function decides whether to allow an operation 
 * (return zero) or not allow it (return a non-zero 
 * which indicates why it is not allowed).
 *
 * The operation can be one of the following values:
 * 0 - Execute (run the "file" - meaningless in our case)
 * 2 - Write (input to the kernel module)
 * 4 - Read (output from the kernel module)
 *
 * This is the real function that checks file 
 * permissions. The permissions returned by ls -l are 
 * for referece only, and can be overridden here. 
 */
static int module_permission(struct inode *inode, int op)
{
  /* We allow everybody to read from our module, but 
   * only root (uid 0) may write to it */ 
  if (op == 4 || (op == 2 && current->euid == 0))
    return 0; 

  /* If it's anything else, access is denied */
  return -EACCES;
}




/* The file is opened - we don't really care about 
 * that, but it does mean we need to increment the 
 * module's reference count. */
int module_open(struct inode *inode, struct file *file)
{
  MOD_INC_USE_COUNT;
 
  return 0;
}


/* The file is closed - again, interesting only because 
 * of the reference count. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
int module_close(struct inode *inode, struct file *file)
#else
void module_close(struct inode *inode, struct file *file)
#endif
{
  MOD_DEC_USE_COUNT;

#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
  return 0;  /* success */
#endif
}


/* Structures to register as the /proc file, with 
 * pointers to all the relevant functions. ********** */



/* File operations for our proc file. This is where we 
 * place pointers to all the functions called when 
 * somebody tries to do something to our file. NULL 
 * means we don't want to deal with something. */
static struct file_operations File_Ops_4_Our_Proc_File =
  {
    NULL,  /* lseek */
    module_output,  /* "read" from the file */
    module_input,   /* "write" to the file */
    NULL,  /* readdir */
    NULL,  /* select */
    NULL,  /* ioctl */
    NULL,  /* mmap */
    module_open,    /* Somebody opened the file */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
    NULL,   /* flush, added here in version 2.2 */
#endif
    module_close,    /* Somebody closed the file */
    /* etc. etc. etc. (they are all given in 
     * /usr/include/linux/fs.h). Since we don't put 
     * anything here, the system will keep the default
     * data, which in Unix is zeros (NULLs when taken as 
     * pointers). */
  };



/* Inode operations for our proc file. We need it so 
 * we'll have some place to specify the file operations 
 * structure we want to use, and the function we use for 
 * permissions. It's also possible to specify functions 
 * to be called for anything else which could be done to 
 * an inode (although we don't bother, we just put 
 * NULL). */
static struct inode_operations Inode_Ops_4_Our_Proc_File =
  {
    &File_Ops_4_Our_Proc_File,
    NULL, /* create */
    NULL, /* lookup */
    NULL, /* link */
    NULL, /* unlink */
    NULL, /* symlink */
    NULL, /* mkdir */
    NULL, /* rmdir */
    NULL, /* mknod */
    NULL, /* rename */
    NULL, /* readlink */
    NULL, /* follow_link */
    NULL, /* readpage */
    NULL, /* writepage */
    NULL, /* bmap */
    NULL, /* truncate */
    module_permission /* check for permissions */
  };


/* Directory entry */
static struct proc_dir_entry Our_Proc_File = 
  {
    0, /* Inode number - ignore, it will be filled by 
        * proc_register[_dynamic] */
    7, /* Length of the file name */
    "rw_test", /* The file name */
    S_IFREG | S_IRUGO | S_IWUSR, 
    /* File mode - this is a regular file which 
     * can be read by its owner, its group, and everybody
     * else. Also, its owner can write to it.
     *
     * Actually, this field is just for reference, it's
     * module_permission that does the actual check. It 
     * could use this field, but in our implementation it
     * doesn't, for simplicity. */
    1,  /* Number of links (directories where the 
         * file is referenced) */
    0, 0,  /* The uid and gid for the file - 
            * we give it to root */
    80, /* The size of the file reported by ls. */
    &Inode_Ops_4_Our_Proc_File, 
    /* A pointer to the inode structure for
     * the file, if we need it. In our case we
     * do, because we need a write function. */
    NULL  
    /* The read function for the file. Irrelevant, 
     * because we put it in the inode structure above */
  }; 



/* Module initialization and cleanup ******************* */

/* Initialize the module - register the proc file */
int init_module()
{
  /* Success if proc_register[_dynamic] is a success, 
   * failure otherwise */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
  /* In version 2.2, proc_register assign a dynamic 
   * inode number automatically if it is zero in the 
   * structure , so there's no more need for 
   * proc_register_dynamic
   */
  return proc_register(&proc_root, &Our_Proc_File);
#else
  return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
}


/* Cleanup - unregister our file from /proc */
void cleanup_module()
{
  proc_unregister(&proc_root, Our_Proc_File.low_ino);
}  

/*  chardev.c - Create an input/output character device
 */

#include <linux/kernel.h>   /* We're doing kernel work */
#include <linux/module.h>   /* Specifically, a module */

/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif        

/* For character devices */

/* The character device definitions are here */
#include <linux/fs.h>

/* A wrapper which does next to nothing at
 * at present, but may help for compatibility
 * with future versions of Linux */
#include <linux/wrapper.h>

			     
/* Our own ioctl numbers */
#include "chardev.h"


/* In 2.2.3 /usr/include/linux/version.h includes a 
 * macro for this, but 2.0.35 doesn't - so I add it 
 * here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif



#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h>  /* for get_user and put_user */
#endif



#define SUCCESS 0


/* Device Declarations ******************************** */


/* The name for our device, as it will appear in 
 * /proc/devices */
#define DEVICE_NAME "char_dev"


/* The maximum length of the message for the device */
#define BUF_LEN 80

/* Is the device open right now? Used to prevent 
 * concurent access into the same device */
static int Device_Open = 0;

/* The message the device will give when asked */
static char Message[BUF_LEN];

/* How far did the process reading the message get? 
 * Useful if the message is larger than the size of the 
 * buffer we get to fill in device_read. */
static char *Message_Ptr;


/* This function is called whenever a process attempts 
 * to open the device file */
static int device_open(struct inode *inode, 
                       struct file *file)
{
#ifdef DEBUG
  printk ("device_open(%p)\n", file);
#endif

  /* We don't want to talk to two processes at the 
   * same time */
  if (Device_Open)
    return -EBUSY;

  /* If this was a process, we would have had to be 
   * more careful here, because one process might have 
   * checked Device_Open right before the other one 
   * tried to increment it. However, we're in the 
   * kernel, so we're protected against context switches.
   *
   * This is NOT the right attitude to take, because we
   * might be running on an SMP box, but we'll deal with
   * SMP in a later chapter.
   */ 

  Device_Open++;

  /* Initialize the message */
  Message_Ptr = Message;

  MOD_INC_USE_COUNT;

  return SUCCESS;
}


/* This function is called when a process closes the 
 * device file. It doesn't have a return value because 
 * it cannot fail. Regardless of what else happens, you 
 * should always be able to close a device (in 2.0, a 2.2
 * device file could be impossible to close).
 */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static int device_release(struct inode *inode, 
                          struct file *file)
#else
static void device_release(struct inode *inode, 
                           struct file *file)
#endif
{
#ifdef DEBUG
  printk ("device_release(%p,%p)\n", inode, file);
#endif
 
  /* We're now ready for our next caller */
  Device_Open --;

  MOD_DEC_USE_COUNT;

#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
  return 0;
#endif
}



/* This function is called whenever a process which 
 * has already opened the device file attempts to 
 * read from it. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_read(
    struct file *file,
    char *buffer, /* The buffer to fill with the data */   
    size_t length,     /* The length of the buffer */
    loff_t *offset) /* offset to the file */
#else
static int device_read(
    struct inode *inode,
    struct file *file,
    char *buffer,   /* The buffer to fill with the data */ 
    int length)     /* The length of the buffer 
                     * (mustn't write beyond that!) */
#endif
{
  /* Number of bytes actually written to the buffer */
  int bytes_read = 0;

#ifdef DEBUG
  printk("device_read(%p,%p,%d)\n", file, buffer, length);
#endif

  /* If we're at the end of the message, return 0 
   * (which signifies end of file) */
  if (*Message_Ptr == 0)
    return 0;

  /* Actually put the data into the buffer */
  while (length && *Message_Ptr)  {

    /* Because the buffer is in the user data segment, 
     * not the kernel data segment, assignment wouldn't 
     * work. Instead, we have to use put_user which 
     * copies data from the kernel data segment to the 
     * user data segment. */
    put_user(*(Message_Ptr++), buffer++);
    length --;
    bytes_read ++;
  }

#ifdef DEBUG
   printk ("Read %d bytes, %d left\n", bytes_read, length);
#endif

   /* Read functions are supposed to return the number 
    * of bytes actually inserted into the buffer */
  return bytes_read;
}


/* This function is called when somebody tries to 
 * write into our device file. */ 
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_write(struct file *file,
                            const char *buffer,
                            size_t length,
                            loff_t *offset)
#else
static int device_write(struct inode *inode,
                        struct file *file,
                        const char *buffer,
                        int length)
#endif
{
  int i;

#ifdef DEBUG
  printk ("device_write(%p,%s,%d)",
    file, buffer, length);
#endif

  for(i=0; i<length && i<BUF_LEN; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
    get_user(Message[i], buffer+i);
#else
    Message[i] = get_user(buffer+i);
#endif  

  Message_Ptr = Message;

  /* Again, return the number of input characters used */
  return i;
}


/* This function is called whenever a process tries to 
 * do an ioctl on our device file. We get two extra 
 * parameters (additional to the inode and file 
 * structures, which all device functions get): the number
 * of the ioctl called and the parameter given to the 
 * ioctl function.
 *
 * If the ioctl is write or read/write (meaning output 
 * is returned to the calling process), the ioctl call 
 * returns the output of this function.
 */
int device_ioctl(
    struct inode *inode,
    struct file *file,
    unsigned int ioctl_num,/* The number of the ioctl */
    unsigned long ioctl_param) /* The parameter to it */
{
  int i;
  char *temp;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
  char ch;
#endif

  /* Switch according to the ioctl called */
  switch (ioctl_num) {
    case IOCTL_SET_MSG:
      /* Receive a pointer to a message (in user space) 
       * and set that to be the device's message. */ 

      /* Get the parameter given to ioctl by the process */
      temp = (char *) ioctl_param;
   
      /* Find the length of the message */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
      get_user(ch, temp);
      for (i=0; ch && i<BUF_LEN; i++, temp++)
        get_user(ch, temp);
#else
      for (i=0; get_user(temp) && i<BUF_LEN; i++, temp++)
	;
#endif

      /* Don't reinvent the wheel - call device_write */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
      device_write(file, (char *) ioctl_param, i, 0);
#else
      device_write(inode, file, (char *) ioctl_param, i);
#endif
      break;

    case IOCTL_GET_MSG:
      /* Give the current message to the calling 
       * process - the parameter we got is a pointer, 
       * fill it. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
      i = device_read(file, (char *) ioctl_param, 99, 0); 
#else
      i = device_read(inode, file, (char *) ioctl_param, 99); 
#endif
      /* Warning - we assume here the buffer length is 
       * 100. If it's less than that we might overflow 
       * the buffer, causing the process to core dump. 
       *
       * The reason we only allow up to 99 characters is 
       * that the NULL which terminates the string also 
       * needs room. */

      /* Put a zero at the end of the buffer, so it 
       * will be properly terminated */
      put_user('\0', (char *) ioctl_param+i);
      break;

    case IOCTL_GET_NTH_BYTE:
      /* This ioctl is both input (ioctl_param) and 
       * output (the return value of this function) */
      return Message[ioctl_param];
      break;
  }

  return SUCCESS;
}


/* Module Declarations *************************** */


/* This structure will hold the functions to be called 
 * when a process does something to the device we 
 * created. Since a pointer to this structure is kept in 
 * the devices table, it can't be local to
 * init_module. NULL is for unimplemented functions. */
struct file_operations Fops = {
  NULL,   /* seek */
  device_read, 
  device_write,
  NULL,   /* readdir */
  NULL,   /* select */
  device_ioctl,   /* ioctl */
  NULL,   /* mmap */
  device_open,
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
  NULL,  /* flush */
#endif
  device_release  /* a.k.a. close */
};


/* Initialize the module - Register the character device */
int init_module()
{
  int ret_val;

  /* Register the character device (atleast try) */
  ret_val = module_register_chrdev(MAJOR_NUM, 
                                 DEVICE_NAME,
                                 &Fops);

  /* Negative values signify an error */
  if (ret_val < 0) {
    printk ("%s failed with %d\n",
            "Sorry, registering the character device ",
            ret_val);
    return ret_val;
  }

  printk ("%s The major device number is %d.\n",
          "Registeration is a success", 
          MAJOR_NUM);
  printk ("If you want to talk to the device driver,\n");
  printk ("you'll have to create a device file. \n");
  printk ("We suggest you use:\n");
  printk ("mknod %s c %d 0\n", DEVICE_FILE_NAME, 
          MAJOR_NUM);
  printk ("The device file name is important, because\n");
  printk ("the ioctl program assumes that's the\n");
  printk ("file you'll use.\n");

  return 0;
}


/* Cleanup - unregister the appropriate file from /proc */
void cleanup_module()
{
  int ret;

  /* Unregister the device */
  ret = module_unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
 
  /* If there's an error, report it */ 
  if (ret < 0)
    printk("Error in module_unregister_chrdev: %d\n", ret);
}  

/*  chardev.h - the header file with the ioctl definitions.
 *
 *  The declarations here have to be in a header file, because
 *  they need to be known both to the kernel module
 *  (in chardev.c) and the process calling ioctl (ioctl.c)
 */

#ifndef CHARDEV_H
#define CHARDEV_H

#include <linux/ioctl.h>



/* The major device number. We can't rely on dynamic 
 * registration any more, because ioctls need to know 
 * it. */
#define MAJOR_NUM 100


/* Set the message of the device driver */
#define IOCTL_SET_MSG _IOR(MAJOR_NUM, 0, char *)
/* _IOR means that we're creating an ioctl command 
 * number for passing information from a user process
 * to the kernel module. 
 *
 * The first arguments, MAJOR_NUM, is the major device 
 * number we're using.
 *
 * The second argument is the number of the command 
 * (there could be several with different meanings).
 *
 * The third argument is the type we want to get from 
 * the process to the kernel.
 */

/* Get the message of the device driver */
#define IOCTL_GET_MSG _IOR(MAJOR_NUM, 1, char *)
 /* This IOCTL is used for output, to get the message 
  * of the device driver. However, we still need the 
  * buffer to place the message in to be input, 
  * as it is allocated by the process.
  */


/* Get the n'th byte of the message */
#define IOCTL_GET_NTH_BYTE _IOWR(MAJOR_NUM, 2, int)
 /* The IOCTL is used for both input and output. It 
  * receives from the user a number, n, and returns 
  * Message[n]. */


/* The name of the device file */
#define DEVICE_FILE_NAME "char_dev"


#endif

/*  ioctl.c - the process to use ioctl's to control the kernel module
 *
 *  Until now we could have used cat for input and output.  But now
 *  we need to do ioctl's, which require writing our own process. 
 */

/* device specifics, such as ioctl numbers and the 
 * major device file. */
#include "chardev.h"    


#include <fcntl.h>      /* open */ 
#include <unistd.h>     /* exit */
#include <sys/ioctl.h>  /* ioctl */



/* Functions for the ioctl calls */

ioctl_set_msg(int file_desc, char *message)
{
  int ret_val;

  ret_val = ioctl(file_desc, IOCTL_SET_MSG, message);

  if (ret_val < 0) {
    printf ("ioctl_set_msg failed:%d\n", ret_val);
    exit(-1);
  }
}



ioctl_get_msg(int file_desc)
{
  int ret_val;
  char message[100]; 

  /* Warning - this is dangerous because we don't tell 
   * the kernel how far it's allowed to write, so it 
   * might overflow the buffer. In a real production 
   * program, we would have used two ioctls - one to tell
   * the kernel the buffer length and another to give 
   * it the buffer to fill
   */
  ret_val = ioctl(file_desc, IOCTL_GET_MSG, message);

  if (ret_val < 0) {
    printf ("ioctl_get_msg failed:%d\n", ret_val);
    exit(-1);
  }

  printf("get_msg message:%s\n", message);
}



ioctl_get_nth_byte(int file_desc)
{
  int i;
  char c;

  printf("get_nth_byte message:");

  i = 0;
  while (c != 0) {
    c = ioctl(file_desc, IOCTL_GET_NTH_BYTE, i++);

    if (c < 0) {
      printf(
      "ioctl_get_nth_byte failed at the %d'th byte:\n", i);
      exit(-1);
    }

    putchar(c);
  } 
  putchar('\n');
}




/* Main - Call the ioctl functions */
main()
{
  int file_desc, ret_val;
  char *msg = "Message passed by ioctl\n";

  file_desc = open(DEVICE_FILE_NAME, 0);
  if (file_desc < 0) {
    printf ("Can't open device file: %s\n", 
            DEVICE_FILE_NAME);
    exit(-1);
  }

  ioctl_get_nth_byte(file_desc);
  ioctl_get_msg(file_desc);
  ioctl_set_msg(file_desc, msg);

  close(file_desc); 
}

/*  syscall.c 
 * 
 *  System call "stealing" sample.
 */


/* Copyright (C) 2001 by Peter Jay Salzman */


/* The necessary header files */

/* Standard in kernel modules */
#include <linux/kernel.h>   /* We're doing kernel work */
#include <linux/module.h>   /* Specifically, a module */

/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif        

#include <sys/syscall.h>  /* The list of system calls */

/* For the current (process) structure, we need
 * this to know who the current user is. */
#include <linux/sched.h>




/* In 2.2.3 /usr/include/linux/version.h includes a 
 * macro for this, but 2.0.35 doesn't - so I add it 
 * here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif



#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h>
#endif



/* The system call table (a table of functions). We 
 * just define this as external, and the kernel will 
 * fill it up for us when we are insmod'ed 
 */
extern void *sys_call_table[];


/* UID we want to spy on - will be filled from the 
 * command line */
int uid;  

#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
MODULE_PARM(uid, "i");
#endif

/* A pointer to the original system call. The reason 
 * we keep this, rather than call the original function 
 * (sys_open), is because somebody else might have 
 * replaced the system call before us. Note that this 
 * is not 100% safe, because if another module 
 * replaced sys_open before us, then when we're inserted 
 * we'll call the function in that module - and it 
 * might be removed before we are.
 *
 * Another reason for this is that we can't get sys_open.
 * It's a static variable, so it is not exported. */
asmlinkage int (*original_call)(const char *, int, int);



/* For some reason, in 2.2.3 current->uid gave me 
 * zero, not the real user ID. I tried to find what went 
 * wrong, but I couldn't do it in a short time, and 
 * I'm lazy - so I'll just use the system call to get the 
 * uid, the way a process would. 
 *
 * For some reason, after I recompiled the kernel this 
 * problem went away. 
 */
asmlinkage int (*getuid_call)();



/* The function we'll replace sys_open (the function 
 * called when you call the open system call) with. To 
 * find the exact prototype, with the number and type 
 * of arguments, we find the original function first 
 * (it's at fs/open.c). 
 *
 * In theory, this means that we're tied to the 
 * current version of the kernel. In practice, the 
 * system calls almost never change (it would wreck havoc 
 * and require programs to be recompiled, since the system
 * calls are the interface between the kernel and the 
 * processes).
 */
asmlinkage int our_sys_open(const char *filename, 
                            int flags, 
                            int mode)
{
  int i = 0;
  char ch;

  /* Check if this is the user we're spying on */
  if (uid == getuid_call()) {  
   /* getuid_call is the getuid system call, 
    * which gives the uid of the user who
    * ran the process which called the system
    * call we got */

    /* Report the file, if relevant */
    printk("Opened file by %d: ", uid); 
    do {
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
      get_user(ch, filename+i);
#else
      ch = get_user(filename+i);
#endif
      i++;
      printk("%c", ch);
    } while (ch != 0);
    printk("\n");
  }

  /* Call the original sys_open - otherwise, we lose 
   * the ability to open files */
  return original_call(filename, flags, mode);
}



/* Initialize the module - replace the system call */
int init_module()
{
  /* Warning - too late for it now, but maybe for 
   * next time... */
  printk("I'm dangerous. I hope you did a ");
  printk("sync before you insmod'ed me.\n");
  printk("My counterpart, cleanup_module(), is even"); 
  printk("more dangerous. If\n");
  printk("you value your file system, it will ");
  printk("be \"sync; rmmod\" \n");
  printk("when you remove this module.\n");

  /* Keep a pointer to the original function in 
   * original_call, and then replace the system call 
   * in the system call table with our_sys_open */
  original_call = sys_call_table[__NR_open];
  sys_call_table[__NR_open] = our_sys_open;

  /* To get the address of the function for system 
   * call foo, go to sys_call_table[__NR_foo]. */

  printk("Spying on UID:%d\n", uid);

  /* Get the system call for getuid */
  getuid_call = sys_call_table[__NR_getuid];

  return 0;
}


/* Cleanup - unregister the appropriate file from /proc */
void cleanup_module()
{
  /* Return the system call back to normal */
  if (sys_call_table[__NR_open] != our_sys_open) {
    printk("Somebody else also played with the ");
    printk("open system call\n");
    printk("The system may be left in ");
    printk("an unstable state.\n");
  }

  sys_call_table[__NR_open] = original_call;
}  

/*  print_string.c - Send output to the tty you're running on, regardless of whether it's
 *     through X11, telnet, etc.  We do this by printing the string to the tty associated
 *     with the current task.
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/sched.h>    // For current
#include <linux/tty.h>      // For the tty declarations
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Peter Jay Salzman");


void print_string(char *str)
{
   struct tty_struct *my_tty;
   my_tty = current->tty;           // The tty for the current task

   /* If my_tty is NULL, the current task has no tty you can print to (this is possible,
    * for example, if it's a daemon).  If so, there's nothing we can do.
    */
   if (my_tty != NULL) { 

      /* my_tty->driver is a struct which holds the tty's functions, one of which (write)
       * is used to write strings to the tty.  It can be used to take a string either
       * from the user's memory segment or the kernel's memory segment.
       *
       * The function's 1st parameter is the tty to write to, because the same function
       * would normally be used for all tty's of a certain type.  The 2nd parameter
       * controls whether the function receives a string from kernel memory (false, 0) or
       * from user memory (true, non zero).  The 3rd parameter is a pointer to a string.
       * The 4th parameter is the length of the string.
       */
      (*(my_tty->driver).write)(
         my_tty,                 // The tty itself
         0,                      // We don't take the string from user space
         str,                    // String
         strlen(str));           // Length

      /* ttys were originally hardware devices, which (usually) strictly followed the
       * ASCII standard.  In ASCII, to move to a new line you need two characters, a
       * carriage return and a line feed.  On Unix, the ASCII line feed is used for both
       * purposes - so we can't just use \n, because it wouldn't have a carriage return
       * and the next line will start at the column right after the line feed. 
       *
       * BTW, this is why text files are different between Unix and MS Windows.  In CP/M
       * and its derivatives, like MS-DOS and MS Windows, the ASCII standard was strictly
       * adhered to, and therefore a newline requirs both a LF and a CR.
       */
      (*(my_tty->driver).write)(my_tty, 0, "\015\012", 2);
   }
}


int print_string_init(void)
{
   print_string("The module has been inserted.  Hello world!");
   return 0;
}


void print_string_exit(void)
{
   print_string("The module has been removed.  Farewell world!");
}  


module_init(print_string_init);
module_exit(print_string_exit);

/*  sched.c - scheduale a function to be called on every timer interrupt.
 *
 *  Copyright (C) 2001 by Peter Jay Salzman
 */

/* The necessary header files */

/* Standard in kernel modules */
#include <linux/kernel.h>                   /* We're doing kernel work */
#include <linux/module.h>                   /* Specifically, a module */

/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif        

/* Necessary because we use the proc fs */
#include <linux/proc_fs.h>

/* We scheduale tasks here */
#include <linux/tqueue.h>

/* We also need the ability to put ourselves to sleep and wake up later */
#include <linux/sched.h>

/* In 2.2.3 /usr/include/linux/version.h includes a macro for this, but
 * 2.0.35 doesn't - so I add it here if necessary.
 */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif

/* The number of times the timer interrupt has been called so far */
static int TimerIntrpt = 0;

/* This is used by cleanup, to prevent the module from being unloaded while
 * intrpt_routine is still in the task queue
 */
static struct wait_queue *WaitQ = NULL;

static void intrpt_routine(void *);

/* The task queue structure for this task, from tqueue.h */
static struct tq_struct Task = {
   NULL,          /* Next item in list - queue_task will do this for us */
   0,             /* A flag meaning we haven't been inserted into a task
                   * queue yet 
                   */
   intrpt_routine, /* The function to run */
   NULL            /* The void* parameter for that function */
};

/* This function will be called on every timer interrupt. Notice the void*
 * pointer - task functions can be used for more than one purpose, each time 
 * getting a different parameter.
 */
static void intrpt_routine(void *irrelevant)
{
   /* Increment the counter */
   TimerIntrpt++;

   /* If cleanup wants us to die */
   if (WaitQ != NULL) 
      wake_up(&WaitQ);               /* Now cleanup_module can return */
   else
      /* Put ourselves back in the task queue */
      queue_task(&Task, &tq_timer);  
}

/* Put data into the proc fs file. */
int procfile_read(char *buffer, 
                  char **buffer_location, off_t offset, 
                  int buffer_length, int zero)
{
   int len;  /* The number of bytes actually used */

   /* It's static so it will still be in memory when we leave this function
    */
   static char my_buffer[80];  

   static int count = 1;

   /* We give all of our information in one go, so if the anybody asks us
    * if we have more information the answer should always be no. 
    */
   if (offset > 0)
      return 0;

   /* Fill the buffer and get its length */
   len = sprintf(my_buffer, "Timer called %d times so far\n", TimerIntrpt);
   count++;

   /* Tell the function which called us where the buffer is */
   *buffer_location = my_buffer;

   /* Return the length */
   return len;
}

struct proc_dir_entry Our_Proc_File = {
   0,  /* Inode number - ignore, it'll be filled by proc_register_dynamic */
   5,                 /* Length of the file name */
   "sched",           /* The file name */
   S_IFREG | S_IRUGO, /* File mode - this is a regular file which can be
                       * read by its owner, its group, and everybody else
                       */
   1,      /* Number of links (directories where the file is referenced) */
   0, 0,             /* The uid and gid for the file - we give it to root */
   80,               /* The size of the file reported by ls. */
   NULL, /* functions which can be done on the inode (linking, removing,
          * etc). - we don't * support any.
          */
   procfile_read,  /* The read function for this file, the function called
									  * when somebody tries to read something from it.
                    */
   NULL /* We could have here a function to fill the file's inode, to
         * enable us to play with permissions, ownership, etc.
         */
  }; 

/* Initialize the module - register the proc file */
int init_module()
{
   /* Put the task in the tq_timer task queue, so it will be executed at
    * next timer interrupt
    */
   queue_task(&Task, &tq_timer);

   /* Success if proc_register_dynamic is a success, failure otherwise */
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,2,0)
   return proc_register(&proc_root, &Our_Proc_File);
#else
   return proc_register_dynamic(&proc_root, &Our_Proc_File);
#endif
}

/* Cleanup */
void cleanup_module()
{
   /* Unregister our /proc file */
   proc_unregister(&proc_root, Our_Proc_File.low_ino);
  
   /* Sleep until intrpt_routine is called one last time. This is necessary,
    * because otherwise we'll deallocate the memory holding intrpt_routine
    * and Task while tq_timer still references them.  Notice that here we
    * don't allow signals to interrupt us. 
    *
    * Since WaitQ is now not NULL, this automatically tells the interrupt
    * routine it's time to die.
    */
   sleep_on(&WaitQ);
}