libct/rootfs: improve some errors

Errors from os.Open, os.Symlink etc do not need to be wrapped, as they
are already wrapped into os.PathError.

Error from unix are bare errnos and need to be wrapped. Same
os.PathError is a good candidate.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/rootfs_linux.go

@@ -11,6 +11,7 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -577,7 +578,7 @@ func checkProcMount(rootfs, dest, source string) error {
 func isProc(path string) (bool, error) {
 	var s unix.Statfs_t
 	if err := unix.Statfs(path, &s); err != nil {
-		return false, err
+		return false, &os.PathError{Op: "statfs", Path: path, Err: err}
 	}
 	return s.Type == unix.PROC_SUPER_MAGIC, nil
 }
@@ -600,7 +601,7 @@ func setupDevSymlinks(rootfs string) error {
 			dst = filepath.Join(rootfs, link[1])
 		)
 		if err := os.Symlink(src, dst); err != nil && !os.IsExist(err) {
-			return fmt.Errorf("symlink %s %s %s", src, dst, err)
+			return err
 		}
 	}
 	return nil
@@ -614,11 +615,11 @@ func reOpenDevNull() error {
 	var stat, devNullStat unix.Stat_t
 	file, err := os.OpenFile("/dev/null", os.O_RDWR, 0)
 	if err != nil {
-		return fmt.Errorf("Failed to open /dev/null - %s", err)
+		return err
 	}
 	defer file.Close() //nolint: errcheck
 	if err := unix.Fstat(int(file.Fd()), &devNullStat); err != nil {
-		return err
+		return &os.PathError{Op: "fstat", Path: file.Name(), Err: err}
 	}
 	for fd := 0; fd < 3; fd++ {
 		if err := unix.Fstat(fd, &stat); err != nil {
@@ -627,7 +628,11 @@ func reOpenDevNull() error {
 		if stat.Rdev == devNullStat.Rdev {
 			// Close and re-open the fd.
 			if err := unix.Dup3(int(file.Fd()), fd, 0); err != nil {
-				return err
+				return &os.PathError{
+					Op:   "dup3",
+					Path: "fd " + strconv.Itoa(int(file.Fd())),
+					Err:  err,
+				}
 			}
 		}
 	}
@@ -808,7 +813,7 @@ func setupPtmx(config *configs.Config) error {
 		return err
 	}
 	if err := os.Symlink("pts/ptmx", ptmx); err != nil {
-		return fmt.Errorf("symlink dev ptmx %s", err)
+		return err
 	}
 	return nil
 }
@@ -824,23 +829,23 @@ func pivotRoot(rootfs string) error {
 
 	oldroot, err := unix.Open("/", unix.O_DIRECTORY|unix.O_RDONLY, 0)
 	if err != nil {
-		return err
+		return &os.PathError{Op: "open", Path: "/", Err: err}
 	}
 	defer unix.Close(oldroot) //nolint: errcheck
 
 	newroot, err := unix.Open(rootfs, unix.O_DIRECTORY|unix.O_RDONLY, 0)
 	if err != nil {
-		return err
+		return &os.PathError{Op: "open", Path: rootfs, Err: err}
 	}
 	defer unix.Close(newroot) //nolint: errcheck
 
 	// Change to the new root so that the pivot_root actually acts on it.
 	if err := unix.Fchdir(newroot); err != nil {
-		return err
+		return &os.PathError{Op: "fchdir", Path: "fd " + strconv.Itoa(newroot), Err: err}
 	}
 
 	if err := unix.PivotRoot(".", "."); err != nil {
-		return fmt.Errorf("pivot_root %s", err)
+		return &os.PathError{Op: "pivot_root", Path: ".", Err: err}
 	}
 
 	// Currently our "." is oldroot (according to the current kernel code).
@@ -849,7 +854,7 @@ func pivotRoot(rootfs string) error {
 	// pivot_root(2).
 
 	if err := unix.Fchdir(oldroot); err != nil {
-		return err
+		return &os.PathError{Op: "fchdir", Path: "fd " + strconv.Itoa(oldroot), Err: err}
 	}
 
 	// Make oldroot rslave to make sure our unmounts don't propagate to the
@@ -867,7 +872,7 @@ func pivotRoot(rootfs string) error {
 
 	// Switch back to our shiny new root.
 	if err := unix.Chdir("/"); err != nil {
-		return fmt.Errorf("chdir / %s", err)
+		return &os.PathError{Op: "chdir", Path: "/", Err: err}
 	}
 	return nil
 }
@@ -937,9 +942,12 @@ func msMoveRoot(rootfs string) error {
 
 func chroot() error {
 	if err := unix.Chroot("."); err != nil {
-		return err
+		return &os.PathError{Op: "chroot", Path: ".", Err: err}
 	}
-	return unix.Chdir("/")
+	if err := unix.Chdir("/"); err != nil {
+		return &os.PathError{Op: "chdir", Path: "/", Err: err}
+	}
+	return nil
 }
 
 // createIfNotExists creates a file or a directory only if it does not already exist.