OS: Membuat Kernel Modul

Sumber:

Pada bagian ini, kita akan menulis dan load sebuah kernel modul sederhana. Dengan menuliskan sendiri modul kita menjadi belajar bagaimana menulis code kernel yang berdiri sendiri, belajar bagaimana menggunakan modul, dan menemukan beberapa aturan bagaimana kernel terkait menjadi satu kesatuan.

Catatan: instruksi disini ditulis untuk kernel 2.6.x yang mungkin tidak cocok untuk versi kernel yang berbeda.

Apakah kernel mendukung modul?

Kernel yang kita gunakan, harus di compile dengan option:

Loadable module support  --->
  [*] Enable loadable module support
  [*]   Module unloading
  [ ]   Module versioning support (EXPERIMENTAL)
  [*]   Automatic kernel module loading

Pastikan pada saat kita mengcompile kernel opsi di atas di aktifkan. Jika tidak, kita perlu mengcompile ulang kernel.

Rangka Kernel Modul Sederhana

Pertama kali, kita perlu mencari lokasi directory tempat kernel di compile, biasanya di /usr/src/linux. Kemudian ubah directory ke drivers/misc dari directory source code Linux.

Sekarang, copy paste code berikut ke file dengan nama, misalnya, mymodule.c

#include <linux/module.h>
#include <linux/config.h>
#include <linux/init.h>

static int __init mymodule_init(void)
{
 printk ("My module worked!\n");
        return 0;
}

static void __exit mymodule_exit(void)
{
 printk ("Unloading my module.\n");
        return;
}

module_init(mymodule_init);
module_exit(mymodule_exit);

MODULE_LICENSE("GPL");

Save the file and edit the Makefile in the same directory. Add this line:

obj-m += mymodule.o

Compile your module:

# make -C [top directory of your kernel source] SUBDIRS=$PWD modules

Load the module. Depending on your kernel version, do that with either:

# insmod ./mymodule.o

Or:

# insmod ./mymodule.ko

And check to see if your message printed out:

# dmesg | tail

You should see this at the end of the output:

My module worked!

Now remove the kernel module:

# rmmod mymodule

Check the output of dmesg again, you should see:

Unloading my module.

You just wrote and ran a new kernel module! Congratulations! The module/kernel interface

Now, let's do some more interesting things with your module. One of the key things to realize is that modules can only "see" functions and variables that the kernel deliberately makes visible to the modules. First, let's try to do things the wrong way.

Edit the file kernel/printk.c and add this line after all the included files and near the other global variable declarations (but outside all functions):

int my_variable = 0;

Now recompile your kernel and reboot into your new kernel. Next, add this to the beginning of your module's mymodule_init function, before the other code:

extern int my_variable;
printk ("my_variable is %d\n", my_variable);
my_variable++;

Save your changes and recompile your module:

# make -C path/to/kernel/src SUBDIRS=$PWD modules

And load the module (this will fail):

# insmod ./mymodule.ko

Loading your module should fail with the message:

insmod: error inserting './mymodule.ko': -1 Unknown symbol in module

What this is saying is that the kernel is not allowing modules to see that variable. When the module loads, it has to resolve all it's external references, like function names or variable names. If it can't find all of it's unresolved names in the list of symbols that the kernel exports, then the module can't write to that variable or call that function. The variable my_variable has space allocated for it somewhere in the kernel, but the module can't figure out where.

To fix this, we're going to add my_variable to the list of symbols that the kernel exports. Many kernel directories have a file specifically for exporting symbols defined in that directory. Bring up the file kernel/printk.c again and add this line after the declaration of your variable:

EXPORT_SYMBOL_NOVERS(my_variable);

Recompile and reboot into your new kernel. Now try to load your module again:

# insmod ./mymodule.ko

This time, when you check dmesg, you should see:

my_variable is 0
My module worked!

Reload your module:

# rmmod mymodule && insmod ./mymodule.ko

Now you should see:

Unloading my module.
my_variable is 1    
My module worked!

Each time you reload the module, my_variable should increase by one. You are reading and writing to a variable which is defined in the main kernel. Your module can access any variable or function in the main kernel, as long as it is explicitly exported via the EXPORT_SYMBOL() declaration. For example, the function printk() is defined in the kernel and exported in the file kernel/printk.c.

A simple loadable kernel module is a fun way to explore the kernel. For example, you can use a module to turn a printk on or off, by defining a variable do_print in the kernel which is initially set to 0. Then make all your printk's dependent on "do_print":

if (do_print)
  printk ("Big long obnoxious message\n");

And turn on do_print only when your module is loaded. You can add a function defined in your module to the list of functions that are called when the kernel receives a certain interrupt (use cat /proc/interrupts to find out what interrupts are in use). The function request_irq() adds your function to the list of handlers for a selected irq line, which you can use to print out a message each time you receive an interrupt on that line. You can investigate the current value of any exported variable by loading a module that reads that value and immediately exits (returns a non-zero value from the module_init() function). The variable jiffies, which increments every 1/100th of a second (on most platforms), is a good candidate for this kind of module.

Play with your new kernel module - modules are fun!

OS: Membuat Kernel Modul

Contents

Apakah kernel mendukung modul?

Rangka Kernel Modul Sederhana

Referensi

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