int main () {
        system ("cat tech.txt");
        return 0;
}

-rwsrws--- 1 techmint techmint 7368 янв.   2 12:38 read
-rwxrwx--- 1 techmint techmint    6 янв.   1 20:55 tech.txt

techmint:x:1004:1004::/home/techmint:/bin/bash
mintol:x:1006:1005:mintol,,,:/home/mintol:/bin/bash

mintol@mirror:/home/techmint$ ./read
-su: ./read: Отказано в доступе

root@ubuntu:/# ls -lR | sort | uniq -w1
.:
----------  1 root  root     0 Nov 24 14:58 crond.reboot
./bin:
brw-rw----+ 1 root cdrom    11,   0 Nov 24 14:58 sr0
c--------- 1 root     root   5,  2 Nov 24 14:58 ptmx
./cdrom:
crw-------  1 root root     10, 137 Nov 24 14:58 vhci
d---------  2 root            root             40 Nov 24 14:58 inaccessible
./dev:
d????????? ? ?        ?          ?            ? gvfs
./etc:
lr-------- 1 colord colord 64 Nov 29 12:18 b638f000-b639b000 -> /lib/i386-linux-gnu/libnss_files-2.21.so
./media:
prw------- 1 root root   0 Nov 24 14:58 1.ref
-r-------- 1 colord colord 0 Nov 29 12:18 0
./root:
-r--r----- 1 root lpadmin 32 Nov 29 10:26 0
./run:
-rw------- 1 colord colord 0 Nov 29 12:18 mem
./sbin:
srw-------  1 root root    0 Nov 24 14:58 control
./sys:
total 0
./usr:
--w------- 1 colord colord 0 Nov 29 12:18 clear_refs

sublimee@ubuntu:/$ ls -l
...
drwxr-xr-x   2 root root  4096 Sep 27 16:46 bin
drwxr-xr-x  18 root root  4280 Nov  9 10:10 dev
drwxr-xr-x 133 root root 12288 Oct 19 12:40 etc
drwxr-xr-x  26 root root   760 Nov  9 11:18 run
...

/*
* 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 <stdio.h>
#include <stdlib.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;
do {
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);
} while (c != 0);
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);
}

/*
* chardev.c - Create an input/output character device
*/
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
#include <linux/fs.h>
#include <asm/uaccess.h> /* for get_user and put_user */
#include "chardev.h"
#define SUCCESS 0
#define DEVICE_NAME "char_dev"
#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 is called whenever a process attempts to open the device file 
*/
static int device_open(struct inode *inode, struct file *file)
{
#ifdef DEBUG
printk(KERN_INFO "device_open(%p)\n", file);
#endif
/* 
* We don't want to talk to two processes at the same time 
*/
if (Device_Open)
return -EBUSY;
Device_Open++;
/*
* Initialize the message 
*/
Message_Ptr = Message;
try_module_get(THIS_MODULE);
return SUCCESS;
}
static int device_release(struct inode *inode, struct file *file)
{
#ifdef DEBUG
printk(KERN_INFO "device_release(%p,%p)\n", inode, file);
#endif
/* 
* We're now ready for our next caller 
*/
Device_Open--;
module_put(THIS_MODULE);
return SUCCESS;
}
/* 
* This function is called whenever a process which has already opened the
* device file attempts to read from it.
*/
static ssize_t device_read(struct file *file, /* see include/linux/fs.h */
char __user * buffer, /* buffer to be
* filled with data */
size_t length, /* length of the buffer */
loff_t * offset)
{
/* 
* Number of bytes actually written to the buffer 
*/
int bytes_read = 0;
#ifdef DEBUG
printk(KERN_INFO "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(KERN_INFO "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. 
*/
static ssize_t
device_write(struct file *file,
const char __user * buffer, size_t length, loff_t * offset)
{
int i;
#ifdef DEBUG
printk(KERN_INFO "device_write(%p,%s,%d)", file, buffer, length);
#endif
for (i = 0; i < length && i < BUF_LEN; i++)
get_user(Message[i], buffer + i);
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,*/ /* see include/linux/fs.h */
struct file *file, /* ditto */
unsigned int ioctl_num, /* number and param for ioctl
*/
unsigned long ioctl_param)
{
int i;
char *temp;
char ch;
/* 
* 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 
*/
get_user(ch, temp);
for (i = 0; ch && i < BUF_LEN; i++, temp++)
get_user(ch, temp);
device_write(file, (char *)ioctl_param, i, 0);
break;
case IOCTL_GET_MSG:
/* 
* Give the current message to the calling process - 
* the parameter we got is a pointer, fill it. 
*/
i = device_read(file, (char *)ioctl_param, 99, 0);
/* 
* 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 = {
.read = device_read,
.write = device_write,
.unlocked_ioctl = device_ioctl,
.open = device_open,
.release = 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 = register_chrdev(MAJOR_NUM, DEVICE_NAME, &Fops);
/* 
* Negative values signify an error 
*/
if (ret_val < 0) {
printk(KERN_ALERT "%s failed with %d\n",
"Sorry, registering the character device ", ret_val);
return ret_val;
}
printk(KERN_INFO "%s The major device number is %d.\n",
"Registeration is a success", MAJOR_NUM);
printk(KERN_INFO "If you want to talk to the device driver,\n");
printk(KERN_INFO "you'll have to create a device file. \n");
printk(KERN_INFO "We suggest you use:\n");
printk(KERN_INFO "mknod %s c %d 0\n", DEVICE_FILE_NAME, MAJOR_NUM);
printk(KERN_INFO "The device file name is important, because\n");
printk(KERN_INFO "the ioctl program assumes that's the\n");
printk(KERN_INFO "file you'll use.\n");
return 0;
}
/* 
* Cleanup - unregister the appropriate file from /proc 
*/
void cleanup_module()
{
//int ret;
/* 
* Unregister the device 
*/
/*ret = */unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
/* 
* If there's an error, report it 
*/
//if (ret < 0)
//printk(KERN_ALERT "Error: unregister_chrdev: %d\n", ret);
}