Add os.md

Author: sneaky-potato

content/index.md

@@ -11,5 +11,6 @@ Some starting points to navigate in this labyrinth:
 - [[hardware-security]] perfect crossover of security and systems
 - [[web-dev]]
 - [[compilers]] a rather deep dive in compilation, execution, binaries
+- [[os]] for system madlads
 - [[cpp]] still learning this humongoes language to get better at computer systems
 - [[reading]]

content/os/container.md

@@ -0,0 +1,105 @@
+## Containerization
+
+But how is Docker is even implemented? What means to spawn a container?
+The answer to both these questions is closely related to linux process management and user space.
+
+## Go: minimal container
+Create the following program in a new folder
+
+```go
+// container/main.go
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+const ROOT_DIR = "./fs"
+
+func run() {
+	fmt.Printf("running %v as %d\n", os.Args[2:], os.Getpid())
+	cmd := exec.Command("/proc/self/exe", append([]string{"child"}, os.Args[2:]...)...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Cloneflags: syscall.CLONE_NEWUTS | syscall.CLONE_NEWPID,
+	}
+
+	cmd.Run()
+}
+
+func child() {
+	fmt.Printf("running %v as %d\n", os.Args[2:], os.Getpid())
+    syscall.Sethostname([]byte("container"))
+	must(syscall.Chroot(ROOT_DIR))
+	must(syscall.Chdir("/"))
+
+	cmd := exec.Command(os.Args[2], os.Args[3:]...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+    cmd.Env = append(os.Environ(), "PATH=/bin")
+
+	must(cmd.Run())
+}
+
+func must(err error) {
+    if err != nil { panic(err) }
+}
+
+func main() {
+	switch os.Args[1] {
+	case "run":
+		run()
+	case "child":
+		child()
+	default:
+		panic("bad command")
+	}
+}
+```
+
+## Minimal filesystem
+We will spawn a container rooted inside the `./fs` directory, for this we need to setup `fs`. Create the following script outside `fs`
+```bash
+#!/bin/bash
+# container/make-rootfs.sh
+
+# 1. Make sure the fs tree exists
+mkdir -p fs/{bin,proc}
+
+# 2. Download a real binary
+if [ ! -f ./fs/bin/busybox ]; then
+    curl -L https://busybox.net/downloads/binaries/1.35.0-x86_64-linux-musl/busybox -o fs/bin/busybox
+fi
+
+# (Check BusyBox site for the latest x86_64 build if the above link 404s)
+# 3. Make it executable and symlink sh
+chmod +x fs/bin/busybox
+for app in sh ls cat echo ps; do
+    if [ ! -f ./fs/bin/$app ]; then
+        ln -s busybox fs/bin/$app
+    fi
+done
+```
+
+This will spawn a child process which exec's the program given by the user (we will try to invoke `./fs/bin/sh`.
+- run the script defined above: `sudo ./make-rootfs.sh`
+- build the container binary: `go build -o container main.go`
+- run the container: `sudo ./container run /bin/sh`
+
+```shell
+$ sudo ./container run /bin/sh
+running [/bin/sh] as 81281
+running [/bin/sh] as 1
+/ # ls -a
+.     ..    bin   proc
+/ #
+```
+
+## Todo: symbolically linking `/proc`

content/os/os.md

@@ -0,0 +1,10 @@
+# Operating Systems
+
+At the time of writing this note, I am running a machine with the following specifications:
+- Operating System: Arch Linux
+- Kernel: Linux 6.12.45-1-lts
+- Architecture: x86-64
+
+And that is why much of the notes found here will be related to linux environment.
+
+- Recently I tried writing a minimal [[container]] program in golang