support unsharing PIDNS (--pidns)

Fix rootless-containers#65

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

README.md

@@ -14,6 +14,7 @@ RootlessKit is a kind of Linux-native "fake root" utility, made for mainly runni
 - [Usage](#usage)
 - [State directory](#state-directory)
 - [Environment variables](#environment-variables)
+- [PID Namespace](#pid-namespace)
 - [Network Drivers](#network-drivers)
   - [`--net=host` (default)](#--nethost-default)
   - [`--net=slirp4netns` (recommended)](#--netslirp4netns-recommended)
@@ -135,15 +136,14 @@ allow
 Full CLI options:
 
 ```console
-
 NAME:
    rootlesskit - the gate to the rootless world
 
 USAGE:
    rootlesskit [global options] command [command options] [arguments...]
 
 VERSION:
-   0.3.0+dev
+   0.4.1+dev
 
 COMMANDS:
      help, h  Shows a list of commands or help for one command
@@ -162,6 +162,7 @@ GLOBAL OPTIONS:
    --copy-up value              mount a filesystem and copy-up the contents. e.g. "--copy-up=/etc" (typically required for non-host network)
    --copy-up-mode value         copy-up mode [tmpfs+symlink] (default: "tmpfs+symlink")
    --port-driver value          port driver for non-host network. [none, socat, slirp4netns, builtin(experimental)] (default: "none")
+   --pidns                      create a PID namespace
    --help, -h                   show help
    --version, -v                print the version
 ```
@@ -184,6 +185,14 @@ The following environment variables will be set for the child process:
 
 Undocumented environment variables are subject to change.
 
+## PID Namespace
+
+When `--pidns` (since v0.5.0) is specified, RootlessKit executes the child process in a new PID namespace.
+The RootlessKit child process becomes the init (PID=1).
+When RootlessKit terminates, all the processes in the namespace are killed with `SIGKILL`.
+
+See also [`pid_namespaces(7)`](http://man7.org/linux/man-pages/man7/pid_namespaces.7.html).
+
 ## Network Drivers
 
 RootlessKit provides several drivers for providing network connectivity:

cmd/rootlesskit/main.go

@@ -101,6 +101,10 @@ func main() {
 			Usage: "port driver for non-host network. [none, socat, slirp4netns, builtin(experimental)]",
 			Value: "none",
 		},
+		cli.BoolFlag{
+			Name:  "pidns",
+			Usage: "create a PID namespace",
+		},
 	}
 	app.Before = func(context *cli.Context) error {
 		if debug {
@@ -163,6 +167,7 @@ func createParentOpt(clicontext *cli.Context, pipeFDEnvKey, stateDirEnvKey strin
 	opt := parent.Opt{
 		PipeFDEnvKey:   pipeFDEnvKey,
 		StateDirEnvKey: stateDirEnvKey,
+		CreatePIDNS:    clicontext.Bool("pidns"),
 	}
 	opt.StateDir = clicontext.String("state-dir")
 	if opt.StateDir == "" {
@@ -298,6 +303,7 @@ func createChildOpt(clicontext *cli.Context, pipeFDEnvKey string, targetCmd []st
 	opt := child.Opt{
 		PipeFDEnvKey: pipeFDEnvKey,
 		TargetCmd:    targetCmd,
+		MountProcfs:  clicontext.Bool("pidns"),
 	}
 	switch s := clicontext.String("net"); s {
 	case "host":

pkg/child/child.go

@@ -68,6 +68,19 @@ func mountSysfs() error {
 	return nil
 }
 
+func mountProcfs() error {
+	cmds := [][]string{{"mount", "-t", "proc", "none", "/proc"}}
+	if err := common.Execs(os.Stderr, os.Environ(), cmds); err != nil {
+		cmdsRo := [][]string{{"mount", "-t", "proc", "-o", "ro", "none", "/proc"}}
+		logrus.Warnf("failed to mount procfs (%v), falling back to read-only mount (%v): %v",
+			cmds, cmdsRo, err)
+		if err := common.Execs(os.Stderr, os.Environ(), cmdsRo); err != nil {
+			logrus.Warnf("failed to mount procfs (%v): %v", cmdsRo, err)
+		}
+	}
+	return nil
+}
+
 func activateLoopback() error {
 	cmds := [][]string{
 		{"ip", "link", "set", "lo", "up"},
@@ -157,6 +170,7 @@ type Opt struct {
 	CopyUpDriver  copyup.ChildDriver  // cannot be nil if len(CopyUpDirs) != 0
 	CopyUpDirs    []string
 	PortDriver    port.ChildDriver
+	MountProcfs   bool // needs to be set if (and only if) parent.Opt.CreatePIDNS is set
 }
 
 func Child(opt Opt) error {
@@ -211,6 +225,11 @@ func Child(opt Opt) error {
 	if err := setupNet(msg, etcWasCopied, opt.NetworkDriver); err != nil {
 		return err
 	}
+	if opt.MountProcfs {
+		if err := mountProcfs(); err != nil {
+			return err
+		}
+	}
 	portQuitCh := make(chan struct{})
 	portErrCh := make(chan error)
 	if opt.PortDriver != nil {

pkg/parent/parent.go

@@ -30,6 +30,7 @@ type Opt struct {
 	StateDirEnvKey string               // optional env key to propagate StateDir value
 	NetworkDriver  network.ParentDriver // nil for HostNetwork
 	PortDriver     port.ParentDriver    // nil for --port-driver=none
+	CreatePIDNS    bool
 }
 
 // Documented state files. Undocumented ones are subject to change.
@@ -85,6 +86,10 @@ func Parent(opt Opt) error {
 	if opt.NetworkDriver != nil {
 		cmd.SysProcAttr.Unshareflags |= syscall.CLONE_NEWNET
 	}
+	if opt.CreatePIDNS {
+		// cannot be Unshareflags (panics)
+		cmd.SysProcAttr.Cloneflags |= syscall.CLONE_NEWPID
+	}
 	cmd.Stdin = os.Stdin
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr