Week 7 System Calls, Kernel Threads, Kernel...

Week 7

System Calls, Kernel

Threads, Kernel Debugging

Sarah Diesburg

Florida State University

First…

� Any questions on

� Part 1 – 5 system calls

� Part 2 – xtime proc module

Story of Kernel

Development

Some context…

In the old days…

� There were no modules or virtual machines

� The kernel is a program

� Has code, can compile, re-compile, make executableexecutable

� When changes needed to be made, developers make changes in source and re-compile

How is the kernel different from a

regular program?� Mostly in how it is executed

� Boot loader loads the kernel image/executable during boot time

� Sets kernel modeSets kernel mode

� Jumps to the entry point in the image/executable

� Remember the generic booting sequence?

Quick Question

� How would you make changes to the kernel

and run those changes?

1. Make changes to the source

2. Re-complie the kernel source2. Re-complie the kernel source

3. Re-install the kernel source

4. Make sure the bootloader sees the new kernel image (grub)

5. Reboot and profit!

Getting more modern..

� Modules were created as bits of code that

can be loaded and unloaded by the kernel in

kernel mode

� Made development easier� Made development easier

� Instead of re-compiling, re-installing, and rebooting into the new kernel, one could just re-compile and load a module

Quick Question

� How would you make changes to a module

and run those changes?

1. Make changes to module source code

2. Re-compile the module2. Re-compile the module

3. Load the new module

Present Day

� Reboots into new kernels and loading new

modules often freezes machines

� Enter virtual machine software

� Process that emulates the hardware necessary to � Process that emulates the hardware necessary to run an OS in user-space

� Guest OS is executed inside the virtual machine process!

New System Calls

Fun but tricky!

Implementing System Calls

int start_elevator(void);

int issue_request(int #1, int #2,

int #3);

int stop_elevator(void);

� Need to implement the functions above. But

int stop_elevator(void);

Adding a System Call to Kernel

� Files to add:� LINUX_DIR/PROJECT_NAME/Makefile

� LINUX_DIR/PROJECT_NAME/PROJECT_NAME.c

� LINUX_DIR/PROJECT_NAME/NEW_SYSCALLS.c

Files to modify:� Files to modify:� LINUX_DIR/arch/x86/kernel/syscall_table_32.S

� LINUX_DIR/include/asm-generic/unistd.h

� LINUX_DIR/include/linux/syscalls.h

� LINUX_DIR/Makefile

Sample System Call

� Let’s add a sample module to the kernel that

defines a sample system call

� test_newsyscall(int test_int);� test_newsyscall(int test_int);

� Takes an int test_int and issues a printk on it

� Returns test_int

� Problem – If our module isn’t loaded, what

happens if we call our sample system call?

Project 2 System Call Model

Elevator module Core kernel

User program

User issues system call,

core kernel looks up

system call in system call

Elevator module

performs system call

action

User program

Elevator module

returns result of

system call

User program

Core kernel forwards

result of system call to

user program

What happens if elevator module is

not loaded?

Core kernel

User program

not loaded?

Core kernel

User program

User issues system call,

core kernel looks up

system call in system call

not loaded?

Elevator module is not

loaded to perform the

action…. OOPS!

Core kernel

User program

Module System Calls

� We must create a wrapper system call!

� Wrapper will call module function if module loaded, else returns an error

� Must be created in a separate, built-in kernel file Must be created in a separate, built-in kernel file in the project folder

Function Pointers

� We will implement our system call wrapper

with a function pointer

� Pointer to a function

� Function pointer can point to any function � Function pointer can point to any function

that you implement that

� Takes the same input variable types

� Returns the same return type

Function Pointers

long (*STUB_test_newsyscall)(int test_int) = NULL;

� Function pointer that

� Returns a long

� Name is STUB_test_newsyscall

� Takes parameter int test_int

� Function pointer set to NULL

� Can set function pointer to a local function you implement

Elevator Project

� Create a file in your elevator project that just

contains the system call information

� KERNEL_DIR/PROJECT_DIR/newsyscalls.c

KERNEL_DIR/PROJECT_DIR/

newsyscalls.c#include <linux/linkage.h>

#include <linux/kernel.h>

#include <linux/module.h>

/* System call stub. We initialize the stub function to be NULL.

long (*STUB_test_newsyscall)(int test_int) = NULL;long (*STUB_test_newsyscall)(int test_int) = NULL;

EXPORT_SYMBOL(STUB_test_newsyscall);

/* System call wrapper. If the stub is not NULL, it will be run,

otherwise returns -ENOSYS */

asmlinkage long sys_test_newsyscall(int test_int)

if (STUB_test_newsyscall)

return STUB_test_newsyscall(test_int)

return -ENOSYS;

Function pointer

return -ENOSYS;

Export the pointer

so we can access

it later

return -ENOSYS;

long (*STUB_test_newsyscall)(int test_int) = NULL;long (*STUB_test_newsyscall)(int test_int) = NULL;

return -ENOSYS;

System call

wrapper

Elevator Project

� Next create a separate file that

� Holds your module code

� Registers the system call pointer

� Actually implements the system call behavior� Actually implements the system call behavior

Inside KERNEL_DIR/PROJECT_DIR/

PROJECT_NAME.C

/* Extern system call stub declarations */

extern long (*STUB_test_newsyscall)(int test_int);

long my_test_newsyscall(int test)

printk("%s: Your int is %i\n", __FUNCTION__, test);

return test;

my_module_init() {

STUB_test_newsyscall=&(my_test_newsyscall);

return 0;

my_module_exit() {

STUB_test_newsyscall=NULL;

PROJECT_NAME.C

return test;

Gain access to stub

function pointer.

my_module_init() {

return 0;

my_module_exit() {

PROJECT_NAME.C

return test;

Local function that

implements syscall

my_module_init() {

return 0;

my_module_exit() {

PROJECT_NAME.C

return test;

} Set stub function pointer }

my_module_init() {

return 0;

my_module_exit() {

Set stub function pointer

to local function in init

PROJECT_NAME.C

return test;

my_module_init() {

return 0;

my_module_exit() {

Reset stub function

pointer to NULL on

module unload

Makefileobj-m := my_module.o

obj-y := newsyscalls.o

KDIR := /lib/modules/2.6.32/build

PWD := $(shell pwd)

default:

$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules

PWD := $(shell pwd)

Compile as a module

default:

PWD := $(shell pwd)

Compile as kernel built-in

default:

Core Kernel Additions

� Add the new system call to the core kernel

system call table

� Modify three files

� Add the project directory to the main Makefile� Add the project directory to the main Makefile

� Modify one file

Modifying syscall_table_32.S

.long sys_preadv

.long sys_pwritev

.long sys_rt_tgsigqueueinfo /* 335 */

.long sys_perf_event_open

Modifying syscall_table_32.S

.long sys_preadv

.long sys_pwritev

.long sys_rt_tgsigqueueinfo /* 335 */

.long sys_perf_event_open

.long sys_test_newsyscall /* 337 */

� Add new system call to the end of the file. � Remember the number – you’ll need it in userspace!

Modifying unistd.h

/* midfile */

#define __NR_perf_event_open 241

__SYSCALL(__NR_perf_event_open, sys_perf_event_open)

#undef __NR_syscalls

#define __NR_syscalls 242

/* midfile */

� Can be found around line 623…

Modifying unistd.h

/* midfile */

#define __NR_test_newsyscall 242

__SYSCALL(__NR_test_newsyscall, sys_test_new_syscall)

/* midfile */

Modifying unistd.h

/* midfile */

#define __NR_test_newsyscall 242

__SYSCALL(__NR_test_newsyscall, sys_test_new_syscall)

/* midfile */

Modifying syscalls.h

asmlinkage long sys_perf_event_open(

struct perf_event_attr __user *attr_uptr,

pid_t pid, int cpu, int group_fd, unsigned

long flags);

#endif

/* EOF *//* EOF */

Modifying syscalls.h

asmlinkage long sys_perf_event_open(

struct perf_event_attr __user *attr_uptr,

pid_t pid, int cpu, int group_fd, unsigned

long flags);

asmlinkage long sys_test_newsyscall(int test_int);asmlinkage long sys_test_newsyscall(int test_int);

#endif

/* EOF */

Modifying

KERNEL_DIR/Makefile# Objects we will link into vmlinux /

subdirs we need to visit

init-y := init/

drivers-y := drivers/ sound/ firmware/

net-y := net/net-y := net/

libs-y := lib/

core-y := usr/

endif # KBUILD_EXTMOD

Modifying

KERNEL_DIR/Makefile# Objects we will link into vmlinux /

subdirs we need to visit

init-y := init/

drivers-y := drivers/ sound/ firmware/

net-y := net/net-y := net/

libs-y := lib/

core-y := usr/ my_module/

endif # KBUILD_EXTMOD

� Found around line 475…

� Can replace “my_module” with the name of your

PROJECT_DIR

Getting it all to work

1. Re-compile the kernel

2. Install modules, install kernel

3. Make new initramfs image

Reboot4. Reboot

5. Test with a user-space program…

Sample User-space Program

#include <stdio.h>

#include <stdlib.h>

#include <sys/syscall.h>

#include <linux/unistd.h>

#define __SYS_TEST_ELEVATOR 337

int main()

int test=5;int test=5;

long ret;

ret=syscall(__SYS_TEST_ELEVATOR, test);

if(ret<0)

perror("system call error");

printf("Function successful, returned %i\n", ret);

return 0;

syscall()

int syscall(int number, ...);

� Performs the system call based on the system call’s

number

� Number can be found in the syscall_table_32.S file (our example � Number can be found in the syscall_table_32.S file (our example was 337)

User-space Program Output

� Output when my_module not loaded

system call error: Function not implemented

� Output when my_module loaded

Function successful, returned 5

Kthreads

Run the main logic of your module in a

kthread!

Refresher: hello.c

#include <linux/init.h>

MODULE_LICENSE(“Dual BSD/GPL”);

static int hello_init(void)

printk(KERN_ALERT “Hello, world!\n”);

return 0;return 0;

static void hello_exit(void)

printk(KERN_ALERT “Goodbye, sleepy world.\n”);

module_init(hello_init);

module_exit(hello_exit);

Kernel Modules

� Remember, kernel modules are very event-

� We need a way to start an independent

thread of execution in response to an eventthread of execution in response to an event

� e.g. start_elevator() for project 2…

kthread_run

kthread_run(threadfn, data, namefmt, ...)

� Creates a new thread and tells it to run� threadfn – the name of the function the thread should run

� data – data pointer for threadfn (can be NULL if the function � data – data pointer for threadfn (can be NULL if the function

does not take any args)

� namefmt – name of the thread (displayed during “ps” command)

� Returns a task_struct

kthread_run example

struct task_struct *t;

t = kthread_run(run, NULL, “my_elevator");

if (IS_ERR(t)){if (IS_ERR(t)){

ret=PTR_ERR(t);

kthread_stop

int kthread_stop(struct task_struct * k);

� Sets kthread_should_stop for k to return true, wakes

the thread, and waits for the thread to exit

Returns the result of the thread function� Returns the result of the thread function

kthread_stop_example

ret=kthread_stop(t);

if(ret != -EINTR)

printk("Main logic tread stopped.\n“);

Thread Function Example

static int run(void *arg)

/* Lock here */

while(!kthread_should_stop()) {

/* Do stuff */

/* Unlock here */

schedule();

/* Lock here */

/* Unlock here */

printk("%s: kernel thread exits.\n", __FUNCTION__);

return 0;

Thread Function Example

static int run(void *arg)

/* Lock here */

while(!kthread_should_stop()) {

/* Do stuff */schedule() is very

important here. Why?/* Unlock here */

schedule();

/* Lock here */

/* Unlock here */

printk("%s: kernel thread exits.\n", __FUNCTION__);

return 0;

important here. Why?

Inefficient Solution

� Thread will continue to run even though it has

nothing to do

� Eats up resources

� Investigate the kthread interface to find ways � Investigate the kthread interface to find ways

� Put thread to sleep

� Wake up thread

� There is more than one way to do this…

Debugging

Kernel Debugging Configurations

� Timing info on printks

� __depreciated logic

� Detection of hung tasks

� SLUB debugging� SLUB debugging

� Kernel memory leak detector

� Mutex/lock debugging

� Kmemcheck

� Check for stack overflow

� Linked list debugging

Select Kernel Hacking

Enable Debugging Options

Debugging through procfs

� Necessary for elevator project!

� General process

� Identify data to monitor in your module

� Create a proc entry to monitor this data� Create a proc entry to monitor this data

� Run your module

� Query /proc/<entry> for that information at any time

Kernel Oops and Other Errors

� Kernel errors often only appear on first tty

(terminal interface)

� Why?

� How can I see my first tty?� How can I see my first tty?

� On regular system – CTRL+ALT+F1

� CTRL+ALT+F7 to go back to X screen

� On VMware – CTRL+ALT+SPACE+F1

� CTRL+ALT+SPACE+F7 to go back to X screen

Reason for failure

for failure

Current drivers

Call Trace

Failed command

command

Defensive Programming

• Infinite loops and deadlocks at the kernel

level hang your machine

– Ctrl-Alt-Del has NO effect

– Ctrl-C does not matter– Ctrl-C does not matter

– Ctrl-D does not matter

– You may only reboot

• How do you protect yourself?

– Use schedule() strategically

– Use preemptable versions of functions

Debugging Tools not Covered

� LTT – Linux Tracing Framework

� gdb – Invoking gbd on the kernel image

� kgdb – A remote debugger for the kernel

Magic SysRq� Magic SysRq

� printk – Rate limiting, turning on/off

Next Time

� Locks

� Linked lists

� Elevator algorithms

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