Linux Kernel Modules: A Beginner’s Guide

The Linux kernel is the core of any Linux operating system, managing hardware resources and providing essential services. One of its most powerful features is its modular design, which allows for dynamic loading and unloading of kernel modules. Let’s explore what kernel modules are and how they work.

Table of Contents

• What Are Kernel Modules?
• Why Use Kernel Modules?
• Basic Module Commands
  • Listing Loaded Modules
  • Getting Module Information
  • Loading a Module
  • Unloading a Module
• Module Location
• Module Configuration
  • Loading Modules at Boot
  • Blacklisting Modules
• Module Parameters
  • Viewing Parameters
  • Setting Parameters at Load Time
  • Setting Persistent Parameters
• Common Issues and Troubleshooting
  • Module Dependencies
  • Missing Module
  • Module Loading Errors
• Best Practices
• Module Security
• Next Steps

What Are Kernel Modules?

Kernel modules are pieces of code that can be loaded and unloaded into the kernel on demand. They extend the functionality of the kernel without requiring the system to be rebooted. Think of them as plugins for your kernel that can be inserted or removed as needed.

Why Use Kernel Modules?

Modular design offers several advantages:

• Dynamic Loading: Add functionality without rebooting
• Smaller Kernel Size: Only load what you need
• Resource Efficiency: Unload modules when not in use
• Flexibility: Easily switch between different drivers
• Development: Test new kernel code without rebuilding

Basic Module Commands

Let’s look at the essential commands for working with kernel modules:

Listing Loaded Modules

lsmod

This command shows all currently loaded modules. The output includes:

• Module name
• Size in memory
• Number of uses
• Dependencies

Getting Module Information

modinfo module_name

This provides detailed information about a specific module:

• Author
• Description
• Parameters
• Dependencies
• File location

Loading a Module

sudo modprobe module_name

This command loads a module and its dependencies. Always use modprobe instead of insmod as it handles dependencies automatically.

Unloading a Module

sudo modprobe -r module_name

This safely removes a module and its unused dependencies.

Module Location

Kernel modules are typically stored in:

/lib/modules/$(uname -r)/kernel
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They’re organized into subdirectories based on their function:

• drivers/
• fs/ (filesystem modules)
• net/ (networking modules)
• sound/ (audio modules)

Module Configuration

Loading Modules at Boot

To load modules automatically at boot:

1. Edit /etc/modules:

sudo nano /etc/modules

1. Add module names, one per line:

module1
module2

Blacklisting Modules

To prevent modules from loading:

1. Create a file in /etc/modprobe.d/:

sudo nano /etc/modprobe.d/blacklist.conf

1. Add blacklist entries:

blacklist module_name

Module Parameters

Many modules accept parameters to customize their behavior:

Viewing Parameters

modinfo -p module_name

Setting Parameters at Load Time

sudo modprobe module_name parameter=value

Setting Persistent Parameters

1. Create a file in /etc/modprobe.d/:

sudo nano /etc/modprobe.d/module_name.conf

1. Add options:

options module_name parameter=value

Common Issues and Troubleshooting

Module Dependencies

If a module won’t load, check dependencies:

sudo depmod -a

This updates the module dependency database.

Missing Module

If modprobe can’t find a module:

1. Verify the kernel headers are installed:

sudo apt install linux-headers-$(uname -r)
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1. Update the module database:

sudo depmod -a

Module Loading Errors

Check system logs for detailed error messages:

dmesg | grep module_name

Best Practices

1. Always use modprobe instead of insmod/rmmod
2. Check module compatibility before loading
3. Monitor system logs when loading new modules
4. Keep kernel and modules updated
5. Document any custom module configurations

Module Security

Kernel modules run with full system privileges, so:

• Only load modules from trusted sources
• Regularly audit loaded modules
• Use module signing for additional security
• Implement proper access controls

For more information about Linux system security, check out our guide on Securing Your Linux Server.

Next Steps

Now that you understand kernel modules, you might want to:

• Learn about building custom kernel modules
• Explore system hardening with module controls
• Study kernel module development

Kernel modules are a fundamental part of Linux system administration. Understanding how to manage them effectively will help you maintain a more efficient and secure system. Start by experimenting with safe modules and gradually work your way up to more complex module management tasks.

Remember to always exercise caution when working with kernel modules, as they operate at the highest privilege level and can affect system stability if not managed properly.