The Linux kernel is just a program
serversfor.dev
Most books and courses introduce Linux through shell commands, leaving the kernel as a mysterious black box doing magic behind the scenes. In this post, we will run some experiments to demystify it: the Linux kernel is just a binary that you can build and run.

I’ve been working on a “Linux Inside Out” series and wrote a post that might interest folks here who like low(ish)-level / OS internals.

The idea is to dissect the components of a Linux OS, layer by layer, and build a mental model of how everything fits together through experiments.

The first part is about the kernel, in the post I:

  • take the same kernel image my distro boots from /boot
  • boot it directly with QEMU (no distro, no init system)
  • watch it panic
  • write a tiny Go program and use it as PID 1
  • build a minimal initramfs around it so the kernel can actually start our process

The goal isn’t to build a real distro, just to give a concrete mental model of:

  • that the Linux kernel is just a compressed file, you can boot it
  • without anything else
  • what the kernel actually does at boot
  • how it hands control to userspace
  • what PID 1 / init is in practice
  • what is kernel space vs user space

Link: https://serversfor.dev/linux-inside-out/the-linux-kernel-is-just-a-program/

I’m the author, would be happy to hear from other devs whether this way of explaining things makes sense, and what you’d add or change for future posts in the series.

Hope you find it useful.

  • non_burglar@lemmy.world
    0·
    1 day ago

    Are you just reverse-engineering this for fun, or are you trying to learn how qemu builds on a bootstrap?

    • zknd@lemmy.worldOP
      3·
      1 day ago

      I am writing a blog post series to make this topic more approachable for others.

      • non_burglar@lemmy.world
        3·
        19 hours ago

        Nice, and good job.

        With respect:

        • You are describing the init process, after the boot stuff is done. Not sure if you meant that or not.
        • vmlinuz is the bootable kernel image distinct from the kernel itself. It contains the compressed bootbale ELF kernel image (which itself contains bvmlinuz) and gzip decompress function, bsetup code, bbootsect system.map, and a handful of commands for the kernel image to bootstrap itself.
        • Kernel space doesn’t stop once user space is allocated memory and initialized, they both run at the same time. Maybe I read that wrong, but it didn’t seem right to me.

        I want to be careful here not to discourage you, this is great exploration!

        I realize I’m handing out unsolicited advice here, but when I was first learning about Unix/Linux kernels in the Solaris and HP/UX days, the thing that helped the process “click” for me was compiling a kernel and building an ELF. And if you’re going to continue on this journey (which I hope you do), you should probably read a bit on memory segmentation and broadly about assembly instructions.

        Good luck!

        • zknd@lemmy.worldOP
          1·
          41 minutes ago

          Thank you for the feedback!

          My goal with this series is to make Linux internals more approachable, so I’m intentionally keeping each post narrow and avoiding details that would overwhelm readers at this stage.

          The primary target audience of these blog posts are developers, and I have a progression plan in mind how to build up layer by layer the necessary knowledge they need to know to understand how they program work/interact with the other parts of the OS. The boot process or the exact composition of the kernel image is irrelevant here in my opinion, they wont need to touch them on a VPS for example or on their dev machine.

          Regarding the kernel space/user space I did not write that the kernel space stopped, I will come back to it in the next post that will be about system calls, we will discuss there that when our programs interact with the kernel we are switching between user and kernel mode back and forth, with the syscall instruction (on x86-64 systems).

          Anyway, I really appreciate your feedback and that you put effort into it, and thank you for reading the post! :)