1. Intro to Kernel Modules and /proc
Sarah Diesburg
CS 3430
Operating Systems

2. Or “drivers”, if you prefer…
Kernel Modules
Or “drivers”, if you prefer…

3. Kernel Module
A kernel module is a portion of kernel functionality that can be dynamically loaded into the operating system at run-time
Example
USB drivers
File system drivers
Disk drivers
Cryptographic libraries

4. Why not just compile everything into the kernel?
Each machine only needs a certain number of drivers
For example, should not have to load every single motherboard driver
Load only the modules you need
Smaller system footprint
Dynamically load modules for new devices
Camera, new printer, etc.

5. Creating a Kernel Module
Hello world example

6. Sample Kernel Module: hello.c
#include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static int hello_init(void) { printk(KERN_ALERT “Hello, world!\n”); return 0; } static void hello_exit(void) printk(KERN_ALERT “Goodbye, sleepy world.\n”); module_init(hello_init); module_exit(hello_exit);

7. Sample Kernel Module: hello.c
Module headers
#include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static int hello_init(void) { printk(KERN_ALERT “Hello, world!\n”); return 0; } static void hello_exit(void) printk(KERN_ALERT “Goodbye, sleepy world.\n”); module_init(hello_init); module_exit(hello_exit);

8. Sample Kernel Module: hello.c
License declaration
#include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static int hello_init(void) { printk(KERN_ALERT “Hello, world!\n”); return 0; } static void hello_exit(void) printk(KERN_ALERT “Goodbye, sleepy world.\n”); module_init(hello_init); module_exit(hello_exit);

9. Sample Kernel Module: hello.c
#include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static int hello_init(void) { printk(KERN_ALERT “Hello, world!\n”); return 0; } static void hello_exit(void) printk(KERN_ALERT “Goodbye, sleepy world.\n”); module_init(hello_init); module_exit(hello_exit);
Initialization function, runs when module loaded
Tells kernel which function to run on load

10. Sample Kernel Module: hello.c
#include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static int hello_init(void) { printk(KERN_ALERT “Hello, world!\n”); return 0; } static void hello_exit(void) printk(KERN_ALERT “Goodbye, sleepy world.\n”); module_init(hello_init); module_exit(hello_exit);
Exit function, runs when module exits
Tells kernel which function to run on exit

11. Sample Kernel Module: Makefile
ifneq ($(KERNELRELEASE),)
obj-m := hello.o
else
KERNELDIR ?= \
/lib/modules/`uname -r`/build/
PWD := `pwd`
default:
$(MAKE) -C $(KERNELDIR) \
M=$(PWD) modules
endif
clean:
rm -f *.ko *.o Module* *mod*

12. Compile the Kernel Module
/usr/src/hello$> make
Creates hello.ko – This is the finished kernel module!

13. Inserting and Removing the Module
insmod – insert a module
/usr/src/hello$> sudo insmod hello.ko
rmmod – remove a module
/usr/src/hello$> sudo rmmod hello.ko

14. Listing Modules lsmod – lists all running modules
/usr/src/hello$>lsmod

15. Where is it printing? Look inside /var/log/syslog
Hint – to watch syslog in realtime, issue the following command in a second terminal:
$> sudo tail –f /var/log/syslog
Demo…

16. Kernel Module vs User Application
All kernel modules are event-driven
Register functions
Wait for requests and service them
Server/client model
No standard C library
Why not?
No floating point support
Segmentation fault could freeze/crash your system
Kernel ‘oops’!

17. Kernel Functions printk() instead of printf()
kmalloc() instead of malloc()
kfree() instead of free()
Where can I find definitions of these kernel functions?

18. Kernel manpages Section 9 of manpages Use manpages
$> sudo apt-get install linux-manual-3.16
$> man printk

19. Kernel Headers #include <linux/init.h> /* module stuff */
#include <linux/module.h> /* module stuff */
#include <asm/semaphore.h> /* locks */
#include <linux/list.h> /* linked lists */
#include <linux/string.h> /* string functions! */
Look inside linux /include/ for more…
Google is also your friend

20. Project 2: Kernel Modules

21. procfs Kernel Module procfs “hello world” example Steps
Creates a read/write procfs entry
Steps
Create entry in module_init function
Registers reading/writing function using file_operations structure
Delete entry in module_cleanup function

22. Testing Procfs
$> sudo insmod hello_proc.ko $> sudo tail /var/log/syslog $> cat /proc/helloworld $> echo 2 > /proc/helloworld $> sudo rmmod hello_proc

23. Part 2: Remember
You will write your own proc module called remember that
Allows the user to write a string to /proc/remember (max length 80)
Allows the user to read /proc/remember and get back the string that was just added
If no string was added, the string EMPTY should be read instead

24. Remember Example #> echo “Hello there” > /proc/remember
#> cat /proc/remember
Hello there
#>