commit c7bbc92dbe8dca2bc40d1f108060b27030e195a4
Author: M1ngdaXie <156019134+M1ngdaXie@users.noreply.github.com>
Date:   Sun Jul 19 14:25:00 2026 +0800

    initial commit

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..9f11b75
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.idea/
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..966d8c0
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module github.com/M1ngdaXie/mini-docker
+
+go 1.26.1
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..aefcc53
--- /dev/null
+++ b/main.go
@@ -0,0 +1,257 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"syscall"
+)
+
+// 这个 demo 没有加 CLONE_NEWUSER(user namespace)。
+// 加了之后容器里的"root"会映射成宿主机上一个无权限的普通用户,
+// mount/mknod 这类系统调用会被内核直接拒绝——从根上堵死设备逃逸这条路。
+// 现在故意不加,是因为这个工具还在用来练习/演示逃逸技术;
+// 如果以后想让它变成一个"正经"的隔离环境,这是必须补的一项。
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintf(os.Stderr, "Usage: %s run [cmd] [args...]\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	switch os.Args[1] {
+	case "run":
+		run()
+	case "child":
+		child()
+	default:
+		panic("Unknown command")
+	}
+}
+
+func run() {
+	args := os.Args[2:]
+	if len(args) == 0 {
+		args = []string{"/bin/sh"}
+	}
+
+	hostPID := os.Getpid()
+	fmt.Printf("[run]   Running %v as PID %d\n", args, hostPID)
+
+	overlayBase := fmt.Sprintf("/tmp/maingo-%d", hostPID)
+	upperDir := overlayBase + "/upper"
+	workDir := overlayBase + "/work"
+	mergedDir := overlayBase + "/merged"
+	vethHost := fmt.Sprintf("veth0-%d", hostPID)
+	vethCtr := fmt.Sprintf("veth1-%d", hostPID)
+	cgroupName := fmt.Sprintf("maingo-%d", hostPID)
+
+	ipSuffix := hostPID % 253
+	hostIP := fmt.Sprintf("10.0.%d.1", ipSuffix)
+	ctrIP := fmt.Sprintf("10.0.%d.2", ipSuffix)
+	subnet := fmt.Sprintf("10.0.%d.0/24", ipSuffix)
+
+	must(os.MkdirAll(upperDir, 0755))
+	must(os.MkdirAll(workDir, 0755))
+	must(os.MkdirAll(mergedDir, 0755))
+
+	// 用 defer 而不是"cmd.Wait() 之后顺序执行"：
+	// must(cmd.Wait()) 在子进程非 0 退出时会 panic,
+	// 之前的写法一旦这里 panic，后面的清理代码一行都不会跑，
+	// 会在宿主机上留下孤儿 veth / iptables 规则 / cgroup / 临时目录。
+	// defer 保证不管中间从哪里 panic，清理都会按 LIFO 顺序跑完。
+	defer safeRemoveAll(overlayBase)
+	defer cleanupCgroup(cgroupName)
+	defer cleanupVeth(vethHost)
+	defer cleanupNAT(subnet)
+
+	r, w, err := os.Pipe()
+	must(err)
+
+	cmd := exec.Command("/proc/self/exe", append([]string{"child"}, args...)...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.ExtraFiles = []*os.File{r}
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Cloneflags: syscall.CLONE_NEWUTS |
+			syscall.CLONE_NEWPID |
+			syscall.CLONE_NEWNS |
+			syscall.CLONE_NEWNET,
+	}
+	cmd.Env = append(os.Environ(),
+		"MAINGO_MERGED="+mergedDir,
+		"MAINGO_UPPER="+upperDir,
+		"MAINGO_WORK="+workDir,
+		"MAINGO_CGROUP="+cgroupName)
+
+	must(cmd.Start())
+	r.Close()
+
+	setupVeth(cmd.Process.Pid, vethHost, vethCtr, hostIP, ctrIP, subnet)
+	setupNAT(cmd.Process.Pid, ctrIP, subnet)
+
+	w.Write([]byte{1})
+	w.Close()
+
+	must(cmd.Wait())
+}
+
+func child() {
+	fmt.Printf("[child] Running %v as PID %d\n", os.Args[2:], os.Getpid())
+
+	must(syscall.Sethostname([]byte("container")))
+
+	// 必须在任何 mount 操作之前执行。
+	// CLONE_NEWNS 只是给了子进程一份 mount 表的独立拷贝，
+	// 但拷贝出来的挂载点默认继承宿主机的 propagation 类型（多数发行版根挂载点是 shared）。
+	// shared 状态下，容器里新增的任何挂载（包括后面的 overlay、以及测试用的宿主机磁盘挂载）
+	// 都会自动同步回宿主机自己的 mount namespace。
+	// 把根挂载点递归改成 MS_PRIVATE，彻底切断这个双向同步，
+	// 容器里再怎么 mount，宿主机那边都看不见、也不会被波及。
+	must(syscall.Mount("", "/", "", syscall.MS_PRIVATE|syscall.MS_REC, ""))
+
+	setupCgroup(os.Getenv("MAINGO_CGROUP"))
+	waitForNetwork()
+
+	mergedDir := os.Getenv("MAINGO_MERGED")
+	upperDir := os.Getenv("MAINGO_UPPER")
+	workDir := os.Getenv("MAINGO_WORK")
+
+	lowerDir := "/home/mingdax/alpine-rootfs"
+	opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", lowerDir, upperDir, workDir)
+	must(syscall.Mount("overlay", mergedDir, "overlay", 0, opts))
+	fmt.Printf("[overlay] mounted\n")
+
+	must(syscall.Chroot(mergedDir))
+	must(syscall.Chdir("/"))
+	must(os.MkdirAll("/proc", 0555))
+	must(syscall.Mount("proc", "proc", "proc", 0, ""))
+
+	cmd := exec.Command(os.Args[2], os.Args[3:]...)
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+
+	must(cmd.Run())
+
+	must(syscall.Unmount("proc", 0))
+}
+
+func setupVeth(childPID int, vethHost, vethCtr, hostIP, ctrIP, subnet string) {
+	pid := strconv.Itoa(childPID)
+
+	runCmdSilent("ip", "link", "del", vethHost)
+
+	runCmd("ip", "link", "add", vethHost, "type", "veth", "peer", "name", vethCtr)
+	runCmd("ip", "link", "set", vethCtr, "netns", pid)
+
+	runCmd("ip", "addr", "add", hostIP+"/24", "dev", vethHost)
+	runCmd("ip", "link", "set", vethHost, "up")
+
+	runCmd("nsenter", "-t", pid, "-n", "ip", "addr", "add", ctrIP+"/24", "dev", vethCtr)
+	runCmd("nsenter", "-t", pid, "-n", "ip", "link", "set", vethCtr, "up")
+	runCmd("nsenter", "-t", pid, "-n", "ip", "link", "set", "lo", "up")
+
+	fmt.Printf("[veth]  pair ready: host=%s, container=%s\n", hostIP, ctrIP)
+}
+
+func setupNAT(childPID int, ctrIP, subnet string) {
+	pid := strconv.Itoa(childPID)
+
+	must(os.WriteFile("/proc/sys/net/ipv4/ip_forward", []byte("1"), 0644))
+	runCmd("iptables", "-t", "nat", "-A", "POSTROUTING", "-s", subnet, "-j", "MASQUERADE")
+	runCmd("nsenter", "-t", pid, "-n", "ip", "route", "add", "default", "via",
+		ctrIP[:len(ctrIP)-1]+"1")
+
+	fmt.Printf("[nat]   NAT enabled: %s → internet\n", subnet)
+}
+
+func cleanupVeth(vethHost string) {
+	runCmdSilent("ip", "link", "del", vethHost)
+}
+
+func cleanupNAT(subnet string) {
+	runCmdSilent("iptables", "-t", "nat", "-D", "POSTROUTING", "-s", subnet, "-j",
+		"MASQUERADE")
+	fmt.Printf("[nat]   cleaned up\n")
+}
+
+func waitForNetwork() {
+	f := os.NewFile(3, "syncpipe")
+	buf := make([]byte, 1)
+	_, err := f.Read(buf)
+	must(err)
+	f.Close()
+	fmt.Printf("[net]   network ready\n")
+}
+
+func setupCgroup(name string) {
+	cg := "/sys/fs/cgroup/" + name
+	must(os.MkdirAll(cg, 0755))
+
+	must(os.WriteFile(cg+"/cpu.max", []byte("100000 100000"), 0644))
+	must(os.WriteFile(cg+"/memory.max", []byte("134217728"), 0644))
+	must(os.WriteFile(cg+"/cgroup.procs", []byte(strconv.Itoa(os.Getpid())), 0644))
+
+	fmt.Printf("[cgroup] limits set: 1 CPU, 128MB memory\n")
+}
+
+func cleanupCgroup(name string) {
+	cg := "/sys/fs/cgroup/" + name
+	os.WriteFile(cg+"/cgroup.kill", []byte("1"), 0644)
+	os.Remove(cg)
+	fmt.Printf("[cgroup] cleaned up\n")
+}
+
+// safeRemoveAll 是 os.RemoveAll 的一层保险。
+// 就算上面的 MS_PRIVATE 隔离出于某种原因失效（比如以后改了挂载逻辑忘记加），
+// 这里也会在真正删除前检查每个子路径的设备号（st_dev），
+// 一旦发现和 overlayBase 本身不是同一个设备，直接拒绝删除，
+// 而不是像 os.RemoveAll 那样无视挂载边界、一路删穿到别的文件系统上。
+func safeRemoveAll(path string) {
+	rootFi, err := os.Lstat(path)
+	if err != nil {
+		return // 目录已经不在了，没什么好删的
+	}
+	rootDev := rootFi.Sys().(*syscall.Stat_t).Dev
+
+	err = filepath.Walk(path, func(p string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		if st, ok := info.Sys().(*syscall.Stat_t); ok && st.Dev != rootDev {
+			return fmt.Errorf("safeRemoveAll: %s 跨越了挂载边界（dev %d != %d），拒绝删除", p, st.Dev, rootDev)
+		}
+		return nil
+	})
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "[overlay] 清理中止，未删除任何内容: %v\n", err)
+		return
+	}
+
+	if err := os.RemoveAll(path); err != nil {
+		fmt.Fprintf(os.Stderr, "[overlay] 清理 %s 失败: %v\n", path, err)
+		return
+	}
+	fmt.Printf("[overlay] cleaned up %s\n", path)
+}
+
+func runCmd(args ...string) {
+	cmd := exec.Command(args[0], args[1:]...)
+	cmd.Stderr = os.Stderr
+	must(cmd.Run())
+}
+
+func runCmdSilent(args ...string) {
+	cmd := exec.Command(args[0], args[1:]...)
+	cmd.Run()
+}
+
+func must(err error) {
+	if err != nil {
+		panic(err)
+	}
+}