The kernel’s task list

1. The kernel’s task list Introduction to process descriptors and their related data-structures for Linux kernel version 2.6.22

2. Multi-tasking • Modern operating systems allow multiple users to share a computer’s resources • Users are allowed to run multiple tasks • The OS kernel must protect each task from interference by other tasks, while allowing every task to take its turn using some of the processor’s available time

3. Stacks and task-descriptors • To manage multitasking, the OS needs to use a data-structure which can keep track of every task’s progress and usage of the computer’s available resources (physical memory, open files, pending signals, etc.) • Such a data-structure is called a ‘process descriptor’ – every active task needs one • Every task needs its own ‘private’ stack

4. What’s on a program’s stack? Upon entering ‘main()’: • A program’s exit-address is on user stack • Command-line arguments on user stack • Environment variables are on user stack During execution of ‘main()’: • Function parameters and return-addresses • Storage locations for ‘automatic’ variables

5. Entering the kernel… A user process enters ‘kernel-mode’: • when it decides to execute a system-call • when it is ‘interrupted’ (e.g. by the timer) • when ‘exceptions’ occur (e.g. divide by 0)

6. Switching to a different stack • Entering kernel-mode involves not only a ‘privilege-level transition’ (from level 3 to level 0), but also a stack-area ‘switch’ • This is necessary for robustness: e.g., user-mode stack might be exhausted • This is desirable for security: e.g, privileged data might be accessible

7. What’s on the kernel stack? Upon entering kernel-mode: • task’s registers are saved on kernel stack (e.g., address of task’s user-mode stack) During execution of kernel functions: • Function parameters and return-addresses • Storage locations for ‘automatic’ variables

8. Supporting structures • So every task, in addition to having its own code and data, will also have a stack-area that is located in user-space, plus another stack-area that is located in kernel-space • Each task also has a process-descriptor which is accessible only in kernel-space

9. A task’s virtual-memory layout process descriptor and kernel-mode stack User space Kernel space Privilege-level 0 User-mode stack-area Privilege-level 3 Shared runtime-libraries Task’s code and data

10. The Linux process descriptor pagedir[] task_struct state mm_struct Each process descriptor contains many fields and some are pointers to other kernel structures which may themselves include fields that point to structures *stack flags *pgd *mm user_struct exit_code *user files_struct pid *files signal_struct *parent *signal

11. Something new in 2.6 • Linux uses part of a task’s kernel-stack page-frame to store ‘thread information’ • The thread-info includes a pointer to the task’s process-descriptor data-structure Task’s kernel-stack struct task_struct 8-KB Task’s process-descriptor Task’s thread-info page-frame aligned

12. Tasks have ’states’ From kernel-header: <linux/sched.h> • #define TASK_RUNNING 0 • #define TASK_INTERRUPTIBLE 1 • #define TASK_UNINTERRUPTIBLE 2 • #define TASK_STOPPED 4 • #define TASK_TRACED 8 • #define TASK_NONINTERACTIVE 64 • #define TASK_DEAD 128

13. Fields in a process-descriptor struct task_struct { volatile long state; void *stack; unsigned long flags; struct mm_struct *mm; struct thread_struct *thread; pid_t pid; char comm[16]; /* plus many other fields */ };

14. Finding a task’s ‘thread-info’ • During a task’s execution in kernel-mode, it’s very quick to find that task’s thread-info object • Just use two assembly-language instructions: movl $0xFFFFF000, %eax andl %esp, %eax Ok, now %eax = the thread-info’s base-address There’s a macro that implements this computation

15. Finding task-related kernel-data • Use a macro ‘task_thread_info( task )’ to get a pointer to the ‘thread_info’ structure: struct thread_info *info = task_thread_info( task ); • Then one more step gets you back to the address of the task’s process-descriptor: struct task_struct *task = info->task;

16. The kernel’s ‘task-list’ • Kernel keeps a list of process descriptors • A ‘doubly-linked’ circular list is used • The ‘init_task’ serves as a fixed header • Other tasks inserted/deleted dynamically • Tasks have forward & backward pointers, implemented as fields in the ‘tasks’ field • To go forward: task = next_task( task ); • To go backward: task = prev_task( task );

17. Doubly-linked circular list next_task init_task (pid=0) newest task … prev_task

18. Demo • We can write a module that lets us create a pseudo-file (named ‘/proc/tasklist’) for viewing the list of all currently active tasks • Our ‘tasklist.c’ module shows the name and process-ID of each task, along with that task’s current ‘state’ (0, 1, 2, 4, 8,…) • Use the command: $ cat /proc/tasklist to display a complete list of the active tasks

19. Maybe a big /proc file… • We can’t know ahead of time how many tasks are active in our system – this will depend on many varying factors, such as who else is logged in, which commands have been issued, whether we’re using text-mode console or graphical desktop • So it’s perfectly possible our pseudo-file might ‘overflow’ its kernel-supplied buffer!

20. How to avoid buffer-overflow • Our module’s ‘get_info()’ callback-function has four parameter-values passed to it by the kernel: • char *buf - address of a small kernel buffer • char **start - address of a pointer variable • off_t offset - current offset of file-pointer • int buflen - size of the kernel buffer • The initial conditions are: offset == 0 and *start == NULL • Kernel’s behavior will vary if we modify *start

21. Normal case • We expect the ‘/proc’ file to deliver a small amount of text-data (not more than would fit in the kernel-supplied buffer (e.g., 3KB) • So we make no change to ‘*start’ • Then kernel will deliver the data it finds in the buffer it had supplied to ‘get_info()’ • The kernel will not call ‘get_info()’ again (unless our file is closed and reopened)

22. Alternative case • Our ‘get_info()’ function modifies the value of the (initially NULL) ‘*start’ pointer – for example, maybe assigning it the address of some buffer we’ve allocated, or even assigning the address of the kernel-buffer: *start = buf; • In this case, the kernel will again call our module’s ‘get_info()’ function, provided we returned a nonzero function-value before!

23. The benefit • Knowing about this alternative option, we can design our ‘get_info()’ function so that it delivers a big amount of data in several small-size chunks, never overflowing the size-limitations on the kernel’s buffer • We just need to think carefully about the differing senarios under which ‘get_info()’ will be repeatedly called

24. First pass • The value of ‘offset’ will be zero • We set *start to a buffer-address where we place a positive number of data-bytes • Kernel delivers those bytes to the ‘reader’, taking them from the *start address, then advances the file-pointer by that amount • Kernel calls our ‘get_info()’ again, but with a non-zero ‘offset’ value this time!

25. Final time • When our ‘get_info()’ function has finally finished delivering all the desired data to the file’s ‘reader’, and still we receive yet another ‘get_info()’ call, then we simply return a function-value equal to zero, telling the kernel that the data has been exhausted -- and so not to call again!

26. Our implementation struct task_struct *task; // ‘global’ variables’ values remembered int my_get_info( char *buf, char **start, off_t offset, int buflen ) { int len = 0; if ( offset == 0 ) // our first time through this function { task = &init_task; // start of circular linked-list } else if ( task == &init_task ) return 0; // our final pass // put some data into the kernel-supplied buffer len += sprintf( buf+len, “pid=%d \n”, task->pid ); *start = buf; // tell kernel where to find data, and to call again task = next_task( task ); // advance to next node of circular list return len; // and tell kernel how far to advance }

27. In-class exercise #1 • Different versions of the 2.6 Linux kernel use slightly different definitions for the task-related kernel data-structures (e.g., the 2.6.10 kernel used a smaller-sized ‘thread-info’ structure than 2.6.9 kernel did) • So, by using the C ‘sizeof’ operator, can you quickly create an LKM that will show us: • the size of a ‘task_struct’ object (in bytes)? • the size of a ‘thread_info’ object (in bytes)?

28. ‘Kernel threads’ • Some tasks don’t have a page-directory of their own – because they don’t need one • They only execute code, and access data, that resides in the kernel’s address space • They can just ‘borrow’ the page-directory that belongs to another task • These ‘kernel thread’ tasks will store the NULL-pointer value (i.e., zero) in the ‘mm’ field of their ‘task_struct’ descriptor

29. In-class exercise #2 • Can you modify our ‘tasklist.c’ module so it will display a list of only those tasks which are ‘kernel threads’? (i.e., task->mm == 0) • How many ‘kernel threads’ on your list?