@@ -1,14 +1,18 @@

 package archive // import "github.com/docker/docker/pkg/archive"

 import (

+	"fmt"

 	"io/ioutil"

 	"os"

+	"os/exec"

 	"path/filepath"

 	"syscall"

 	"testing"

+	"github.com/docker/docker/pkg/reexec"

 	"github.com/docker/docker/pkg/system"

 	rsystem "github.com/opencontainers/runc/libcontainer/system"

+	"github.com/pkg/errors"

 	"golang.org/x/sys/unix"

 	"gotest.tools/assert"

 	"gotest.tools/skip"

@@ -162,3 +166,129 @@ func TestOverlayTarAUFSUntar(t *testing.T) {

 	checkFileMode(t, filepath.Join(dst, "d2", "f1"), 0660)

 	checkFileMode(t, filepath.Join(dst, "d3", WhiteoutPrefix+"f1"), 0600)

 }

+

+func unshareCmd(cmd *exec.Cmd) {

+	cmd.SysProcAttr = &syscall.SysProcAttr{

+		Cloneflags: syscall.CLONE_NEWUSER | syscall.CLONE_NEWNS,

+		UidMappings: []syscall.SysProcIDMap{

+			{

+				ContainerID: 0,

+				HostID:      os.Geteuid(),

+				Size:        1,

+			},

+		},

+		GidMappings: []syscall.SysProcIDMap{

+			{

+				ContainerID: 0,

+				HostID:      os.Getegid(),

+				Size:        1,

+			},

+		},

+	}

+}

+

+const (

+	reexecSupportsUserNSOverlay = "docker-test-supports-userns-overlay"

+	reexecMknodChar0            = "docker-test-userns-mknod-char0"

+	reexecSetOpaque             = "docker-test-userns-set-opaque"

+)

+

+func supportsOverlay(dir string) error {

+	lower := filepath.Join(dir, "l")

+	upper := filepath.Join(dir, "u")

+	work := filepath.Join(dir, "w")

+	merged := filepath.Join(dir, "m")

+	for _, s := range []string{lower, upper, work, merged} {

+		if err := os.MkdirAll(s, 0700); err != nil {

+			return err

+		}

+	}

+	mOpts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", lower, upper, work)

+	if err := syscall.Mount("overlay", merged, "overlay", uintptr(0), mOpts); err != nil {

+		return errors.Wrapf(err, "failed to mount overlay (%s) on %s", mOpts, merged)

+	}

+	if err := syscall.Unmount(merged, 0); err != nil {

+		return errors.Wrapf(err, "failed to unmount %s", merged)

+	}

+	return nil

+}

+

+// supportsUserNSOverlay returns nil error if overlay is supported in userns.

+// Only Ubuntu and a few distros support overlay in userns (by patching the kernel).

+// https://lists.ubuntu.com/archives/kernel-team/2014-February/038091.html

+// As of kernel 4.19, the patch is not merged to the upstream.

+func supportsUserNSOverlay() error {

+	tmp, err := ioutil.TempDir("", "docker-test-supports-userns-overlay")

+	if err != nil {

+		return err

+	}

+	defer os.RemoveAll(tmp)

+	cmd := reexec.Command(reexecSupportsUserNSOverlay, tmp)

+	unshareCmd(cmd)

+	out, err := cmd.CombinedOutput()

+	if err != nil {

+		return errors.Wrapf(err, "output: %q", string(out))

+	}

+	return nil

+}

+

+// isOpaque returns nil error if the dir has trusted.overlay.opaque=y.

+// isOpaque needs to be called in the initial userns.

+func isOpaque(dir string) error {

+	xattrOpaque, err := system.Lgetxattr(dir, "trusted.overlay.opaque")

+	if err != nil {

+		return errors.Wrapf(err, "failed to read opaque flag of %s", dir)

+	}

+	if string(xattrOpaque) != "y" {

+		return errors.Errorf("expected \"y\", got %q", string(xattrOpaque))

+	}

+	return nil

+}

+

+func TestReexecUserNSOverlayWhiteoutConverter(t *testing.T) {

+	skip.If(t, os.Getuid() != 0, "skipping test that requires root")

+	skip.If(t, rsystem.RunningInUserNS(), "skipping test that requires initial userns")

+	if err := supportsUserNSOverlay(); err != nil {

+		t.Skipf("skipping test that requires kernel support for overlay-in-userns: %v", err)

+	}

+	tmp, err := ioutil.TempDir("", "docker-test-userns-overlay")

+	assert.NilError(t, err)

+	defer os.RemoveAll(tmp)

+

+	char0 := filepath.Join(tmp, "char0")

+	cmd := reexec.Command(reexecMknodChar0, char0)

+	unshareCmd(cmd)

+	out, err := cmd.CombinedOutput()

+	assert.NilError(t, err, string(out))

+	assert.NilError(t, isChar0(char0))

+

+	opaqueDir := filepath.Join(tmp, "opaquedir")

+	err = os.MkdirAll(opaqueDir, 0755)

+	assert.NilError(t, err, string(out))

+	cmd = reexec.Command(reexecSetOpaque, opaqueDir)

+	unshareCmd(cmd)

+	out, err = cmd.CombinedOutput()

+	assert.NilError(t, err, string(out))

+	assert.NilError(t, isOpaque(opaqueDir))

+}

+

+func init() {

+	reexec.Register(reexecSupportsUserNSOverlay, func() {

+		if err := supportsOverlay(os.Args[1]); err != nil {

+			panic(err)

+		}

+	})

+	reexec.Register(reexecMknodChar0, func() {

+		if err := mknodChar0Overlay(os.Args[1]); err != nil {

+			panic(err)

+		}

+	})

+	reexec.Register(reexecSetOpaque, func() {

+		if err := replaceDirWithOverlayOpaque(os.Args[1]); err != nil {

+			panic(err)

+		}

+	})

+	if reexec.Init() {

+		os.Exit(0)

+	}

+}