How to Compile a Kernel with Custom Patches on FreeBSD Operating System

Introduction

FreeBSD is a powerful, open-source Unix-like operating system known for its stability, performance, and advanced networking capabilities. One of its key strengths is the ability to customize the kernel to suit specific needs, whether for performance optimization, hardware support, or security enhancements. Compiling a custom kernel with patches allows users to modify kernel behavior, apply security fixes, or add experimental features not yet available in the default kernel.

This guide provides a step-by-step process for compiling a FreeBSD kernel with custom patches. We will cover prerequisites, obtaining the kernel source, applying patches, configuring the kernel, and compiling and installing the modified kernel.

Prerequisites

Before proceeding, ensure you have the following:

1. A FreeBSD System – This guide assumes you are running FreeBSD 13 or later, though the process is similar for older versions.

2. Root Access – Kernel compilation requires administrative privileges.

3. Sufficient Disk Space – The kernel source and build files require several gigabytes of free space.

4. Basic Command-Line Knowledge – Familiarity with shell commands and text editors (e.g., vim, nano) is necessary.

5. Installed Development Tools – Ensure the following packages are installed:

   pkg install git svn patch
   

Step 1: Obtain the FreeBSD Kernel Source

FreeBSD provides multiple ways to access the kernel source:

Option 1: Using git (Recommended)

The FreeBSD project maintains a Git repository for the kernel source.

1. Clone the FreeBSD source repository:

   git clone --branch main --depth 1 https://git.freebsd.org/src.git /usr/src
   

   • --branch main fetches the latest stable branch (adjust if using a release branch like stable/13).
   • --depth 1 reduces download size by fetching only the latest revision.

2. Verify the source tree:

   cd /usr/src
   ls
   

   You should see directories like sys/ (kernel source) and stand/ (bootloader).

Option 2: Using svn

If you prefer Subversion:

svn checkout https://svn.freebsd.org/base/stable/13 /usr/src

(Replace stable/13 with your desired branch.)

Option 3: Using freebsd-update (For Release Versions)

If you only need the source matching your installed release:

freebsd-update fetch
freebsd-update install

Step 2: Apply Custom Patches

Before modifying the kernel, identify the patches you want to apply. These could be:

• Security fixes from FreeBSD’s security advisories.
• Performance tweaks from community forums.
• Experimental features from developer mailing lists.

Applying a Patch File

1. Download the patch (e.g., custom_kernel.patch).

2. Apply it using the patch command:

   cd /usr/src
   patch -p1 < /path/to/custom_kernel.patch
   

   • -p1 strips the first directory level from paths in the patch file.

Manual Modifications

If you’re making changes manually:

1. Navigate to the relevant kernel source directory (e.g., /usr/src/sys/amd64/conf for AMD64 configurations).

2. Edit files with a text editor:

   vim /usr/src/sys/kern/sched_ule.c
   

   (Example: Modify the ULE scheduler for different task prioritization.)

Step 3: Configure the Kernel

FreeBSD uses kernel configuration files to define which features and drivers are included.

Option 1: Use the Default Configuration

1. Copy the default configuration for your architecture:

   cd /usr/src/sys/amd64/conf
   cp GENERIC MYKERNEL
   

   (Replace amd64 with your architecture, e.g., arm64, i386.)

Option 2: Customize the Configuration

1. Edit the new configuration file (MYKERNEL):

   vim MYKERNEL
   

   • Add or remove device drivers (e.g., device sound).
   • Enable/disable kernel options (e.g., options TCP_DEBUG).

Example modifications:

# Enable additional debugging
options KDB
options DDB
options INVARIANTS

# Disable unused drivers
# nodevice snd_hda  # Example: Disable HD Audio driver

Step 4: Compile the Kernel

Once the configuration is ready, compile the kernel:

1. Navigate to /usr/src:

   cd /usr/src
   

2. Build the kernel (replace MYKERNEL with your config name):

   make -j$(sysctl -n hw.ncpu) buildkernel KERNCONF=MYKERNEL
   

   • -j$(sysctl -n hw.ncpu) speeds up compilation by using all CPU cores.

3. Resolve any compilation errors:

   • Missing headers? Ensure /usr/src is complete.
   • Patch conflicts? Manually resolve them by editing affected files.

Step 5: Install the New Kernel

After successful compilation, install the kernel:

1. Run:

   make installkernel KERNCONF=MYKERNEL
   

   This copies the new kernel (/boot/kernel/kernel) and modules.

2. Verify the installation:

   uname -a
   

   (The changes will appear after reboot.)

Step 6: Reboot and Test

1. Reboot the system:

   shutdown -r now
   

2. Confirm the new kernel is running:

   uname -a
   

3. Test functionality:

   • Check if patches are applied (e.g., dmesg | grep "Your_Patch").
   • Verify hardware/driver changes.

Troubleshooting

Kernel Panics on Boot

1. Boot the last working kernel from the loader menu (select kernel.old).
2. Revert changes and recompile.

Patch Application Failures

• Use patch --dry-run to test before applying.
• Manually merge conflicts using diff and vimdiff.

Performance Issues

• Revert to GENERIC kernel if instability occurs.
• Consult FreeBSD forums/mailing lists for patch-specific issues.

Conclusion

Compiling a custom kernel with patches on FreeBSD allows for deep system customization, improved performance, and enhanced security. By following this guide, you can safely modify, compile, and install a tailored kernel suited to your needs. Always test changes in a non-production environment before deployment.

For further reading, consult the FreeBSD Handbook and developer resources.