Kernel Module Cross Compilation Notes

🧭 Overview

External kernel module development requires a matching build environment with the target kernel.

Matching includes:

• Kernel version (uname -r)
• Configuration (.config)
• Symbol version (Module.symvers)

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🏗️ Two Worlds in Cross Compilation

Type	Runs on	Purpose
Host tools	x86 (WSL)	Build system (fixdep, modpost)
Target code	ARM (Pi)	Kernel module (.ko)

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⚙️ Kernel Release Matching

Kernel release format:

<version><LOCALVERSION>

Example:

6.12.47+rpt-rpi-2712

Key Config

CONFIG_LOCALVERSION
CONFIG_LOCALVERSION_AUTO

Remove extra '+'

make LOCALVERSION= kernelrelease

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🔧 Build Preparation

make prepare
make modules_prepare

Purpose:

• Generate headers
• Build host tools
• Prepare external module interface

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🧩 Module.symvers

What it is

• Symbol version database
• Maps exported kernel symbols

Why it matters

Used during:

modpost

Without it:

• Undefined symbols
• Version mismatch
• Module load failure

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❗ Symbol Version Mismatch

Example error:

disagrees about version of symbol _printk

Cause

• Kernel and module built with different symbol versions

Fix

• Use exact Module.symvers from target kernel

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🔍 vermagic

Check with:

modinfo hello.ko

Example:

vermagic: 6.12.47+rpt-rpi-2712 SMP preempt mod_unload

Note

Matching vermagic does NOT guarantee success.

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⚠️ Common Errors

Exec format error

Cause:

• Running ARM binary on x86

Fix:

make prepare

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Invalid module format

Cause:

• Kernel mismatch
• vermagic mismatch

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Unknown symbol

Cause:

• Missing or incorrect Module.symvers

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🧪 Debugging Workflow

insmod hello.ko
dmesg | tail

Useful:

dmesg -w

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🧭 Best Practice

• Always use target kernel config
• Always align Module.symvers
• Avoid mixing different kernel builds
• Do not rely only on version string

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🎯 Summary

A working kernel module requires matching:

• Kernel version
• Configuration
• Symbol version

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