Day05 - Kernel Module Hello World

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Objective

• Build a Linux kernel module
• Load and unload module
• Observe kernel behavior via logs

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Environment

• Board: Raspberry Pi 5
• OS: Debian (Raspberry Pi OS Lite)
• Architecture: arm64

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Step 1 — Create Source Code

File: hello.c

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

static int __init hello_init(void)
{
printk(KERN_INFO "Hello, Kernel Module Loaded!\n");
return 0;
}

static void __exit hello_exit(void)
{
printk(KERN_INFO "Goodbye, Kernel Module Unloaded!\n");
}

module_init(hello_init);
module_exit(hello_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Chia-Wei Chien");
MODULE_DESCRIPTION("Simple Hello World Kernel Module");

/* End of File */

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Step 2 — Create Makefile

obj-m += hello.o

KDIR := /lib/modules/$(shell uname -r)/build
PWD  := $(shell pwd)

all:
$(MAKE) -C $(KDIR) M=$(PWD) modules

clean:
$(MAKE) -C $(KDIR) M=$(PWD) clean

# End of File

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Step 3 — Build Module

make

Output:

• hello.ko

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Step 4 — Insert Module

sudo insmod hello.ko

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Step 5 — Check Kernel Log

dmesg | tail

Expected:

Hello, Kernel Module Loaded!

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Step 6 — Remove Module

sudo rmmod hello

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Step 7 — Verify Removal

dmesg | tail

Expected:

Goodbye, Kernel Module Unloaded!

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Step 8 — Inspect Module

lsmod
modinfo hello.ko

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Observations

• Module is dynamically inserted into kernel
• printk outputs to kernel log
• No reboot required

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Key Learnings

• Kernel modules run in kernel space
• .ko is a loadable object
• insmod / rmmod control module lifecycle
• Debugging relies on dmesg

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Common Issues

1. Missing kernel headers

Error:

No such file or directory: /lib/modules/.../build

Solution:

sudo apt install linux-headers-$(uname -r)

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2. Permission denied

Use:

sudo insmod hello.ko

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Next Step

• Implement character device driver
• Add read/write interface
• Connect user space with kernel

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