overlay: use idmapped lower layers where supported

drivers/overlay/check.go

@@ -1,3 +1,4 @@
+//go:build linux
 // +build linux
 
 package overlay
@@ -11,6 +12,7 @@ import (
 	"syscall"
 
 	"github.com/containers/storage/pkg/archive"
+	"github.com/containers/storage/pkg/idtools"
 	"github.com/containers/storage/pkg/ioutils"
 	"github.com/containers/storage/pkg/mount"
 	"github.com/containers/storage/pkg/system"
@@ -218,3 +220,55 @@ func doesVolatile(d string) (bool, error) {
 	}()
 	return true, nil
 }
+
+// supportsIdmappedLowerLayers checks if the kernel supports mounting overlay on top of
+// a idmapped lower layer.
+func supportsIdmappedLowerLayers(home string) (bool, error) {
+	layerDir, err := ioutil.TempDir(home, "compat")
+	if err != nil {
+		return false, err
+	}
+	defer func() {
+		_ = os.RemoveAll(layerDir)
+	}()
+
+	mergedDir := filepath.Join(layerDir, "merged")
+	lowerDir := filepath.Join(layerDir, "lower")
+	lowerMappedDir := filepath.Join(layerDir, "lower-mapped")
+	upperDir := filepath.Join(layerDir, "upper")
+	workDir := filepath.Join(layerDir, "work")
+
+	_ = idtools.MkdirAs(mergedDir, 0700, 0, 0)
+	_ = idtools.MkdirAs(lowerDir, 0700, 0, 0)
+	_ = idtools.MkdirAs(lowerMappedDir, 0700, 0, 0)
+	_ = idtools.MkdirAs(upperDir, 0700, 0, 0)
+	_ = idtools.MkdirAs(workDir, 0700, 0, 0)
+
+	idmap := []idtools.IDMap{
+		{
+			ContainerID: 0,
+			HostID:      0,
+			Size:        1,
+		},
+	}
+	pid, cleanupFunc, err := createUsernsProcess(idmap, idmap)
+	if err != nil {
+		return false, err
+	}
+	defer cleanupFunc()
+
+	if err := createIDMappedMount(lowerDir, lowerMappedDir, int(pid)); err != nil {
+		return false, errors.Wrapf(err, "create mapped mount")
+	}
+	defer unix.Unmount(lowerMappedDir, unix.MNT_DETACH)
+
+	opts := fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", lowerMappedDir, upperDir, workDir)
+	flags := uintptr(0)
+	if err := unix.Mount("overlay", mergedDir, "overlay", flags, opts); err != nil {
+		return false, err
+	}
+	defer func() {
+		_ = unix.Unmount(mergedDir, unix.MNT_DETACH)
+	}()
+	return true, nil
+}

drivers/overlay/idmapped_utils.go

@@ -0,0 +1,160 @@
+//go:build linux
+// +build linux
+
+package overlay
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"syscall"
+	"unsafe"
+
+	"github.com/containers/storage/pkg/idtools"
+	"github.com/pkg/errors"
+	"golang.org/x/sys/unix"
+)
+
+type attr struct {
+	attrSet     uint64
+	attrClr     uint64
+	propagation uint64
+	userNs      uint64
+}
+
+const (
+	// _MOUNT_ATTR_IDMAP - Idmap mount to @userns_fd in struct mount_attr
+	_MOUNT_ATTR_IDMAP = 0x00100000 //nolint:golint
+
+	// _OPEN_TREE_CLONE - Clone the source path mount
+	_OPEN_TREE_CLONE = 0x00000001 //nolint:golint
+
+	// _MOVE_MOUNT_F_EMPTY_PATH - Move the path referenced by the fd
+	_MOVE_MOUNT_F_EMPTY_PATH = 0x00000004 //nolint:golint
+)
+
+// openTree is a wrapper for the open_tree syscall
+func openTree(path string, flags int) (fd int, err error) {
+	var _p0 *byte
+
+	if _p0, err = syscall.BytePtrFromString(path); err != nil {
+		return 0, err
+	}
+
+	r, _, e1 := syscall.Syscall6(uintptr(unix.SYS_OPEN_TREE), uintptr(0), uintptr(unsafe.Pointer(_p0)),
+		uintptr(flags), 0, 0, 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return int(r), nil
+}
+
+// moveMount is a wrapper for the the move_mount syscall.
+func moveMount(fdTree int, target string) (err error) {
+	var _p0, _p1 *byte
+
+	empty := ""
+
+	if _p0, err = syscall.BytePtrFromString(target); err != nil {
+		return err
+	}
+	if _p1, err = syscall.BytePtrFromString(empty); err != nil {
+		return err
+	}
+
+	flags := _MOVE_MOUNT_F_EMPTY_PATH
+
+	_, _, e1 := syscall.Syscall6(uintptr(unix.SYS_MOVE_MOUNT),
+		uintptr(fdTree), uintptr(unsafe.Pointer(_p1)),
+		0, uintptr(unsafe.Pointer(_p0)), uintptr(flags), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// mountSetAttr is a wrapper for the mount_setattr syscall
+func mountSetAttr(dfd int, path string, flags uint, attr *attr, size uint) (err error) {
+	var _p0 *byte
+
+	if _p0, err = syscall.BytePtrFromString(path); err != nil {
+		return err
+	}
+
+	_, _, e1 := syscall.Syscall6(uintptr(unix.SYS_MOUNT_SETATTR), uintptr(dfd), uintptr(unsafe.Pointer(_p0)),
+		uintptr(flags), uintptr(unsafe.Pointer(attr)), uintptr(size), 0)
+	if e1 != 0 {
+		err = e1
+	}
+	return
+}
+
+// createIDMappedMount creates a IDMapped bind mount from SOURCE to TARGET using the user namespace
+// for the PID process.
+func createIDMappedMount(source, target string, pid int) error {
+	path := fmt.Sprintf("/proc/%d/ns/user", pid)
+	userNsFile, err := os.Open(path)
+	if err != nil {
+		return errors.Wrapf(err, "unable to get user ns file descriptor for %q", path)
+	}
+
+	var attr attr
+	attr.attrSet = _MOUNT_ATTR_IDMAP
+	attr.attrClr = 0
+	attr.propagation = 0
+	attr.userNs = uint64(userNsFile.Fd())
+
+	defer userNsFile.Close()
+
+	targetDirFd, err := openTree(source, _OPEN_TREE_CLONE|unix.AT_RECURSIVE)
+	if err != nil {
+		return err
+	}
+	defer unix.Close(targetDirFd)
+
+	if err := mountSetAttr(targetDirFd, "", unix.AT_EMPTY_PATH|unix.AT_RECURSIVE,
+		&attr, uint(unsafe.Sizeof(attr))); err != nil {
+		return err
+	}
+	if err := os.Mkdir(target, 0700); err != nil && !os.IsExist(err) {
+		return err
+	}
+	return moveMount(targetDirFd, target)
+}
+
+// createUsernsProcess forks the current process and creates a user namespace using the specified
+// mappings.  It returns the pid of the new process.
+func createUsernsProcess(uidMaps []idtools.IDMap, gidMaps []idtools.IDMap) (int, func(), error) {
+	pid, _, err := syscall.Syscall6(uintptr(unix.SYS_CLONE), unix.CLONE_NEWUSER|uintptr(unix.SIGCHLD), 0, 0, 0, 0, 0)
+	if err != 0 {
+		return -1, nil, err
+	}
+	if pid == 0 {
+		_ = unix.Prctl(unix.PR_SET_PDEATHSIG, uintptr(unix.SIGKILL), 0, 0, 0)
+		// just wait for the SIGKILL
+		for {
+			syscall.Syscall6(uintptr(unix.SYS_PAUSE), 0, 0, 0, 0, 0, 0)
+		}
+	}
+	cleanupFunc := func() {
+		unix.Kill(int(pid), unix.SIGKILL)
+		_, _ = unix.Wait4(int(pid), nil, 0, nil)
+	}
+	writeMappings := func(fname string, idmap []idtools.IDMap) error {
+		mappings := ""
+		for _, m := range idmap {
+			mappings = mappings + fmt.Sprintf("%d %d %d\n", m.ContainerID, m.HostID, m.Size)
+		}
+		return ioutil.WriteFile(fmt.Sprintf("/proc/%d/%s", pid, fname), []byte(mappings), 0600)
+	}
+	if err := writeMappings("uid_map", uidMaps); err != nil {
+		cleanupFunc()
+		return -1, nil, err
+	}
+	if err := writeMappings("gid_map", gidMaps); err != nil {
+		cleanupFunc()
+		return -1, nil, err
+	}
+
+	return int(pid), cleanupFunc, nil
+}

drivers/overlay/overlay.go

@@ -121,6 +121,8 @@ type Driver struct {
 	supportsVolatile *bool
 	usingMetacopy    bool
 	locker           *locker.Locker
+
+	supportsIDMappedMounts *bool
 }
 
 type additionalLayerStore struct {
@@ -205,6 +207,26 @@ func checkSupportVolatile(home, runhome string) (bool, error) {
 	return usingVolatile, nil
 }
 
+// checkAndRecordIDMappedSupport checks and stores if the kernel supports mounting overlay on top of a
+// idmapped lower layer.
+func checkAndRecordIDMappedSupport(home, runhome string) (bool, error) {
+	feature := "idmapped-lower-dir"
+	overlayCacheResult, overlayCacheText, err := cachedFeatureCheck(runhome, feature)
+	if err == nil {
+		if overlayCacheResult {
+			logrus.Debugf("Cached value indicated that overlay is supported")
+			return true, nil
+		}
+		logrus.Debugf("Cached value indicated that overlay is not supported")
+		return false, errors.New(overlayCacheText)
+	}
+	supportsIDMappedMounts, err := supportsIdmappedLowerLayers(home)
+	if err2 := cachedFeatureRecord(runhome, feature, supportsIDMappedMounts, ""); err2 != nil {
+		return false, errors.Wrap(err2, "recording overlay idmapped mounts support status")
+	}
+	return supportsIDMappedMounts, err
+}
+
 func checkAndRecordOverlaySupport(fsMagic graphdriver.FsMagic, home, runhome string) (bool, error) {
 	var supportsDType bool
 
@@ -1485,6 +1507,51 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 		}
 	}
 
+	if d.supportsIDmappedMounts() && len(options.UidMaps) > 0 && len(options.GidMaps) > 0 {
+		var newAbsDir []string
+		mappedRoot := filepath.Join(d.home, id, "mapped")
+		if err := os.MkdirAll(mappedRoot, 0700); err != nil {
+			return "", err
+		}
+
+		pid, cleanupFunc, err := createUsernsProcess(options.UidMaps, options.GidMaps)
+		if err != nil {
+			return "", err
+		}
+		defer cleanupFunc()
+
+		idMappedMounts := make(map[string]string)
+
+		// rewrite the lower dirs to their idmapped mount.
+		c := 0
+		for _, absLower := range absLowers {
+			mappedMountSrc := getMappedMountRoot(absLower)
+
+			root, found := idMappedMounts[mappedMountSrc]
+			if !found {
+				root = filepath.Join(mappedRoot, fmt.Sprintf("%d", c))
+				c++
+				if err := createIDMappedMount(mappedMountSrc, root, int(pid)); err != nil {
+					return "", errors.Wrapf(err, "create mapped mount for %q on %q", mappedMountSrc, root)
+				}
+				idMappedMounts[mappedMountSrc] = root
+
+				// overlay takes a reference on the mount, so it is safe to unmount
+				// the mapped idmounts as soon as the final overlay file system is mounted.
+				defer unix.Unmount(root, unix.MNT_DETACH)
+			}
+
+			// relative path to the layer through the id mapped mount
+			rel, err := filepath.Rel(mappedMountSrc, absLower)
+			if err != nil {
+				return "", err
+			}
+
+			newAbsDir = append(newAbsDir, filepath.Join(root, rel))
+		}
+		absLowers = newAbsDir
+	}
+
 	var opts string
 	if readWrite {
 		opts = fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", strings.Join(absLowers, ":"), diffDir, workdir)
@@ -1587,6 +1654,18 @@ func (d *Driver) Put(id string) error {
 
 	unmounted := false
 
+	mappedRoot := filepath.Join(d.home, id, "mapped")
+	// It should not happen, but cleanup any mapped mount if it was leaked.
+	if _, err := os.Stat(mappedRoot); err == nil {
+		mounts, err := ioutil.ReadDir(mappedRoot)
+		if err == nil {
+			// Go through all of the mapped mounts.
+			for _, m := range mounts {
+				_ = unix.Unmount(filepath.Join(mappedRoot, m.Name()), unix.MNT_DETACH)
+			}
+		}
+	}
+
 	if d.options.mountProgram != "" {
 		// Attempt to unmount the FUSE mount using either fusermount or fusermount3.
 		// If they fail, fallback to unix.Unmount
@@ -1958,12 +2037,31 @@ func (d *Driver) UpdateLayerIDMap(id string, toContainer, toHost *idtools.IDMapp
 	return nil
 }
 
+// supportsIDmappedMounts returns whether the kernel supports using idmapped mounts with
+// overlay lower layers.
+func (d *Driver) supportsIDmappedMounts() bool {
+	if d.supportsIDMappedMounts != nil {
+		return *d.supportsIDMappedMounts
+	}
+
+	supportsIDMappedMounts, err := checkAndRecordIDMappedSupport(d.home, d.runhome)
+	d.supportsIDMappedMounts = &supportsIDMappedMounts
+	if err == nil {
+		return supportsIDMappedMounts
+	}
+	logrus.Debugf("Check for idmapped mounts support %v", err)
+	return false
+}
+
 // SupportsShifting tells whether the driver support shifting of the UIDs/GIDs in an userNS
 func (d *Driver) SupportsShifting() bool {
 	if os.Getenv("_TEST_FORCE_SUPPORT_SHIFTING") == "yes-please" {
 		return true
 	}
-	return d.options.mountProgram != ""
+	if d.options.mountProgram != "" {
+		return true
+	}
+	return d.supportsIDmappedMounts()
 }
 
 // dumbJoin is more or less a dumber version of filepath.Join, but one which
@@ -2132,3 +2230,15 @@ func redirectDiffIfAdditionalLayer(diffPath string) (string, error) {
 	}
 	return diffPath, nil
 }
+
+// getMappedMountRoot is a heuristic that calculates the parent directory where
+// the idmapped mount should be applied.
+// It is useful to minimize the number of idmapped mounts and at the same time use
+// a common path as long as possible to reduce the length of the mount data argument.
+func getMappedMountRoot(path string) string {
+	dirName := filepath.Dir(path)
+	if filepath.Base(dirName) == linkDir {
+		return filepath.Dir(dirName)
+	}
+	return dirName
+}