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Week 6 Intro to Kernel Modules, Project 2

Week 6 Intro to Kernel Modules, Project 2. Sarah Diesburg Florida State University. Kernel Logistics. Where should I put the kernel source? /usr/src/ Creates /usr/src/linux-2.6.32/ Where do I issue kernel building commands (e.g. ‘make oldconfig’, ‘make menuconfig’, ‘make’, …)?

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Week 6 Intro to Kernel Modules, Project 2

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  1. Week 6 Intro to Kernel Modules, Project 2 Sarah Diesburg Florida State University

  2. Kernel Logistics • Where should I put the kernel source? • /usr/src/ • Creates /usr/src/linux-2.6.32/ • Where do I issue kernel building commands (e.g. ‘make oldconfig’, ‘make menuconfig’, ‘make’, …)? • Inside /usr/src/linux-2.6.32/

  3. Kernel Logistics • Where is the kernel image installed? • Inside /boot/ • Starts with vmlinuz… • Where does the initramfs image go? • Inside /boot/ • Where is the grub file? • /boot/grub/menu.lst

  4. Kernel Logistics • Where should I develop my new kernel modules? • Inside /usr/src/linux-2.6.32/<module_name>/

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

  6. 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

  7. 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.

  8. Creating a Kernel Module • Hello world example

  9. Sample Kernel Module: hello.c #include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static inthello_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);

  10. Sample Kernel Module: hello.c Module headers #include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static inthello_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);

  11. Sample Kernel Module: hello.c License declaration #include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static inthello_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);

  12. Sample Kernel Module: hello.c #include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static inthello_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

  13. Sample Kernel Module: hello.c #include <linux/init.h> #include <linux/module.h> MODULE_LICENSE(“Dual BSD/GPL”); static inthello_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

  14. 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*

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

  16. 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

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

  18. 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…

  19. 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’!

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

  21. Kernel manpages • Section 9 of manpages • Must install manually for our development kernel $>wget http://ftp.us.debian.org/debian/pool/main/l/linux-2.6/linux-manual-2.6.32_2.6.32-22_all.deb $> sudo dpkg –i linux-manual-2.6.32_2.6.32-22_all.deb

  22. 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-2.6.32/include/ for more… • Google is also your friend

  23. How can I explore the kernel? • Use lxr (“Linux Cross Referencer”): • http://lxr.linux.no/ • Select your kernel version and enter search terms • Use grep on your kernel source $> grep –Rn xtime /usr/src/linux-2.6.32 • R = recursive, n = display line number

  24. Project 2: /Proc Kernel Module and Elevator

  25. procfs Kernel Module • procfs “hello world” example • Creates a read-only procfs entry • Steps • Create entry in module_init function • Register reading function with procfs_read • Delete entry in module_cleanup function • Reference • Linux Kernel Module Programming Guide: Proc FS

  26. Procfs: Headers and Global Data #include <linux/module.h> #include <linux/kernel.h> #include <linux/proc_fs.h> MODULE_LICENSE(“GPL”); #define ENTRY_NAME “helloworld” #define PERMS 0644 #define PARENT NULL structproc_dir_entry *proc_entry; intprocfile_read(char *buf, char **buf_location, off_t offset, intbuffer_length, int *eof, void *data);

  27. Procfs: Creation inthello_proc_init(void) { proc_entry = create_proc_entry(ENTRY_NAME, PERMS,PARENT); /* check proc_entry != NULL */ proc_entry->read_proc = procfile_read; proc_entry->mode = S_IFREG | S_IRUGO; proc_entry->uid = 0; proc_entry->gid = 0; proc_entry->size = 11; printk(“/proc/%s created\n”, ENTRY_NAME); return 0; }

  28. Procfs: Reading intprocfile_read(char *buf, char **buf_location, off_t offset, intbuffer_length, int *eof, void *data) { int ret; printk(“/proc/%s read called.\n”, ENTRY_NAME); /* Setting eof. We exhaust all data in one shot */ *eof = 1; ret = sprintf(buf, “Hello World!\n”); return ret; }

  29. Procfs: Deletion void hello_proc_exit(void) { remove_proc_entry(ENTRY_NAME, NULL); printk(“Removing /proc/%s.\n”, ENTRY_NAME); }

  30. Procfs: Registration module_init(hello_proc_init); module_exit(hello_proc_exit);

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

  32. Part 2: Kernel Time • Implement a procfs entry to display the value of xtime • Hint: You may not be able to directly read xtime from your module, but maybe something else can…

  33. Part 3: Elevator Scheduling

  34. Part 3: Elevator Scheduling • Implement a kernel module that simulates an elevator system • Implement system calls to interact with your elevator • Implement a procfs entry to display debugging information • Test using a set of user-space programs to exercise your system

  35. Why Elevator Scheduling? • Classic producer/consumer analogy • Similar to disk elevators • File system produces read/write requests • Disk consumes requests, optimized for disk head position, rotational delays, etc.

  36. Your Elevator • One elevator • Five floors • Four types of people • Adults • Children • Delivery people • Maintenance people • The elevator cannot exceed its maximum weight load

  37. Your Elevator • People will line up at each floor in a first-in, first-out (FIFO) order • Each person has a starting floor and a destination floor • The elevator must pause for a period of time to collect people and move between floors • Once the elevator reaches a passenger’s destination floor, that passenger gets out and ceases to exist

  38. Passengers will line up (FIFO)

  39. Each passenger has a destination floor in mind… I want to go to floor 3

  40. The elevator must be started to service passengers… Start!

  41. The elevator must be started to service passengers… Elevator starts on the first floor

  42. Passengers enter in FIFO order Make sure passengers don’t exceed weight limit!

  43. Passengers enter in FIFO order More passengers can be queuing up!

  44. Elevator can move to any floor Red and black has destination floor 3, blue has destination floor 2 Going to floor 3!

  45. Elevator can move to any floor Must take certain amount of time between floors…

  46. Elevator can move to any floor Must take certain amount of time between floors…

  47. Elevator can move to any floor Must take certain amount of time between floors…

  48. Elevator can move to any floor Must take certain amount of time between floors…

  49. Elevator can move to any floor Must take certain amount of time between floors…

  50. Passengers disappear when they exit…

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