• GPL - GNU Public License v2 or later
• GPL v2 - GNU Public License v2
• GPL and additional rights - GNU Public License v2 rights and more
• Dual BSD/GPL - GNU Public License v2  or BSD license choice
• Dual MIT/GPL - GNU Public License v2 or MIT license choice
• Dual MPL/GPL - GNU Public License v2 or Mozilla license choice

Como vocês podem notar, há componentes que podem ser definidos com licenças duplas, contudo quando executado no Linux apenas a GPL é relevante. Algumas razões para definir a licença:

1. O modinfo pode mostrar informações para usuários que desejam avaliar as licenças dos módulos sua instalação.
2. A comunidade pode ignorar relatórios de bugs dos módulos proprietários.
3. Os fabricantes podem fazer o mesmo com suas próprias políticas.

Para inserir a licença, basta colocar no seu código-fonte a macro  ”MODULE_LICENCE”. Exemplo:

  MODULE_LICENCE("GPL");

Lembre-se que alguns recursos do kernel são disponíveis apenas se seu código é livre.

Um exemplo é o sysfs (através da macro EXPORT_SYMBOL_GPL) que por questões de manutenção e consistência exige que você licencie seu módulo em alguma licença compatível com a GPL.

Now that I know that somebody at Academia reads my posts I’ll put code with a _little_ documentation information together. Today we will do a quick review about kernel threads or to be more precisely, a way to perform some operations in the background. Kernel threads are standard process that:

1. Exist solely in kernel-space.
2. Don’t have an address space.
3. Don’t context switch to user-space.
4. Are schedulable and preemptable as normal process.

And let’s see an example:

#include <linux/init.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/delay.h>

static int thread3(void *unused) {

	int count3 = 0;
	while (count3 < 1000) {
		msleep(100);
		printk("Thread 3: %d\n", count3++);
	}
	return 0;
} 

static int thread2(void *unused) {

	int count2 = 0;
	while (count2 < 1000) {
		msleep(10);
		printk("Thread 2: %d\n", count2++);
	}
	return 0;
}

static int __init threads_init(void)
{
	int count1;
	int ret1, ret2;

	count1 = 0;
	ret1 = kernel_thread(thread2, NULL, CLONE_FS | CLONE_FILES | CLONE_SIGHAND | SIGCHLD );
	ret2 = kernel_thread(thread3, NULL, CLONE_FS | CLONE_FILES | CLONE_SIGHAND | SIGCHLD );

	printk("-- kernel thread: module init\n");
	printk("-- kernel thread: spawning thread 1 ret=%d\n", ret1);
	printk("-- kernel thread: spawning thread 2 ret=%d\n", ret2);	

	/*
	   Don't use "long" loops on init().
	   If you uncomment the snippet below the insmod will lock until while finishes
	*/ 

	/*
	while (count1 < 1000) {
		msleep(10);
		printk("Thread 1: %d\n", count1++);
     	}
	*/

    return 0;
}

static void __exit threads_exit(void)
{
    printk("-- kernel thread: module removed\n");
}

module_init(threads_init);
module_exit(threads_exit);

MODULE_AUTHOR("Tiago Maluta <maluta@unifei.edu.br");
MODULE_DESCRIPTION("kthreads examples");
MODULE_LICENSE("GPL");

Things from code that are found in docs…

• CLONE_FS means that parent and child share filesystem information.
• CLONE_FILES means that parent and child open files.
• CLONE_SIGHAND means that parent and child share signal handlers.

I use arbitrarily msleep() just to allow user “see” what is happening when dumping kernel ring buffer.

• procfs is a generic window into kernel internals
• sysfs is specific to device model

Information such as process descriptors and sysctls parameters belong to procfs and not sysfs. Note that: Sysfs is not a replacement for procfs.

Lets create two useless (in a practical way) that presents the “idea” behind these constructions.

1. Procfs

The snippet  below creates a /proc/coding allowing read or write some content.

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

/* for proc_dir_entry and create_proc_entry */
#include <linux/proc_fs.h>

/* For sprintf and snprintf */
#include <linux/string.h>

/* For copy_from_user */
#include <linux/uaccess.h>

static char internal_buffer[256];

int buf_read(char *buf, char **start, off_t offset, int count, int *eof, void *data)
{
	int len;
	len = snprintf(buf, count, "%s", internal_buffer);
	return len;
}

static int buf_write(struct file *file, const char *buf, unsigned long count, void *data)
{
	if(count > 255) /* to avoid overflowwwwwwwwww */
		count = 255;

	/* Copies data from user space to kernel space */
	copy_from_user(internal_buf, buf, count);

	/* inserting NULL to end the string */
	internal_buf[count] = '\0';
	return count;
}

int __init proc_init(void) {
	/* Simple */
	struct proc_dir_entry *de = create_proc_entry("coding", 0667, 0);

	/* Set pointers to our functions */
	de->read_proc = buf_read; /* reading */
	de->write_proc = buf_write; /* writing */ 

	/* We initialize our internal_buffer with some text. */
	sprintf(internal_buffer, "www.coding.com.br");
	return 0 ;
}

void __exit proc_cleanup(void) {

	/* We delete our entry */
	remove_proc_entry("coding", NULL);

}

module_init(proc_init);
module_exit(proc_cleanup);

MODULE_LICENSE("GPL");

2. Sysfs

This example only creates a /sys/class/<module-name> directory with nothing inside. This may not be clear now but remember when I show – in the next posts – some practical use with device node (/dev).

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

/* for sysfs class creation */
#include <linux/platform_device.h>

static struct class *sysfs_class;

int __init sysfs_init(void) {

	sysfs_class = class_create(THIS_MODULE, "sysfs" );
	return 0 ;
}

void __exit sysfs_cleanup(void) {

	/* We delete our entry */
	class_destroy(sysfs_class);
}

module_init(sysfs_init);
module_exit(sysfs_cleanup);

MODULE_LICENSE("GPL");

/* keyboard-dump.c
 *
 * This is a "notify chain" example that dumps keyboard on kernel message
 *
 *	(C) Copyright 2010, Tiago Maluta
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 */

#include <linux/notifier.h>
#include <linux/module.h>
#include <linux/keyboard.h>

static int debug = 1;

#define dbg(fmt, arg...)						\
	do {								\
		if (debug)						\
			printk (KERN_DEBUG "%s: %s: " fmt "\n",		\
				"test" , __FUNCTION__ , ## arg);	\
	} while (0)

int keyboard_event_handler(struct notifier_block *self,
                         unsigned long val,
			 void *data) 

{

	struct keyboard_notifier_param *param = data; 

	unsigned int value = param->value;

	if ((!param->down) && (value > 0xf000))
		printk("%c", KVAL(param->value));

	return NOTIFY_DONE ;
}

static struct notifier_block keyboard_notifier = {

	.notifier_call = keyboard_event_handler,
};

static int __init keyboard_init(void) 

{

	register_keyboard_notifier(&keyboard_notifier);

	dbg();

	return 0;
}

static void __exit keyboard_exit(void)

{
	unregister_keyboard_notifier(&keyboard_notifier);
	dbg();
}

module_init(keyboard_init);
module_exit(keyboard_exit);

MODULE_LICENSE("GPL");

<body bgcolor="#ffffff" text="#000000">
<pre>
<font color="#444444">/* keyboard-dump.c
 *
 * This is a &quot;notify chain&quot; example that dumps keyboard on kernel message
 *
 *	(C) Copyright 2010, Tiago Maluta
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 */</font>

<font color="#0000ff"><strong>#include <font color="#008000">&lt;linux/notifier.h&gt;</font></strong></font>

<font color="#0000ff"><strong>#include <font color="#008000">&lt;linux/module.h&gt;</font></strong></font>
<font color="#0000ff"><strong>#include <font color="#008000">&lt;linux/keyboard.h&gt;</font></strong></font>

<strong>static</strong> <strong>int</strong> <font color="#2040a0">debug</font> <font color="#4444ff">=</font> <font color="#ff0000">1</font><font color="#4444ff">;</font>

<font color="#0000ff"><strong>#define dbg(fmt, arg...)						\</strong></font>
	<strong>do</strong> <font color="#4444ff"><strong>{</strong></font>								\
		<strong>if</strong> <font color="#4444ff">(</font><font color="#2040a0">debug</font><font color="#4444ff">)</font>						\
			<font color="#2040a0">printk</font> <font color="#4444ff">(</font><font color="#2040a0">KERN_DEBUG</font> <font color="#008000">&quot;%s: %s: &quot;</font> <font color="#2040a0">fmt</font> <font color="#008000">&quot;<font color="#77dd77">\n</font>&quot;</font>,		\
				<font color="#008000">&quot;test&quot;</font> , <font color="#2040a0">__FUNCTION__</font> , ## <font color="#2040a0">arg</font><font color="#4444ff">)</font><font color="#4444ff">;</font>	\
	<font color="#4444ff"><strong>}</strong></font> <strong>while</strong> <font color="#4444ff">(</font><font color="#ff0000">0</font><font color="#4444ff">)</font>

<strong>int</strong> <font color="#2040a0">keyboard_event_handler</font><font color="#4444ff">(</font><strong>struct</strong> <font color="#2040a0">notifier_block</font> <font color="#4444ff">*</font><font color="#2040a0">self</font>,
                         <strong>unsigned</strong> <strong>long</strong> <font color="#2040a0">val</font>,
			 <strong>void</strong> <font color="#4444ff">*</font><font color="#2040a0">data</font><font color="#4444ff">)</font>

<font color="#4444ff"><strong>{</strong></font>

	<strong>struct</strong> <font color="#2040a0">keyboard_notifier_param</font> <font color="#4444ff">*</font><font color="#2040a0">param</font> <font color="#4444ff">=</font> <font color="#2040a0">data</font><font color="#4444ff">;</font> 

	<strong>unsigned</strong> <strong>int</strong> <font color="#2040a0">value</font> <font color="#4444ff">=</font> <font color="#2040a0">param</font><font color="#4444ff">-</font><font color="#4444ff">&gt;</font><font color="#2040a0">value</font><font color="#4444ff">;</font>

	<strong>if</strong> <font color="#4444ff">(</font><font color="#4444ff">(</font><font color="#4444ff">!</font><font color="#2040a0">param</font><font color="#4444ff">-</font><font color="#4444ff">&gt;</font><font color="#2040a0">down</font><font color="#4444ff">)</font> <font color="#4444ff">&amp;</font><font color="#4444ff">&amp;</font> <font color="#4444ff">(</font><font color="#2040a0">value</font> <font color="#4444ff">&gt;</font> <font color="#ff0000">0xf000</font><font color="#4444ff">)</font><font color="#4444ff">)</font>
		<font color="#2040a0">printk</font><font color="#4444ff">(</font><font color="#008000">&quot;%c&quot;</font>, <font color="#2040a0">KVAL</font><font color="#4444ff">(</font><font color="#2040a0">param</font><font color="#4444ff">-</font><font color="#4444ff">&gt;</font><font color="#2040a0">value</font><font color="#4444ff">)</font><font color="#4444ff">)</font><font color="#4444ff">;</font>

	<strong>return</strong> <font color="#2040a0">NOTIFY_DONE</font> <font color="#4444ff">;</font>
<font color="#4444ff"><strong>}</strong></font>

<strong>static</strong> <strong>struct</strong> <font color="#2040a0">notifier_block</font> <font color="#2040a0">keyboard_notifier</font> <font color="#4444ff">=</font> <font color="#4444ff"><strong>{</strong></font>

	.<font color="#2040a0">notifier_call</font> <font color="#4444ff">=</font> <font color="#2040a0">keyboard_event_handler</font>,
<font color="#4444ff"><strong>}</strong></font><font color="#4444ff">;</font>

<strong>static</strong> <strong>int</strong> <font color="#2040a0">__init</font> <font color="#2040a0">keyboard_init</font><font color="#4444ff">(</font><strong>void</strong><font color="#4444ff">)</font>

<font color="#4444ff"><strong>{</strong></font>

	<font color="#2040a0">register_keyboard_notifier</font><font color="#4444ff">(</font><font color="#4444ff">&amp;</font><font color="#2040a0">keyboard_notifier</font><font color="#4444ff">)</font><font color="#4444ff">;</font>
	<font color="#2040a0">dbg</font><font color="#4444ff">(</font><font color="#4444ff">)</font><font color="#4444ff">;</font>

	<strong>return</strong> <font color="#ff0000">0</font><font color="#4444ff">;</font>
<font color="#4444ff"><strong>}</strong></font>
<strong>static</strong> <strong>void</strong> <font color="#2040a0">__exit</font> <font color="#2040a0">keyboard_exit</font><font color="#4444ff">(</font><strong>void</strong><font color="#4444ff">)</font>

<font color="#4444ff"><strong>{</strong></font>
	<font color="#2040a0">unregister_keyboard_notifier</font><font color="#4444ff">(</font><font color="#4444ff">&amp;</font><font color="#2040a0">keyboard_notifier</font><font color="#4444ff">)</font><font color="#4444ff">;</font>
	<font color="#2040a0">dbg</font><font color="#4444ff">(</font><font color="#4444ff">)</font><font color="#4444ff">;</font>
<font color="#4444ff"><strong>}</strong></font>

<font color="#2040a0">module_init</font><font color="#4444ff">(</font><font color="#2040a0">keyboard_init</font><font color="#4444ff">)</font><font color="#4444ff">;</font>
<font color="#2040a0">module_exit</font><font color="#4444ff">(</font><font color="#2040a0">keyboard_exit</font><font color="#4444ff">)</font><font color="#4444ff">;</font>

<font color="#2040a0">MODULE_LICENSE</font><font color="#4444ff">(</font><font color="#008000">&quot;GPL&quot;</font><font color="#4444ff">)</font><font color="#4444ff">;</font>

</pre>

The output will be visible (i.e: dmesg) on kernel ring buffer.

If you like try don’t cut&paste the code above but use I gist repository here.

Um exemplo com o ext4:

./scripts/get_maintainer.pl -f  fs/ext4/

“Theodore Ts’o” <tytso@…>

Aneesh Kumar K.V <aneesh.kumar@…>

Eric Sandeen <sandeen@…>

linux-kernel@vger.kernel.org (lembre-se de se cadastrar e sempre de copiar a lista de email junto)

Um exemplo com o driver de video da Intel:

./scripts/get_maintainer.pl -f drivers/video/intelfb/

Sylvain Meyer <sylvain.meyer@…>

Andrew Morton <akpm@…>

Thomas Hilber <sparkie@…>

Stefan Husemann <shusemann@…>

Hannes Eder <hannes@…>

Paul Menzel <paulepanter@…>

linux-fbdev-devel@lists.sourceforge.net

linux-kernel@vger.kernel.org

Dar o retorno do uso de algum programa é ótimo para o desenvolvedor e garante a qualidade do sotfware.