Building a Minimalistic Linux Distribution

Overview

• Presenter: Victor Engelmann
• Objective: Build an extremely minimalistic Linux distribution (~12 MB) that provides a shell for basic file navigation and program execution.
• Tools: BusyBox and the Linux kernel.
• Key Steps: Building a kernel, creating a root partition containing BusyBox.

Key Components

1. Linux Kernel

   • Essential for any Linux system.
   • Provides instructions and drivers on startup.
   • Download from kernel.org (use version 5.x).
   • Steps to build:
     • Download and extract the kernel source code.
     • Configure with make defconfig and compile with make -j8.

2. BusyBox

   • Explanation: A single binary providing common Unix utilities.
   • Functionality: Compact replacement for a full suite of Unix utilities.
   • Download from busybox.net (version 1.34.1 recommended).
   • Steps to build:
     • Download and extract BusyBox source code.
     • Configure with make defconfig.
     • Modify configuration for a static build (use CONFIG_STATIC=y).
     • Compile using make -j8.

Building Process

1. Preparation

   • Create text files (busybox.sh and temp.sh) for step-by-step compilation.
   • Goal: Consolidate into a single script for automatic building.

2. Downloading and Extracting Sources

   • Use variables for versioning to simplify changes.
   • Create a src directory to download and extract files.

3. Kernel Compilation

   • Download and compile the kernel follows the process described above.
   • Use default configurations to simplify the setup.

4. BusyBox Compilation

   • Ensure a static build to avoid dependency on dynamic link libraries.
   • Update configuration using scripting tools for efficiency (sed for modifying .config).
   • For Manjaro systems, installed additional packages musl and kernel-headers-musl.
   • Use musl-gcc to compile BusyBox.

Combining Kernel and BusyBox

1. File Structure

   • Create directories: initrd, bin, dev, proc, sys.
   • Copy BusyBox binary to bin.
   • Create symbolic links for each BusyBox command in bin.

2. Init Script

   • Shell script to mount virtual filesystems and start a shell (bin/sh).
   • Optional: Add poweroff -f to avoid kernel panic on exit.

3. Packaging

   • Use cpio to create an initial RAM file system (initrd.img).
   • Combine with the kernel image for initialization.

Running the System

1. QEMU Configuration

   • Use qemu-system-x86_64 to boot the system.
   • Commands: -kernel for the kernel image, -initrd for the initial ramdisk image.
   • Optional: -nographic to run without a graphical window.

2. Debugging Tips

   • Catch kernel output using console redirection.
   • Handle kernel panics by adding poweroff command in the init script.

Conclusion

• Result: A minimalistic but functional Linux system.
• Code Summary: Approximately 63 lines of shell scripting.
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