overlay: use xattrs perms with network FSs

when running in a user namespace on a network file system,
automatically switch on the permission xattrs feature so that users
won't have to tweak their storage.conf file with difficult to guess
options.

The list of remote file systems was taken from coreutils stat:

https://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=blob;f=src/stat.c;h=0c34501e34410b9096edf377654b9ff27fe9fa24;hb=HEAD#l260

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

drivers/overlay/overlay.go

@@ -248,6 +248,23 @@ func (d *Driver) getSupportsVolatile() (bool, error) {
 	return supportsVolatile, nil
 }
 
+// isNetworkFileSystem checks if the specified file system is supported by native overlay
+// as backing store when running in a user namespace.
+func isNetworkFileSystem(fsMagic graphdriver.FsMagic) bool {
+	switch fsMagic {
+	// a bunch of network file systems...
+	case graphdriver.FsMagicNfsFs, graphdriver.FsMagicSmbFs, graphdriver.FsMagicAcfs,
+		graphdriver.FsMagicAfs, graphdriver.FsMagicCephFs, graphdriver.FsMagicCIFS,
+		graphdriver.FsMagicFHGFSFs, graphdriver.FsMagicGPFS, graphdriver.FsMagicIBRIX,
+		graphdriver.FsMagicKAFS, graphdriver.FsMagicLUSTRE, graphdriver.FsMagicNCP,
+		graphdriver.FsMagicNFSD, graphdriver.FsMagicOCFS2, graphdriver.FsMagicPANFS,
+		graphdriver.FsMagicPRLFS, graphdriver.FsMagicSMB2, graphdriver.FsMagicSNFS,
+		graphdriver.FsMagicVBOXSF, graphdriver.FsMagicVXFS:
+		return true
+	}
+	return false
+}
+
 // Init returns the a native diff driver for overlay filesystem.
 // If overlay filesystem is not supported on the host, a wrapped graphdriver.ErrNotSupported is returned as error.
 // If an overlay filesystem is not supported over an existing filesystem then a wrapped graphdriver.ErrIncompatibleFS is returned.
@@ -266,18 +283,27 @@ func Init(home string, options graphdriver.Options) (graphdriver.Driver, error)
 	}
 
 	if opts.mountProgram != "" {
+		if unshare.IsRootless() && isNetworkFileSystem(fsMagic) && opts.forceMask == nil {
+			m := os.FileMode(0700)
+			opts.forceMask = &m
+			logrus.Debugf("Network file system detected as backing store.  Enforcing overlay option `force_mask=\"%o\"`.", m)
+		}
+
 		if err := ioutil.WriteFile(getMountProgramFlagFile(home), []byte("true"), 0600); err != nil {
 			return nil, err
 		}
 	} else {
-		// check if they are running over btrfs, aufs, zfs, overlay, or ecryptfs
 		if opts.forceMask != nil {
 			return nil, errors.New("'force_mask' is supported only with 'mount_program'")
 		}
+		// check if they are running over btrfs, aufs, zfs, overlay, or ecryptfs
 		switch fsMagic {
 		case graphdriver.FsMagicAufs, graphdriver.FsMagicZfs, graphdriver.FsMagicOverlay, graphdriver.FsMagicEcryptfs:
 			return nil, errors.Wrapf(graphdriver.ErrIncompatibleFS, "'overlay' is not supported over %s, a mount_program is required", backingFs)
 		}
+		if unshare.IsRootless() && isNetworkFileSystem(fsMagic) {
+			return nil, errors.Wrapf(graphdriver.ErrIncompatibleFS, "A network file system with user namespaces is not supported.  Please use a mount_program")
+		}
 	}
 
 	rootUID, rootGID, err := idtools.GetRootUIDGID(options.UIDMaps, options.GIDMaps)
@@ -1431,6 +1457,11 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 				label = d.optsAppendMappings(label, options.UidMaps, options.GidMaps)
 			}
 
+			// if forceMask is in place, tell fuse-overlayfs to write the permissions mask to an unprivileged xattr as well.
+			if d.options.forceMask != nil {
+				label = label + ",xattr_permissions=2"
+			}
+
 			mountProgram := exec.Command(d.options.mountProgram, "-o", label, target)
 			mountProgram.Dir = d.home
 			var b bytes.Buffer