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+---
+title: Running Golang application as PID 1 with Linux kernel
+permalink: /running-golang-application-as-pid1.html
+date: 2021-12-25T12:00:00+02:00
+layout: post
+type: post
+draft: false
+---
+
+## Unikernels, kernels, and alike
+
+I have been reading a lot about
+[unikernernels](https://en.wikipedia.org/wiki/Unikernel) lately and found them
+very intriguing. When you push away all the marketing speak and look at the
+idea, it makes a lot of sense.
+
+> A unikernel is a specialized, single address space machine image constructed
+> by using library operating systems. ([Wikipedia](https://en.wikipedia.org/wiki/Unikernel))
+
+I really like the explanation from the article
+[Unikernels: Rise of the Virtual Library Operating System](https://queue.acm.org/detail.cfm?id=2566628).
+Really worth a read.
+
+If we compare a normal operating system to a unikernel side by side, they would
+look something like this.
+
+![Virtual machines vs Containers vs Unikernels](/assets/posts/pid1/unikernels.webp)
+
+From this image, we can see how the complexity significantly decreases with
+the use of Unikernels. This comes with a price, of course. Unikernels are hard
+to get running and require a lot of work since you don't have an actual proper
+kernel running in the background providing network access and drivers etc.
+
+So as a half step to make the stack simpler, I started looking into using
+Linux kernel as a base and going from there. I came across this
+[Youtube video talking about Building the Simplest Possible Linux System](https://www.youtube.com/watch?v=Sk9TatW9ino)
+by [Rob Landley](https://landley.net) and apart from statically compiling the
+application to be run as PID1 there was really no other obstacles.
+
+## What is PID 1?
+
+PID 1 is the first process that Linux kernel starts after the boot process.
+It also has a couple of unique properties that are unique to it.
+
+- When the process with PID 1 dies for any reason, all other processes are
+  killed with KILL signal.
+- When any process having children dies for any reason, its children are
+  re-parented to process with PID 1.
+- Many signals which have default action of Term do not have one for PID 1.
+- When the process with PID 1 dies for any reason, kernel panics, which
+  result in system crash.
+
+PID 1 is considered as an Init application which takes care of running other
+and handling services like:
+
+- sshd,
+- nginx,
+- pulseaudio,
+- etc.
+
+If you are on a Linux machine, you can check what your process is with PID 1
+by running the following.
+
+```sh
+$ cat /proc/1/status
+Name:   systemd
+Umask:  0000
+State:  S (sleeping)
+Tgid:   1
+Ngid:   0
+Pid:    1
+PPid:   0
+...
+```
+
+As we can see on my machine the process with id of 1 is [systemd](https://systemd.io/)
+which is a software suite that provides an array of system components for Linux
+operating systems. If you look closely you can also see that the `PPid`
+(process id of the parent process) is `0` which additionally confirms that
+this process doesn't have a parent.
+
+## So why even run application as PID 1 instead of just using a container?
+
+Containers are wonderful, but they come with a lot of baggage. And because they
+are in their nature layered, the images require quite a lot of space and also a
+lot of additional software to handle them. They are not as lightweight as they
+seem, and many popular images require 500 MB plus disk space.
+
+The idea of running this as PID 1 would result in a significantly smaller footprint,
+as we will see later in the post.
+
+> You could run a simple init system inside Docker container described more
+> in this article [Docker and the PID 1 zombie reaping problem](https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem/).
+
+## The master plan
+
+1. Compile Linux kernel with the default definitions.
+2. Prepare a Hello World application in Golang that is statically compiled.
+3. Run it with [QEMU](https://www.qemu.org/) and providing Golang application
+   as init application / PID 1.
+
+For the sake of simplicity we will not be cross-compiling any of it and just
+use the 64bit version.
+
+## Compiling Linux kernel
+
+```sh
+$ wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.7.tar.xz
+$ tar xf linux-5.15.7.tar.xz
+
+$ cd linux-5.15.7
+
+$ make clean
+
+# read more about this https://stackoverflow.com/a/41886394
+$ make defconfig
+
+$ time make -j `nproc`
+
+$ cd ..
+```
+
+At this point we have kernel image that is located in `arch/x86_64/boot/bzImage`.
+We will use this in QEMU later.
+
+To make our lives a bit easier lets move the kernel image to another place.
+Lets create a folder `bin/` in the root of our project with `mkdir -p bin`.
+
+
+At this point we can copy `bzImage` to `bin/` folder with
+`cp linux-5.15.7/arch/x86_64/boot/bzImage bin/bzImage`.
+
+The folder structure of this experiment should look like this.
+
+```txt
+pid1/
+  bin/
+    bzImage
+  linux-5.15.7/
+  linux-5.15.7.tar.xz
+```
+
+## Preparing PID 1 application in Golang
+
+This step is relatively easy. The only thing we must have in mind that we will
+need to compile the binary as a static one.
+
+Let's create `init.go` file in the root of the project.
+
+```go
+package main
+
+import (
+  "fmt"
+  "time"
+)
+
+func main() {
+  for {
+    fmt.Println("Hello from Golang")
+    time.Sleep(1 * time.Second)
+  }
+}
+```
+
+If you notice, we have a forever loop in the main, with a simple sleep of 1
+second to not overwhelm the CPU. This is because PID 1 should never complete
+and/or exit. That would result in a kernel panic. Which is BAD!
+
+There are two ways of compiling Golang application. Statically and dynamically.
+
+To statically compile the binary, use the following command.
+
+```sh
+$ go build -ldflags="-extldflags=-static" init.go
+```
+
+We can also check if the binary is statically compiled with:
+
+```sh
+$ file init
+init: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, Go BuildID=Ypu8Zw_4NBxm1Yxg2OYO/H5x721rQ9uTPiDVh-VqP/vZN7kXfGG1zhX_qdHMgH/9vBfmK81tFrygfOXDEOo, not stripped
+
+$ ldd init
+not a dynamic executable
+```
+
+At this point, we need to create [initramfs](https://www.linuxfromscratch.org/blfs/view/svn/postlfs/initramfs.html)
+(abbreviated from "initial RAM file system", is the successor of initrd. It
+is a cpio archive of the initial file system that gets loaded into memory
+during the Linux startup process).
+
+```sh
+$ echo init | cpio -o --format=newc > initramfs
+$ mv initramfs bin/initramfs
+```
+
+The projects at this stage should look like this.
+
+```txt
+pid1/
+  bin/
+    bzImage
+    initramfs
+  linux-5.15.7/
+  linux-5.15.7.tar.xz
+  init.go
+```
+
+## Running all of it with QEMU
+
+[QEMU](https://www.qemu.org/) is a free and open-source hypervisor. It emulates
+the machine's processor through dynamic binary translation and provides a set
+of different hardware and device models for the machine, enabling it to run a
+variety of guest operating systems.
+
+```sh
+$ qemu-system-x86_64 -serial stdio -kernel bin/bzImage -initrd bin/initramfs -append "console=ttyS0" -m 128
+```
+
+```sh
+$ qemu-system-x86_64 -serial stdio -kernel bin/bzImage -initrd bin/initramfs -append "console=ttyS0" -m 128
+[    0.000000] Linux version 5.15.7 (m@khan) (gcc (GCC) 11.2.1 20211203 (Red Hat 11.2.1-7), GNU ld version 2.37-10.fc35) #7 SMP Mon Dec 13 10:23:25 CET 2021
+[    0.000000] Command line: console=ttyS0
+[    0.000000] x86/fpu: x87 FPU will use FXSAVE
+[    0.000000] signal: max sigframe size: 1440
+[    0.000000] BIOS-provided physical RAM map:
+[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
+[    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
+[    0.000000] BIOS-e820: [mem 0x00000000000f0000-0x00000000000fffff] reserved
+[    0.000000] BIOS-e820: [mem 0x0000000000100000-0x0000000007fdffff] usable
+[    0.000000] BIOS-e820: [mem 0x0000000007fe0000-0x0000000007ffffff] reserved
+[    0.000000] BIOS-e820: [mem 0x00000000fffc0000-0x00000000ffffffff] reserved
+[    0.000000] NX (Execute Disable) protection: active
+[    0.000000] SMBIOS 2.8 present.
+[    0.000000] DMI: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-6.fc35 04/01/2014
+[    0.000000] tsc: Fast TSC calibration failed
+...
+[    2.016106] ALSA device list:
+[    2.016329]   No soundcards found.
+[    2.053176] Freeing unused kernel image (initmem) memory: 1368K
+[    2.056095] Write protecting the kernel read-only data: 20480k
+[    2.058248] Freeing unused kernel image (text/rodata gap) memory: 2032K
+[    2.058811] Freeing unused kernel image (rodata/data gap) memory: 500K
+[    2.059164] Run /init as init process
+Hello from Golang
+[    2.386879] tsc: Refined TSC clocksource calibration: 3192.032 MHz
+[    2.387114] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x2e02e31fa14, max_idle_ns: 440795264947 ns
+[    2.387380] clocksource: Switched to clocksource tsc
+[    2.587895] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3
+Hello from Golang
+Hello from Golang
+Hello from Golang
+```
+
+The whole [log file here](/assets/posts/pid1/qemu.log).
+
+## Size comparison
+
+The cool thing about this approach is that the Linux kernel and the application
+together only take around 12 MB, which is impressive as hell. And we need to
+also know that the size of bzImage (Linux kernel) could be greatly decreased
+by going into `make menuconfig` and removing a ton of features from the kernel,
+making the size even smaller. I managed to get kernel size down to 2 MB and
+still working properly.
+
+```sh
+total 12M
+-rw-r--r--. 1 m m 9.3M Dec 13 10:24 bzImage
+-rw-r--r--. 1 m m 1.9M Dec 27 01:19 initramfs
+```
+
+## Creating ISO image and running it with Gnome Boxes
+
+First we need to create proper folder structure with `mkdir -p iso/boot/grub`.
+
+Then we need to download the [grub binary](https://github.com/littleosbook/littleosbook/raw/master/files/stage2_eltorito).
+You can read more about this program on https://github.com/littleosbook/littleosbook.
+
+```sh
+$ wget -O iso/boot/grub/stage2_eltorito https://github.com/littleosbook/littleosbook/raw/master/files/stage2_eltorito
+```
+
+```sh
+$ tree iso/boot/
+iso/boot/
+├── bzImage
+├── grub
+│   ├── menu.lst
+│   └── stage2_eltorito
+└── initramfs
+```
+
+Let's copy files into proper folders.
+
+
+```sh
+$ cp stage2_eltorito iso/boot/grub/
+$ cp bin/bzImage iso/boot/
+$ cp bin/initramfs iso/boot/
+```
+
+Lets create a GRUB config file at `nano iso/boot/grub/menu.lst` with contents.
+
+```ini
+default=0
+timeout=5
+
+title GoAsPID1
+kernel /boot/bzImage
+initrd /boot/initramfs
+```
+
+Let's create iso file by using genisoimage:
+
+```sh
+genisoimage -R                              \
+            -b boot/grub/stage2_eltorito    \
+            -no-emul-boot                   \
+            -boot-load-size 4               \
+            -A os                           \
+            -input-charset utf8             \
+            -quiet                          \
+            -boot-info-table                \
+            -o GoAsPID1.iso                 \
+            iso
+```
+
+This will produce `GoAsPID1.iso` which you can use with [Virtualbox](https://www.virtualbox.org/)
+or [Gnome Boxes](https://apps.gnome.org/app/org.gnome.Boxes/).
+
+<video src="/assets/posts/pid1/boxes.mp4" controls></video>
+
+## Is running applications as PID 1 even worth it?
+
+Well, the answer to this is not as simple as one would think. Sometimes it is
+and sometimes it's not. For embedded systems and very specialized applications
+it is worth for sure. But in normal uses, I don't think so. It was an interesting
+exercise in compiling kernels and looking at the guts of the Linux kernel,
+but sticking to containers for most of the things is a better option in my
+opinion.
+
+An interesting experiment would be creating an image that supports networking
+and could be deployed to AWS as an EC2 instance and observing how it fares.
+But in that case, we would need to write some sort of supervisor that would
+run on a separate EC2 that would check if other EC2 instances are running
+properly. Remember that if your application fails, kernel panics and the
+whole machine is inoperable in this case.