debugfs

In this program, you are going to learn

How to debug the kernel code ?

How to use below APIs ?

Topics in this section,

Explanation of program part by part

List of headers

Module macros

Initialize variables and functions

Module init function

Module exit function

debugfs APIs

mention init and exit function

See the full program below

Makefile

Compile and Load

Summary of debugfs APIs

Explanation of program part by part

Here is the explanation of the source code.

List of headers

Add the list of headers to refer the APIs used in the source code.

#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kdev_t.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/slab.h>
#include <linux/uaccess.h>
#include <linux/sysfs.h>
#include <linux/kobject.h>
#include <linux/err.h>

Modules macros

Add the module macros which contains the information about macros such as author, description and license.

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux_usr");
MODULE_DESCRIPTION("Simple Linux Device Driver - debugfs");

Initialize function and variables

Initialize the functions and the variables which are used in the source code.

u64 intvalue, hexvalue;

struct dentry *dirret, *fileret, *u64int, *u64hex;
char ker_buf[len];
int filevalue;

Module init functions

Add the module init function to execute once when the module is loaded.

static int __init init_debug(void)
{
    /* create a directory by the name dell in /sys/kernel/debugfs */
    dirret = debugfs_create_dir("dell", NULL);

    /* create a file in the above directory. This requires read and write file operations */
    fileret = debugfs_create_file("text", 0004, dirret, &filevalue, &fops_debug);

    /* create a file which takes in a int(64) value */
    u64int = debugfs_create_u64("number", 0004, dirret, &intvalue);
    if (!u64int) {
        pr_err("error creating int file");
        return -ENODEV;
    }
    /* takes a hex decimal value */
    u64hex = debugfs_create_x64("hexnum", 0004, dirret, &hexvalue);
    if (!u64hex) {
        pr_err("error creating hex file");
        return -ENODEV;
    }
    pr_info("Device Driver Loaded..\n");
    return 0;
}

To be able to use debugfs, we start by creating a directory within /sys/kernel/debug, which is an ideal way to start. The rest of the files can be placed within this directory.

dirret = debugfs_create_dir("dell", NULL);

If you need to create a single file within debugfs, you can the debugfs_create_file.
To create a file in the above directory. This requires read and write file operation

fileret = debugfs_create_file("text", 0004, dirret, &filevalue, &fops_debug);

To create a file which takes in a int(64) value

u64int = debugfs_create_u64("number", 0004, dirret, &intvalue)

It takes a hex decimal value

u64hex = debugfs_create_x64("hexnum", 0004, dirret, &hexvalue

Module exit function

Add the module exit function which is executed when the module is unloaded from the kernel.

static void __exit exit_debug(void)
{
    debugfs_remove_recursive(dirret);
    pr_info("Device Driver Unloaded..\n");
}

debugfs_remove_recursive used to cleanup all the files and directories created.

debugfs_remove_recursive(dirret);

Add debugfs APIs

Add debugfs APIs

static ssize_t myreader(struct file *fp, char __user *user_buffer, size_t count, loff_t *position)
{
    return simple_read_from_buffer(user_buffer, count, position, ker_buf, len);
}

/* write file operation */
static ssize_t mywriter(struct file *fp, const char __user *user_buffer, size_t count, loff_t *position)
{
    if (count > len)
        return -EINVAL;

    return simple_write_to_buffer(ker_buf, len, position, user_buffer, count);
}

static const struct file_operations fops_debug = {
    .read = myreader,
    .write = mywriter,
};

Mention the module init and exit function

Add the mention init and exit function which is executed the module is loaded and unloaded.

module_init(init_debug);
module_exit(exit_debug);

See the full program below

driver_debugfs.c

#include <linux/init.h>
#include <linux/module.h>
#include <linux/debugfs.h> /*this is for DebugFS libraries*/
#include <linux/fs.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Linux_usr");
MODULE_DESCRIPTION("sample Device Driver - debugfs");

#define len 200

u64 intvalue, hexvalue;

struct dentry *dirret, *fileret, *u64int, *u64hex;
char ker_buf[len];
int filevalue;

/* read file operation */
static ssize_t myreader(struct file *fp, char __user *user_buffer, size_t count, loff_t *position)
{
	return simple_read_from_buffer(user_buffer, count, position, ker_buf, len);
}

/* write file operation */
static ssize_t mywriter(struct file *fp, const char __user *user_buffer, size_t count, loff_t *position)
{
	if (count > len)
		return -EINVAL;

	return simple_write_to_buffer(ker_buf, len, position, user_buffer, count);
}

static const struct file_operations fops_debug = {
	.read = myreader,
	.write = mywriter,
};

static int __init init_debug(void)
{
	/* create a directory by the name dell in /sys/kernel/debugfs */
	dirret = debugfs_create_dir("dell", NULL);

	/* create a file in the above directory. This requires read and write file operations */
	fileret = debugfs_create_file("text", 0004, dirret, &filevalue, &fops_debug);

	/* create a file which takes in a int(64) value */
	u64int = debugfs_create_u64("number", 0004, dirret, &intvalue);
	if (!u64int) {
		pr_err("error creating int file");
		return -ENODEV;
	}
	/* takes a hex decimal value */
	u64hex = debugfs_create_x64("hexnum", 0004, dirret, &hexvalue);
	if (!u64hex) {
		pr_err("error creating hex file");
		return -ENODEV;
	}
	pr_info("Device Driver Loaded..\n");
	return 0;
}

static void __exit exit_debug(void)
{
	debugfs_remove_recursive(dirret);
	pr_info("Device Driver Unloaded..\n");
}

module_init(init_debug);
module_exit(exit_debug);

Makefile

obj-m += driver_debugfs.o
all:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
	make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Compile and Load

Run Make to compile the module.

$make
make -C /lib/modules/5.4.0-150-generic/build M=$HOME/kernel_driver_debugfs modules
make[1]: Entering directory '/usr/src/linux-headers-5.4.0-150-generic'
  CC [M]  $HOME/kernel_driver_debugfs/driver_debugfs.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC [M]  $HOME/kernel_driver_debugfs/driver_debugfs.mod.o
  LD [M]  $HOME/kernel_driver_debugfs/driver_debugfs.ko
make[1]: Leaving directory '/usr/src/linux-headers-5.4.0-150-generic'

Run ls to check if driver_debugfs.ko is generated or not.

$ ls -l
total 36
-rw-rw-r-- 1 test test  154 Feb 26 13:18 Makefile
-rw-rw-r-- 1 test test   47 Feb 26 13:18 modules.order
-rw-rw-r-- 1 test test    0 Feb 26 13:18 Module.symvers
-rw-rw-r-- 1 test test  820 Feb 26 13:18 driver_debugfs.c
-rw-rw-r-- 1 test test 5880 Feb 26 13:18 driver_debugfs.ko
-rw-rw-r-- 1 test test   47 Feb 26 13:18 driver_debugfs.mod
-rw-rw-r-- 1 test test  919 Feb 26 13:18 driver_debugfs.mod.c
-rw-rw-r-- 1 test test 3448 Feb 26 13:18 driver_debugfs.mod.o
-rw-rw-r-- 1 test test 3320 Feb 26 13:18 driver_debugfs.o

Run insmod to load the module.

$ sudo insmod ./driver_debugfs.ko

Once the module is loaded into kernel check for the files created.

$ ls -l /sys/kernel/debug/dell/
total 0
-rw-r--r-- 1 root root 0 Jul 14 20:08 hexnum
-rw-r--r-- 1 root root 0 Jul 14 20:08 number
-rw-r--r-- 1 root root 0 Jul 14 20:08 text

Run rmmod to unload the module.

$ sudo rmmod driver_debugfs

Check dmesg and we can clearly see the read and write operations are executed.

[ 9523.186611] Device Driver Loaded..
[ 9534.912912] Device Driver Unloaded..

Summary

API	Learning
debugfs_create_dir	To create a directory within /sys/kernel/debug
debugfs_create_file	To create a file in the above directory
debugfs_create_u64	To create a file which takes in a int(64) value
debugfs_remove_recursive	To create a file which takes hex decimal value