gvisor/gofer: Use pivot_root instead of chroot

PiperOrigin-RevId: 231864273
Change-Id: I8545b72b615f5c2945df374b801b80be64ec3e13

1 files changed, 24 insertions, 21 deletions

diff --git a/runsc/cmd/chroot.go b/runsc/cmd/chroot.go
index c1acbf26b..ed1dafef1 100644
--- a/runsc/cmd/chroot.go
+++ b/runsc/cmd/chroot.go
@@ -36,6 +36,29 @@ func mountInChroot(chroot, src, dst, typ string, flags uint32) error {
 	return nil
 }
 
+func pivotRoot(root string) error {
+	if err := os.Chdir(root); err != nil {
+		return fmt.Errorf("error changing working directory: %v", err)
+	}
+	// pivot_root(new_root, put_old) moves the root filesystem (old_root)
+	// of the calling process to the directory put_old and makes new_root
+	// the new root filesystem of the calling process.
+	//
+	// pivot_root(".", ".") makes a mount of the working directory the new
+	// root filesystem, so it will be moved in "/" and then the old_root
+	// will be moved to "/" too. The parent mount of the old_root will be
+	// new_root, so after umounting the old_root, we will see only
+	// the new_root in "/".
+	if err := syscall.PivotRoot(".", "."); err != nil {
+		return fmt.Errorf("error changing root filesystem: %v", err)
+	}
+
+	if err := syscall.Unmount(".", syscall.MNT_DETACH); err != nil {
+		return fmt.Errorf("error umounting the old root file system: %v", err)
+	}
+	return nil
+}
+
 // setUpChroot creates an empty directory with runsc mounted at /runsc and proc
 // mounted at /proc.
 func setUpChroot(pidns bool) error {
@@ -66,29 +89,9 @@ func setUpChroot(pidns bool) error {
 		}
 	}
 
-	if err := os.Chdir(chroot); err != nil {
-		return fmt.Errorf("error changing working directory: %v", err)
-	}
-
 	if err := syscall.Mount("", chroot, "", syscall.MS_REMOUNT|syscall.MS_RDONLY|syscall.MS_BIND, ""); err != nil {
 		return fmt.Errorf("error remounting chroot in read-only: %v", err)
 	}
-	// pivot_root(new_root, put_old) moves the root filesystem (old_root)
-	// of the calling process to the directory put_old and makes new_root
-	// the new root filesystem of the calling process.
-	//
-	// pivot_root(".", ".") makes a mount of the working directory the new
-	// root filesystem, so it will be moved in "/" and then the old_root
-	// will be moved to "/" too. The parent mount of the old_root will be
-	// new_root, so after umounting the old_root, we will see only
-	// the new_root in "/".
-	if err := syscall.PivotRoot(".", "."); err != nil {
-		return fmt.Errorf("error changing root filesystem: %v", err)
-	}
 
-	if err := syscall.Unmount(".", syscall.MNT_DETACH); err != nil {
-		return fmt.Errorf("error umounting the old root file system: %v", err)
-	}
-
-	return nil
+	return pivotRoot(chroot)
 }