Linux kernel modules

Linux Kernel Modules
Let’s solve the puzzle
Dheryta Jaisinghani
Workshop on Computer Systems, Ashoka University
Dec 9, 2018
(Note: Most of the codes in the slides are from references in the end)

Basics of Operating
Systems
From user space to kernel space

Basic Operating System Structure
Hardware
Kernel
User Services
User Applications
& Processes
System Call
Interface
● What is an operating system?
○ An operating system is system software
that manages computer hardware and
software resources and provides
common services for computer
programs. [Wikipedia]
● What is Kernel?
○ Core of the operating system
○ Loads at startup and takes care of
everything else -
resources/memory/scheduling and many
more
○ Types: Monolithic, Microkernel, Modular,
Nano, Exo 3

What is a Kernel Module?
● Piece of code - Runtime Load/Unload
● Examples - Device Drivers - printer
driver, WLAN driver, vbox driver and
many more
● Actual kernel image is small - Modules
make it big
○ Monolithic kernels would have
been huge
User-level
Programs
System Call Interface
Kernel Services
Device Modules and Drivers
Physical Devices
User space
Kernel space
4

Module vs Program
● Program
○ main() - sequentially executes instructions and terminates
● Kernel Modules
○ init_module() or module_init() - Entry function
■ Initial setup to tell kernel about this module
■ Kernel executes it when needed
○ cleanup_module() or module_exit - Exit function
■ Unregister the module
5

Example 1
(user space to kernel
space)

User Space (Library Functions) → Kernel Space (System
Calls)
#include <stdio.h>
int main(void)
{
printf("hello");
return 0;
}
SystemCalls
7

Example 2
My First Kernel
Module
Example 2.1 from Ref [1]

Prepare the system
● Update the system
○ sudo apt-get update
● Search appropriate headers
○ apt-cache search linux-headers-$(uname -r)
● Download and install correct Linux headers
○ sudo apt-get install linux-headers-$(uname -r)
● Check if installed correctly
○ cd /usr/src/linux-headers-$(uname -r)
○ ls - should show the header files
● Follow steps under Prepare the System for Building LKMs, presented here
(http://derekmolloy.ie/writing-a-linux-kernel-module-part-1-introduction/)
9

Hello World Kernel Module
#include <linux/module.h>
#include <linux/kernel.h>
DRIVER_AUTHOR "Dheryta Jaisinghani"
DRIVER_DESC "A Hello World kernel module."
MODULE_LICENSE("GPL");
MODULE_AUTHOR(DRIVER_AUTHOR);
MODULE_DESCRIPTION(DRIVER_DESC);
int init_module(void)
{
printk(KERN_INFO "Hello Worldn");
return 0;
}
void cleanup_module(void)
{
printk(KERN_INFO "Goodbye Worldn");
}
10
Header files to make it a kernel module
and log levels
Module Information
Load the module
0 - Success, Else - Failure
Unload the module

Example 3
Different Log Levels

printk
● Log at kernel
● 8 priority levels (See: include/linux/kern_levels.h)
○ KERN_EMERG 0 system is unusable
○ KERN_ALERT 1 action must be taken immediately
○ KERN_CRIT 2 critical conditions
○ KERN_ERR 3 error conditions
○ KERN_WARNING 4 warning conditions
○ KERN_NOTICE 5 normal but significant condition
○ KERN_INFO 6 informational
○ KERN_DEBUG 7 debug-level messages
12

Module Makefile
obj−m += hello.o
all:
make −C /lib/modules/$(shell uname −r)/build M=$(PWD) modules
clean:
make −C /lib/modules/$(shell uname −r)/build M=$(PWD) clean
13
Multiple modules can be built with single makefile. Refer to
Section 2.2 in The Linux Kernel Module Programming Guide

Example 4
Extracting info about
modules

All about modules
● lsmod - Show all loaded modules
○ lsmod
● insmod - Insert a Module (excludes dependencies)
○ sudo insmod <module_name>
● modprobe - Insert a Module (includes dependencies)
○ sudo modprobe <module_name>
● modinfo - Show information about a module
○ modinfo <module_name.ko>
● depmod - Build module dependency database
○ /lib/modules/$(uname -r)/modules.dep
● rmmod - Remove a module
○ rmmod <module_name.ko>
● Show the log
○ dmesg or cat /var/log/syslog
15

Example 5
Moving away from
default init and exit

Not Using Default init and cleanup
#include <linux/module.h>
static int __init myInit(void)
{
printk(KERN_INFO "Hello Worldn");
/*
* A non 0 return means init_module failed; module can't be loaded.
*/
return 0;
}
static void __exit myExit(void)
{
}
module_init(myInit);
module_exit(myExit);
17
● __init and __exit are Macros
● Kernel can free memory
when initialization or module
unloading is done

Example 6
Variables in Kernel
Modules

Declaring init variables in kernel
modules#include <linux/module.h>
static int myData __initdata = 3;
static int __init myInit(void)
{
printk(KERN_INFO "Hello World, My Data %dn", myData);
return 0;}
static void __exit myExit(void)
{
}
module_init(myInit);
module_exit(myExit); 19
● Static init variable
● __initdata is a macro
● Notice the change from standard
c program

Example 7
Arguments in Kernel
Modules

Passing command line arguments
21
Conventional argc and argv[] does not work with kernel modules
● Register Parameters
○ //Name, Type, Permission bits
○ module_param(myDataInt, int, 0000);
○ MODULE_PARM_DESC(myDataInt, "An
Integer");
○ module_param(myDataStr, charp, 0000);
○ MODULE_PARM_DESC(myDataStr, "A String");
○ //Name, Type, Pointer to variable to array length,
Permission bits
○ module_param_array(myDataArr, int,
&myDataArrCount, 0000);
○ MODULE_PARM_DESC(myDataArr, "An Array");
● Declare static variables
○ static int myDataInt = 1;
○ static char *myDataStr = "WoCS";
○ static int myDataArr[4] = {0,0,0,0};
○ static int myDataArrCount = 0;

Passing command line arguments
$ sudo insmod helloworld-command_line_args.ko myDataStr="Dheryta"
myDataArrCount=3 myDataInt=10 myDataArr=1,2,3
[23492.223323] Hello World
******
[23492.223327] myDataInt is an integer: 10
[23492.223329] myDataStr is a string: Dheryta
[23492.223331] myDataArr[0] = 1
[23492.223332] myDataArr[1] = 2
[23492.223334] myDataArr[2] = 3
[23492.223336] got 3 arguments for myDataArrCount.
22

Device Drivers vs Device Files
● Everything is a file or a directory
● Every device is represented by a file in /dev/
● Device Driver: Kernel Module that controls a device
● Device File:
○ Interface for the Device Driver to read from or
write to a physical device
○ Also known as Device Nodes
○ Created with mknod system call [ex. mknod
<c/b> <major> <minor>]
Device File
(/dev/xxx)
Device Driver
Physical Device
User space
Kernel space
24

Types of Device Files
● Character Files
○ Stream of data one character at a time
○ No restriction on number of bytes
● Block Files
○ Random access to block of data
○ Can buffer and schedule the requests
$ ls -a /dev/
$crw-rw---- 1 root dialout 4, 64 Nov 30 09:51 ttyS0
$brw-rw---- 1 root dialout 4, 64 Nov 30 09:52 sdd
26

Major and Minor Device Number
root@iiitd-HP-Compaq-8200-Elite-MT-PC:/home/iiitd# ls -l /dev/sda*
brw-rw---- 1 root disk 8, 0 Dec 3 11:48 /dev/sda
brw-rw---- 1 root disk 8, 1 Dec 3 11:48 /dev/sda1
● Major Number
○ The driver for the hardware
○ Unique for each driver
● Minor Number
○ The number of unique hardware a driver manages
27

Example 9
Some Fun with Device
Files

Writing to/Read from device file
$cat /usr/share/sounds/ubuntu/notifications/Amsterdam.ogg > /dev/audio
$cat /dev/urandom | padsp tee /dev/audio > /dev/null
$echo “Hello World” > /dev/tty1
$cat /dev/psaux
29

Example 10
System Runtime
Information

Getting Runtime Information of System
● proc file system
○ Reports runtime information about various resources
○ Memory usage/hardware/modules loaded and many more
● Try
○ ls /proc/ (Numbered directories correspond to processes!)
○ cat /proc/devices
○ cat /proc/modules
○ cat /proc/cpuinfo
○ cat /proc/meminfo
● More Details: http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html
31

Example 11
My First Character
Device Driver

Developing a Character Device Driver
Device Driver → Kernel Module Device File → /dev/<filename>
Register the device -
● register_chrdev
● mknod
● struct file_operations
Do the operations -
read/write/open/close
Unregister the device -
● unregister_chrdev
Try Example - chardev.c
33
● Try_module_get
● module_put

Example 12
Dancing Lights with
Keyboard

Interacting with External Devices
● printk does not always help
● Device specific commands should be known
● Interfacing with Keyboard
○ Periodic Lighting Keyboard - Example 10.2 from Ref[1]
○ Dancing Lights Keyboard - Modify Example 10.2 (see help on next slide)
35

Dancing Lights with Keyboard
● Define 3 Macros
○ #define ONE_LEDS_ON 0x04
○ #define TWO_LEDS_ON 0x06
○ #define THREE_LEDS_ON 0x07
36
● Modify my_timer_func
static void my_timer_func(unsigned long ptr)
{
int *pstatus = (int *)ptr;
if (*pstatus == RESTORE_LEDS)
*pstatus = ONE_LEDS_ON;
else if (*pstatus == ONE_LEDS_ON)
*pstatus = TWO_LEDS_ON;
else if (*pstatus == TWO_LEDS_ON)
*pstatus = THREE_LEDS_ON;
else
*pstatus = RESTORE_LEDS;
(my_driver->ops->ioctl) (vc_cons[fg_console].d->port.tty, KDSETLED,
*pstatus);
(my_driver->ops->ioctl) (vc_cons[fg_console].d->port.tty, KIOCSOUND,
CLOCK_TICK_RATE/DEFAULT_FREQ);
my_timer.expires = jiffies + BLINK_DELAY;
add_timer(&my_timer);
}

Interrupt Handlers
37
Processor Hardware
Instructions
How? → Interrupts
Received an interrupt
Pause current work.
Save the state (How?)
Address the interrupt
with Interrupt Handler
Resume the work
request_irq() → Create entry in /proc/interrupts
request_irq() → Create entry in /proc/interrupts

Example 13
Interrupt Handling
Refer Example 7.1 for chardev.c and Ref [8] for interrupts

Raise and Capture Interrupt on a
Character Device
● chardev.c
● Raise the interrupt whenever file is read [asm instruction]
● Capture the interrupt
● Tell user that the interrupt is captured
39

Example 14
Practice with IOCTL
(Do it on your own)
Ref [7]

Interacting with Physical Devices
Kernel Modules
41
/proc/ file system
/dev/ file system
Device read/write
IOCTLs
System Calls
Interrupt Handlers
Use Existing or Write Your Own Custom Calls

What and Why of Shell Scripts
● Shell is an interface to allow the use of operating system services
● All commands are executed in a shell through a terminal
● Bash shell is the most common shell
● Shell scripts allow to
○ Automate the execution of repetitive shell commands
○ Routine procedures such as backups
○ System monitoring
43

Shell Script Structure
● Every shell script starts with a shell name like → # !/bin/bash
● As per convention, a shell script file extension is .sh
● A shell script should be made executable with chmod command
● A shell script can have constructs such as for, while, if-elseif-else, switch
● A shell script can read/write from files
● A shell script can call another program may it be a python or C or any other
● In summary - shell scripts are very powerful
44

Control CD Drive
#!/bin/bash
while :
do
eject
eject -t
done
Ref:https://www.quora.com/What-is-the-most-interesting-shell-script-you-have-ever-written
46

GRE Prep
1. sudo apt-get install cowsay
2. Prepare a dictionary
a. Apple == red fruit
b. LadyFinger == green vegetable
c. Clock == A device to show time
d. English == It is a language
3. In .bashrc → shuf -n 1 MyDictionary.txt | cowsay
Ref:https://www.quora.com/What-is-the-most-interesting-shell-script-you-have-ever-
written 47

System Health
● Use standard commands to summarize all vitals
48
Ref: https://www.tecmint.com/linux-server-health-monitoring-script/

Conclusion
● Kernel Modules: An introduction
● Hello world modules: Creating, Compiling, Inserting, Initialization and Passing
variables
● Proc, Device file system
● Device drivers, files
● Interaction with devices
● Interrupt handling
● Shell Scripts
49

Linux WiFi Subsytem
● Tutorial: Opening Nuts and Bolts of Linux WiFi Subsytem
● Slides:
https://www.slideshare.net/DherytaJaisinghani/tutorial-wifi-driver-code-opening
-nuts-and-bolts-of-linux-wifi-subsystem
● Video: https://www.youtube.com/watch?v=pa1oEyc7Dm0
50

Contact
● Web: www.dheryta.co.in
● Email: dherytaj@iiitd.ac.in
51

References
1. https://www.tldp.org/LDP/lkmpg/2.6/lkmpg.pdf
2. http://derekmolloy.ie/writing-a-linux-kernel-module-part-1-introduction/
3. http://www.cs.colby.edu/maxwell/courses/tutorials/maketutor/
4. https://elinux.org/Debugging_by_printing
5. ftp://ftp.lpp.polytechnique.fr/jeandet/keep/sync/LINUX/interrupt/t3.pdf
6. http://lwn.net/Kernel/LDD3/
7. https://embetronicx.com/tutorials/linux/device-drivers/ioctl-tutorial-in-linux/
8. https://embetronicx.com/tutorials/linux/device-drivers/linux-device-driver-tutori
al-part-13-interrupt-example-program-in-linux-kernel/
9. http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html
10. https://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/dev.html
11. https://01.org/linuxgraphics/gfx-docs/drm/driver-api/index.html
52

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