Week 7
System Calls, Kernel
Threads, Kernel Debugging
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Sarah Diesburg
Florida State University
First…
� Any questions on
� Part 1 – 5 system calls
� Part 2 – xtime proc module
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Story of Kernel
Development
Some context…
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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
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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?
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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!
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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
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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
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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!
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New System Calls
Fun but tricky!
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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
how?
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?
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Project 2 System Call Model
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Elevator module Core kernel
User program
Project 2 System Call Model
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Elevator module Core kernel
User program
User issues system call,
core kernel looks up
system call in system call
table
Project 2 System Call Model
Elevator module
performs system call
action
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Elevator module Core kernel
User program
Project 2 System Call Model
Elevator module
returns result of
system call
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Elevator module Core kernel
User program
Project 2 System Call Model
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Elevator module Core kernel
User program
Core kernel forwards
result of system call to
user program
What happens if elevator module is
not loaded?
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Core kernel
User program
What happens if elevator module is
not loaded?
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Core kernel
User program
User issues system call,
core kernel looks up
system call in system call
table
What happens if elevator module is
not loaded?
Elevator module is not
loaded to perform the
action…. OOPS!
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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
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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
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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
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Elevator Project
� Create a file in your elevator project that just
contains the system call information
� KERNEL_DIR/PROJECT_DIR/newsyscalls.c
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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)
else
return -ENOSYS;
}26
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;
Function pointer
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)
else
return -ENOSYS;
}27
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;
Export the pointer
so we can access
it later
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)
else
return -ENOSYS;
}28
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)
else
return -ENOSYS;
}29
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
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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;
}
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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;
}
Gain access to stub
function pointer.
}
my_module_init() {
STUB_test_newsyscall=&(my_test_newsyscall);
return 0;
}
my_module_exit() {
STUB_test_newsyscall=NULL;
}
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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;
}
Local function that
implements syscall
}
my_module_init() {
STUB_test_newsyscall=&(my_test_newsyscall);
return 0;
}
my_module_exit() {
STUB_test_newsyscall=NULL;
}
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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;
} Set stub function pointer }
my_module_init() {
STUB_test_newsyscall=&(my_test_newsyscall);
return 0;
}
my_module_exit() {
STUB_test_newsyscall=NULL;
}
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Set stub function pointer
to local function in init
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;
}
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Reset stub function
pointer to NULL on
module unload
KERNEL_DIR/PROJECT_DIR/
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
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KERNEL_DIR/PROJECT_DIR/
Makefileobj-m := my_module.o
obj-y := newsyscalls.o
KDIR := /lib/modules/2.6.32/build
PWD := $(shell pwd)
Compile as a module
default:
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
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KERNEL_DIR/PROJECT_DIR/
Makefileobj-m := my_module.o
obj-y := newsyscalls.o
KDIR := /lib/modules/2.6.32/build
PWD := $(shell pwd)
Compile as kernel built-in
default:
$(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
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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
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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_perf_event_open 241
__SYSCALL(__NR_perf_event_open, sys_perf_event_open)
#define __NR_test_newsyscall 242
__SYSCALL(__NR_test_newsyscall, sys_test_new_syscall)
#undef __NR_syscalls
#define __NR_syscalls 242
/* midfile */
Modifying unistd.h
/* midfile */
#define __NR_perf_event_open 241
__SYSCALL(__NR_perf_event_open, sys_perf_event_open)
#define __NR_test_newsyscall 242
__SYSCALL(__NR_test_newsyscall, sys_test_new_syscall)
#undef __NR_syscalls
#define __NR_syscalls 243
/* 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 */
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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 */
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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
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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
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� 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");
else
printf("Function successful, returned %i\n", ret);
return 0;
}
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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)
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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
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Kthreads
Run the main logic of your module in a
kthread!
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Refresher: 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;return 0;
}
static void hello_exit(void)
{
printk(KERN_ALERT “Goodbye, sleepy world.\n”);
}
module_init(hello_init);
module_exit(hello_exit);
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Kernel Modules
� Remember, kernel modules are very event-
based
� 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…
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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
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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);
}
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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
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kthread_stop_example
ret=kthread_stop(t);
if(ret != -EINTR)
printk("Main logic tread stopped.\n“);
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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;
}
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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;
}
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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
to
� Put thread to sleep
� Wake up thread
� There is more than one way to do this…
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Debugging
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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
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Select Kernel Hacking
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Enable Debugging Options
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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
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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
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Oops!
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Reason for failure
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for failure
Current drivers
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Call Trace
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Call Trace
Failed command
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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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