Day 7 — Cross Compile Kernel Module¶

📌 What I did¶

Set up cross-compilation environment on Ubuntu 24.04 (WSL)
Downloaded Raspberry Pi kernel source (matching 6.12.47)
Applied running kernel config from Raspberry Pi
Prepared kernel build tree for external module
Built kernel module (hello.ko) on WSL
Deployed module to Raspberry Pi via scp
Successfully loaded module using insmod

⚠️ Issues Encountered¶

1. fixdep: Exec format error¶

/bin/sh: 1: scripts/basic/fixdep: Exec format error

Cause:

Kernel build tools were copied from Raspberry Pi (ARM)
Attempted to execute on WSL (x86)

Fix:

make prepare
make modules_prepare

👉 Rebuilt host tools for x86

2. kernelrelease mismatch¶

Initial result:

6.12.47-v8-16k+

Target:

6.12.47+rpt-rpi-2712

Cause:

CONFIG_LOCALVERSION mismatch
Dirty git tree adds +

Fix:

scripts/config --set-str LOCALVERSION "+rpt-rpi-2712"
scripts/config --disable LOCALVERSION_AUTO
make LOCALVERSION= kernelrelease

3. Module load failure (Invalid parameters)¶

insmod: ERROR: could not insert module hello.ko: Invalid parameters

Real cause from dmesg:

disagrees about version of symbol _printk
Unknown symbol _printk

Cause:

Symbol version mismatch
Module.symvers not aligned with target kernel

Fix:

Copied Module.symvers from Raspberry Pi

scp pi:/usr/src/linux-headers-6.12.47+rpt-rpi-2712/Module.symvers .

✅ Result¶

Successfully loaded module:

[ 6663.120808] Hello Kernel Module Loaded

Verified:

lsmod | grep hello

Unloaded:

sudo rmmod hello

🧠 Key Takeaways¶

Cross-compilation includes host tools + target binaries
Kernel version string must match exactly (uname -r)
LOCALVERSION affects module compatibility
+ indicates dirty tree and can break matching
Module.symvers is required for symbol version alignment
vermagic matching is necessary but not sufficient

Module Setup & Cleanup Scripts¶

setup_mydev.sh¶

#!/bin/bash
#
# @file setup_mydev.sh
# @brief Load mydev kernel module and create device node
#

# Exit immediately if any command fails (except those explicitly handled)
set -e

MODULE="mydev"
DEVICE="/dev/mydev"

echo "=== Loading module ==="

# Try to remove existing module
# - '2>/dev/null' suppresses error output (e.g., module not loaded)
# - '|| true' ensures script continues even if rmmod fails
#   (important because set -e would otherwise stop execution)
sudo rmmod $MODULE 2>/dev/null || true

# Insert module
sudo insmod ./${MODULE}.ko

echo "=== Getting major number ==="

# Extract major number from /proc/devices
#
# Step-by-step:
# 1. grep $MODULE /proc/devices
#    → find the line containing "mydev"
#      e.g., "509 mydev"
#
# 2. awk '{print $1}'
#    → extract first column (major number)
#
# 3. $( ... )
#    → command substitution: store output into variable
MAJOR=$(grep $MODULE /proc/devices | awk '{print $1}')

# Check if MAJOR is empty
# -z means "string length is zero"
if [ -z "$MAJOR" ]; then
    echo "ERROR: Failed to get major number"
    exit 1
fi

echo "Major number: $MAJOR"

echo "=== Creating device node ==="

# If device node already exists, remove it
# This prevents mismatch if major number changed
if [ -e $DEVICE ]; then
    sudo rm -f $DEVICE
fi

# Create character device node
# Syntax: mknod <path> <type> <major> <minor>
# 'c' = character device
sudo mknod $DEVICE c $MAJOR 0

# Set permissions to rw-rw-rw-
# This allows non-root users to read/write the device (for testing)
sudo chmod 666 $DEVICE

echo "=== Device ready ==="
ls -l $DEVICE

echo "=== dmesg ==="

# Show last 10 kernel log lines
dmesg | tail -n 10

# End of File

cleanup_mydev.sh¶

#!/bin/bash
#
# @file cleanup_mydev.sh
# @brief Remove mydev kernel module and device node
#

# Exit immediately if any command fails
set -e

MODULE="mydev"
DEVICE="/dev/mydev"

echo "=== Removing module ==="

sudo rmmod $MODULE

echo "=== Removing device node ==="

# Check if device node exists
# -e means "file exists"
if [ -e $DEVICE ]; then
    sudo rm -f $DEVICE
fi

echo "=== Cleanup done ==="

# End of File

Test `mydev` using Python Script¶

test_mydev.py¶

#!/usr/bin/env python3
"""
@file test_mydev.py
@brief Test script for /dev/mydev character device

Usage:
    python3 test_mydev.py basic
    python3 test_mydev.py offset
    python3 test_mydev.py boundary
    python3 test_mydev.py busy
    python3 test_mydev.py stress
"""

import os
import sys
import time
import threading

DEV_PATH = "/dev/mydev"


def check_device():
    """Ensure device exists"""
    if not os.path.exists(DEV_PATH):
        print(f"ERROR: {DEV_PATH} not found")
        sys.exit(1)


# =========================================================
# Basic Test
# =========================================================
def test_basic():
    print("=== Basic Test ===")

    data = b"hello kernel"

    with open(DEV_PATH, "wb") as f:
        f.write(data)

    with open(DEV_PATH, "rb") as f:
        read_data = f.read()

    print(f"Write: {data}")
    print(f"Read : {read_data}")

    print("PASS" if read_data == data else "FAIL")


# =========================================================
# Offset Test
# =========================================================
def test_offset():
    print("=== Offset Test ===")

    data = b"abcdef"

    with open(DEV_PATH, "wb") as f:
        f.write(data)

    f = open(DEV_PATH, "rb")
    r1 = f.read(3)
    r2 = f.read(3)
    r3 = f.read(3)
    f.close()

    print(f"Read1: {r1}")
    print(f"Read2: {r2}")
    print(f"Read3: {r3}")

    if r1 == b"abc" and r2 == b"def" and r3 == b"":
        print("PASS")
    else:
        print("FAIL")


# =========================================================
# Boundary Test
# =========================================================
def test_boundary():
    print("=== Boundary Test ===")

    data = b"A" * 200  # larger than BUFFER_SIZE (128)

    with open(DEV_PATH, "wb") as f:
        f.write(data)

    with open(DEV_PATH, "rb") as f:
        read_data = f.read()

    print(f"Written: {len(data)} bytes")
    print(f"Read   : {len(read_data)} bytes")

    if len(read_data) <= 128:
        print("PASS (correct truncation)")
    else:
        print("FAIL (overflow detected)")


# =========================================================
# Busy Test
# =========================================================
def test_busy():
    print("=== Busy Test ===")

    fd1 = os.open(DEV_PATH, os.O_RDWR)
    print("fd1 opened")

    try:
        fd2 = os.open(DEV_PATH, os.O_RDWR)
        print("FAIL: second open should not succeed")
        os.close(fd2)
    except OSError as e:
        print(f"PASS: second open failed as expected ({e})")

    time.sleep(2)
    os.close(fd1)
    print("fd1 closed")


# =========================================================
# Stress Test
# =========================================================
def writer(stop_event, errors):
    while not stop_event.is_set():
        try:
            with open(DEV_PATH, "wb") as f:
                f.write(b"stress-data")
        except Exception:
            errors.append("write")


def reader(stop_event, errors):
    while not stop_event.is_set():
        try:
            with open(DEV_PATH, "rb") as f:
                f.read()
        except Exception:
            errors.append("read")


def test_stress():
    print("=== Stress Test (5 sec) ===")

    stop_event = threading.Event()
    errors = []

    threads = []

    for _ in range(2):
        threads.append(threading.Thread(target=writer, args=(stop_event, errors)))
        threads.append(threading.Thread(target=reader, args=(stop_event, errors)))

    for t in threads:
        t.start()

    time.sleep(5)
    stop_event.set()

    for t in threads:
        t.join()

    print("Stress test finished")

    if errors:
        print(f"Errors occurred: {len(errors)}")
    else:
        print("PASS (no userspace errors)")

    print("👉 Check dmesg for kernel issues")


# =========================================================
# Main
# =========================================================
def main():
    check_device()

    if len(sys.argv) != 2:
        print("Usage: python3 test_mydev.py [basic|offset|boundary|busy|stress]")
        sys.exit(1)

    cmd = sys.argv[1]

    if cmd == "basic":
        test_basic()
    elif cmd == "offset":
        test_offset()
    elif cmd == "boundary":
        test_boundary()
    elif cmd == "busy":
        test_busy()
    elif cmd == "stress":
        test_stress()
    else:
        print("Unknown command")


if __name__ == "__main__":
    main()

# End of File

test_run.sh¶

sh setup_mydev.sh

chmod +x test_mydev.py

python3 test_mydev.py basic
python3 test_mydev.py offset
python3 test_mydev.py boundary
python3 test_mydev.py busy
python3 test_mydev.py stress

sh cleanup_mydev.sh

🚀 Next Steps¶

Convert hello module to character device driver
Add module parameters
Learn modprobe and depmod
Use dmesg -w for real-time debugging

Day 7 — Cross Compile Kernel Module¶

📌 What I did¶

⚠️ Issues Encountered¶

1. fixdep: Exec format error¶

2. kernelrelease mismatch¶

3. Module load failure (Invalid parameters)¶

✅ Result¶

🧠 Key Takeaways¶

Module Setup & Cleanup Scripts¶

setup_mydev.sh¶

cleanup_mydev.sh¶

Test mydev using Python Script¶

test_mydev.py¶

test_run.sh¶

🚀 Next Steps¶

Test `mydev` using Python Script¶