// +build linux
 
 package overlay2
 
 import (
 	"fmt"
 	"io/ioutil"
 	"os"
 	"path"
 	"path/filepath"
 
 	"github.com/docker/docker/pkg/system"
 	"github.com/pkg/errors"

 	"github.com/sirupsen/logrus"

 	"golang.org/x/sys/unix"

 )
 
 // hasOpaqueCopyUpBug checks whether the filesystem has a bug
 // which copies up the opaque flag when copying up an opaque
 // directory. When this bug exists naive diff should be used.
 func hasOpaqueCopyUpBug(d string) error {
 	td, err := ioutil.TempDir(d, "opaque-bug-check")
 	if err != nil {
 		return err
 	}
 	defer func() {
 		if err := os.RemoveAll(td); err != nil {
 			logrus.Warnf("Failed to remove check directory %v: %v", td, err)
 		}
 	}()
 
 	// Make directories l1/d, l2/d, l3, work, merged
 	if err := os.MkdirAll(filepath.Join(td, "l1", "d"), 0755); err != nil {
 		return err
 	}
 	if err := os.MkdirAll(filepath.Join(td, "l2", "d"), 0755); err != nil {
 		return err
 	}
 	if err := os.Mkdir(filepath.Join(td, "l3"), 0755); err != nil {
 		return err
 	}
 	if err := os.Mkdir(filepath.Join(td, "work"), 0755); err != nil {
 		return err
 	}
 	if err := os.Mkdir(filepath.Join(td, "merged"), 0755); err != nil {
 		return err
 	}
 
 	// Mark l2/d as opaque
 	if err := system.Lsetxattr(filepath.Join(td, "l2", "d"), "trusted.overlay.opaque", []byte("y"), 0); err != nil {
 		return errors.Wrap(err, "failed to set opaque flag on middle layer")
 	}
 
 	opts := fmt.Sprintf("lowerdir=%s:%s,upperdir=%s,workdir=%s", path.Join(td, "l2"), path.Join(td, "l1"), path.Join(td, "l3"), path.Join(td, "work"))

 	if err := unix.Mount("overlay", filepath.Join(td, "merged"), "overlay", 0, opts); err != nil {

 		return errors.Wrap(err, "failed to mount overlay")
 	}
 	defer func() {

 		if err := unix.Unmount(filepath.Join(td, "merged"), 0); err != nil {

 			logrus.Warnf("Failed to unmount check directory %v: %v", filepath.Join(td, "merged"), err)
 		}
 	}()
 
 	// Touch file in d to force copy up of opaque directory "d" from "l2" to "l3"
 	if err := ioutil.WriteFile(filepath.Join(td, "merged", "d", "f"), []byte{}, 0644); err != nil {
 		return errors.Wrap(err, "failed to write to merged directory")
 	}
 
 	// Check l3/d does not have opaque flag
 	xattrOpaque, err := system.Lgetxattr(filepath.Join(td, "l3", "d"), "trusted.overlay.opaque")
 	if err != nil {
 		return errors.Wrap(err, "failed to read opaque flag on upper layer")
 	}
 	if string(xattrOpaque) == "y" {
 		return errors.New("opaque flag erroneously copied up, consider update to kernel 4.8 or later to fix")
 	}
 
 	return nil
 }