1. Linux Kernel Driver

2. Char Driver: Basics Contact details exported to user space using /dev:
bash$ ls -l /dev
total 0
brw-rw root disk 8, :01 /dev/sda
brw-rw root disk 8, :01 /dev/sda1
brw-rw root disk 8, :01 /dev/sda2
...
crw--w root root 4, :01 /dev/tty0
crw root root 4, :01 /dev/tty1
crw root root 4, :01 /dev/tty2
Block device
Char device
Major number
Minor number

3. Char Driver: Codeflow Initialization
Tie driver to the rest of the kernel
I/O system entry points
Read, write, open, seek, close
(Interrupt routines)
Cleanup

4. Char Driver: Initialization
#define DEVICE_NAME “rot13crypt”
struct cdev crypt_cdev;
static dev_t crypt_dev;
static struct file_operations crypt_fops = { ... };
static int __init crypt_init(void) {
int result;
result = alloc_chrdev_region(&crypt_dev, 0, 1, DEVICE_NAME);
/* dynamic allocation van device major number */
/* connect file operations with character dev */
cdev_init(&crypt_cdev, &crypt_fops);
result = cdev_add(&crypt_cdev, crypt_dev, 1);
return 0; }
module_init(crypt_init)
Kernel abstraction for character drivers
Addresses of IO system entry points

5. Char Driver: Cleanup static void __exit crypt_cleanup(void) {
/* Remove the cdev */
cdev_del(&crypt_cdev);
/* Release the major number */
unregister_chrdev_region(MAJOR(crypt_dev), 1); }
module_exit(crypt_cleanup)

6. Char Driver: Entry points
static struct file_operations crypt_fops = {
.owner = THIS_MODULE,
.read = crypt_read,
.write = crypt_write,
.open = crypt_open,
.release = crypt_close, };
/* Called when application opens device node */
int crypt_open(struct inode *inode, struct file *file) {
return 0; }
/* Called when application closes device node */
int crypt_release(struct inode *inode, struct file *file) {

7. Char Device: read/write
char *kbuf = NULL;
static ssize_t crypt_write(struct file * file, const char * buf, size_t count, loff_t *ppos) {
kbuf = kmalloc(count, GFP_KERNEL);
if( buf == NULL )
return -ENOMEM;
/* Copy the characters to kernel memory */
if( copy_from_user(kbuf, buf, count) ) {
kfree(kbuf);
return -EFAULT; }
return nbyteswritten;
static ssize_t crypt_read(struct file * file, char * buf, size_t count, loff_t *ppos) {
if( copy_to_user(buf, kbuf, count) ) {
return -EIO;
return nbytesread;

8. Kernel Memory kmalloc, kfree Flag specifies the type of memory
copy_[from|to]_user
Copy data between user and kernel memory
Between different virtual memory mappings

9. Practical Driver prints major number (see dmesg)
Or 'cat /proc/devices | grep rot13crypt'
minor = 0
mknod /dev/rot13crypt c major minor
echo “string” > /dev/rot13crypt
Calls open, write and close
cat /dev/rot13crypt
Calls open, read, close

10. Implementation Implement rot13 “encryption”
The transformation done using a lookup table:
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm

11. Build Makefile bash$ make obj-m := rot13crypt.o
kernel_release=`uname -r`
all:
make -C /lib/modules/${kernel_release}/build/ SUBDIRS=${PWD} modules