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overlay.go
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overlay.go
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// +build linux
package overlay // import "github.com/docker/docker/daemon/graphdriver/overlay"
import (
"fmt"
"io"
"io/ioutil"
"os"
"path"
"path/filepath"
"strconv"
"strings"
"github.com/docker/docker/daemon/graphdriver"
"github.com/docker/docker/daemon/graphdriver/copy"
"github.com/docker/docker/daemon/graphdriver/overlayutils"
"github.com/docker/docker/pkg/archive"
"github.com/docker/docker/pkg/containerfs"
"github.com/docker/docker/pkg/fsutils"
"github.com/docker/docker/pkg/idtools"
"github.com/docker/docker/pkg/parsers"
"github.com/docker/docker/pkg/system"
"github.com/moby/locker"
"github.com/moby/sys/mount"
"github.com/opencontainers/selinux/go-selinux/label"
"github.com/sirupsen/logrus"
"golang.org/x/sys/unix"
)
// This is a small wrapper over the NaiveDiffWriter that lets us have a custom
// implementation of ApplyDiff()
var (
// ErrApplyDiffFallback is returned to indicate that a normal ApplyDiff is applied as a fallback from Naive diff writer.
ErrApplyDiffFallback = fmt.Errorf("Fall back to normal ApplyDiff")
backingFs = "<unknown>"
)
// ApplyDiffProtoDriver wraps the ProtoDriver by extending the interface with ApplyDiff method.
type ApplyDiffProtoDriver interface {
graphdriver.ProtoDriver
// ApplyDiff writes the diff to the archive for the given id and parent id.
// It returns the size in bytes written if successful, an error ErrApplyDiffFallback is returned otherwise.
ApplyDiff(id, parent string, diff io.Reader) (size int64, err error)
}
type naiveDiffDriverWithApply struct {
graphdriver.Driver
applyDiff ApplyDiffProtoDriver
}
// NaiveDiffDriverWithApply returns a NaiveDiff driver with custom ApplyDiff.
func NaiveDiffDriverWithApply(driver ApplyDiffProtoDriver, uidMaps, gidMaps []idtools.IDMap) graphdriver.Driver {
return &naiveDiffDriverWithApply{
Driver: graphdriver.NewNaiveDiffDriver(driver, uidMaps, gidMaps),
applyDiff: driver,
}
}
// ApplyDiff creates a diff layer with either the NaiveDiffDriver or with a fallback.
func (d *naiveDiffDriverWithApply) ApplyDiff(id, parent string, diff io.Reader) (int64, error) {
b, err := d.applyDiff.ApplyDiff(id, parent, diff)
if err == ErrApplyDiffFallback {
return d.Driver.ApplyDiff(id, parent, diff)
}
return b, err
}
// This backend uses the overlay union filesystem for containers
// plus hard link file sharing for images.
// Each container/image can have a "root" subdirectory which is a plain
// filesystem hierarchy, or they can use overlay.
// If they use overlay there is a "upper" directory and a "lower-id"
// file, as well as "merged" and "work" directories. The "upper"
// directory has the upper layer of the overlay, and "lower-id" contains
// the id of the parent whose "root" directory shall be used as the lower
// layer in the overlay. The overlay itself is mounted in the "merged"
// directory, and the "work" dir is needed for overlay to work.
// When an overlay layer is created there are two cases, either the
// parent has a "root" dir, then we start out with an empty "upper"
// directory overlaid on the parents root. This is typically the
// case with the init layer of a container which is based on an image.
// If there is no "root" in the parent, we inherit the lower-id from
// the parent and start by making a copy in the parent's "upper" dir.
// This is typically the case for a container layer which copies
// its parent -init upper layer.
// Additionally we also have a custom implementation of ApplyLayer
// which makes a recursive copy of the parent "root" layer using
// hardlinks to share file data, and then applies the layer on top
// of that. This means all child images share file (but not directory)
// data with the parent.
type overlayOptions struct{}
// Driver contains information about the home directory and the list of active mounts that are created using this driver.
type Driver struct {
home string
uidMaps []idtools.IDMap
gidMaps []idtools.IDMap
ctr *graphdriver.RefCounter
supportsDType bool
locker *locker.Locker
}
func init() {
graphdriver.Register("overlay", Init)
}
// Init returns the NaiveDiffDriver, a native diff driver for overlay filesystem.
// If overlay filesystem is not supported on the host, the error
// graphdriver.ErrNotSupported is returned.
// If an overlay filesystem is not supported over an existing filesystem then
// error graphdriver.ErrIncompatibleFS is returned.
func Init(home string, options []string, uidMaps, gidMaps []idtools.IDMap) (graphdriver.Driver, error) {
_, err := parseOptions(options)
if err != nil {
return nil, err
}
// Perform feature detection on /var/lib/docker/overlay if it's an existing directory.
// This covers situations where /var/lib/docker/overlay is a mount, and on a different
// filesystem than /var/lib/docker.
// If the path does not exist, fall back to using /var/lib/docker for feature detection.
testdir := home
if _, err := os.Stat(testdir); os.IsNotExist(err) {
testdir = filepath.Dir(testdir)
}
if err := overlayutils.SupportsOverlay(testdir, false); err != nil {
logrus.WithField("storage-driver", "overlay").Error(err)
return nil, graphdriver.ErrNotSupported
}
fsMagic, err := graphdriver.GetFSMagic(testdir)
if err != nil {
return nil, err
}
if fsName, ok := graphdriver.FsNames[fsMagic]; ok {
backingFs = fsName
}
supportsDType, err := fsutils.SupportsDType(testdir)
if err != nil {
return nil, err
}
if !supportsDType {
if !graphdriver.IsInitialized(home) {
return nil, overlayutils.ErrDTypeNotSupported("overlay", backingFs)
}
// allow running without d_type only for existing setups (#27443)
logrus.WithField("storage-driver", "overlay").Warn(overlayutils.ErrDTypeNotSupported("overlay", backingFs))
}
// Create the driver home dir
if err := idtools.MkdirAllAndChown(home, 0701, idtools.CurrentIdentity()); err != nil {
return nil, err
}
d := &Driver{
home: home,
uidMaps: uidMaps,
gidMaps: gidMaps,
ctr: graphdriver.NewRefCounter(graphdriver.NewFsChecker(graphdriver.FsMagicOverlay)),
supportsDType: supportsDType,
locker: locker.New(),
}
return NaiveDiffDriverWithApply(d, uidMaps, gidMaps), nil
}
func parseOptions(options []string) (*overlayOptions, error) {
o := &overlayOptions{}
for _, option := range options {
key, _, err := parsers.ParseKeyValueOpt(option)
if err != nil {
return nil, err
}
key = strings.ToLower(key)
switch key {
default:
return nil, fmt.Errorf("overlay: unknown option %s", key)
}
}
return o, nil
}
func (d *Driver) String() string {
return "overlay"
}
// Status returns current driver information in a two dimensional string array.
// Output contains "Backing Filesystem" used in this implementation.
func (d *Driver) Status() [][2]string {
return [][2]string{
{"Backing Filesystem", backingFs},
{"Supports d_type", strconv.FormatBool(d.supportsDType)},
}
}
// GetMetadata returns metadata about the overlay driver such as root,
// LowerDir, UpperDir, WorkDir and MergeDir used to store data.
func (d *Driver) GetMetadata(id string) (map[string]string, error) {
dir := d.dir(id)
if _, err := os.Stat(dir); err != nil {
return nil, err
}
metadata := make(map[string]string)
// If id has a root, it is an image
rootDir := path.Join(dir, "root")
if _, err := os.Stat(rootDir); err == nil {
metadata["RootDir"] = rootDir
return metadata, nil
}
lowerID, err := ioutil.ReadFile(path.Join(dir, "lower-id"))
if err != nil {
return nil, err
}
metadata["LowerDir"] = path.Join(d.dir(string(lowerID)), "root")
metadata["UpperDir"] = path.Join(dir, "upper")
metadata["WorkDir"] = path.Join(dir, "work")
metadata["MergedDir"] = path.Join(dir, "merged")
return metadata, nil
}
// Cleanup any state created by overlay which should be cleaned when daemon
// is being shutdown. For now, we just have to unmount the bind mounted
// we had created.
func (d *Driver) Cleanup() error {
return mount.RecursiveUnmount(d.home)
}
// CreateReadWrite creates a layer that is writable for use as a container
// file system.
func (d *Driver) CreateReadWrite(id, parent string, opts *graphdriver.CreateOpts) error {
return d.Create(id, parent, opts)
}
// Create is used to create the upper, lower, and merge directories required for overlay fs for a given id.
// The parent filesystem is used to configure these directories for the overlay.
func (d *Driver) Create(id, parent string, opts *graphdriver.CreateOpts) (retErr error) {
if opts != nil && len(opts.StorageOpt) != 0 {
return fmt.Errorf("--storage-opt is not supported for overlay")
}
dir := d.dir(id)
rootUID, rootGID, err := idtools.GetRootUIDGID(d.uidMaps, d.gidMaps)
if err != nil {
return err
}
root := idtools.Identity{UID: rootUID, GID: rootGID}
currentID := idtools.CurrentIdentity()
if err := idtools.MkdirAllAndChown(path.Dir(dir), 0701, currentID); err != nil {
return err
}
if err := idtools.MkdirAndChown(dir, 0701, currentID); err != nil {
return err
}
defer func() {
// Clean up on failure
if retErr != nil {
os.RemoveAll(dir)
}
}()
// Toplevel images are just a "root" dir
if parent == "" {
// This must be 0755 otherwise unprivileged users will in the container will not be able to read / in the container
return idtools.MkdirAndChown(path.Join(dir, "root"), 0755, root)
}
parentDir := d.dir(parent)
// Ensure parent exists
if _, err := os.Lstat(parentDir); err != nil {
return err
}
// If parent has a root, just do an overlay to it
parentRoot := path.Join(parentDir, "root")
if s, err := os.Lstat(parentRoot); err == nil {
if err := idtools.MkdirAndChown(path.Join(dir, "upper"), s.Mode(), root); err != nil {
return err
}
if err := idtools.MkdirAndChown(path.Join(dir, "work"), 0700, root); err != nil {
return err
}
return ioutil.WriteFile(path.Join(dir, "lower-id"), []byte(parent), 0600)
}
// Otherwise, copy the upper and the lower-id from the parent
lowerID, err := ioutil.ReadFile(path.Join(parentDir, "lower-id"))
if err != nil {
return err
}
if err := ioutil.WriteFile(path.Join(dir, "lower-id"), lowerID, 0600); err != nil {
return err
}
parentUpperDir := path.Join(parentDir, "upper")
s, err := os.Lstat(parentUpperDir)
if err != nil {
return err
}
upperDir := path.Join(dir, "upper")
if err := idtools.MkdirAndChown(upperDir, s.Mode(), root); err != nil {
return err
}
if err := idtools.MkdirAndChown(path.Join(dir, "work"), 0700, root); err != nil {
return err
}
return copy.DirCopy(parentUpperDir, upperDir, copy.Content, true)
}
func (d *Driver) dir(id string) string {
return path.Join(d.home, id)
}
// Remove cleans the directories that are created for this id.
func (d *Driver) Remove(id string) error {
if id == "" {
return fmt.Errorf("refusing to remove the directories: id is empty")
}
d.locker.Lock(id)
defer d.locker.Unlock(id)
return system.EnsureRemoveAll(d.dir(id))
}
// Get creates and mounts the required file system for the given id and returns the mount path.
func (d *Driver) Get(id, mountLabel string) (_ containerfs.ContainerFS, err error) {
d.locker.Lock(id)
defer d.locker.Unlock(id)
dir := d.dir(id)
if _, err := os.Stat(dir); err != nil {
return nil, err
}
// If id has a root, just return it
rootDir := path.Join(dir, "root")
if _, err := os.Stat(rootDir); err == nil {
return containerfs.NewLocalContainerFS(rootDir), nil
}
mergedDir := path.Join(dir, "merged")
if count := d.ctr.Increment(mergedDir); count > 1 {
return containerfs.NewLocalContainerFS(mergedDir), nil
}
defer func() {
if err != nil {
if c := d.ctr.Decrement(mergedDir); c <= 0 {
if mntErr := unix.Unmount(mergedDir, 0); mntErr != nil {
logrus.WithField("storage-driver", "overlay").Debugf("Failed to unmount %s: %v: %v", id, mntErr, err)
}
// Cleanup the created merged directory; see the comment in Put's rmdir
if rmErr := unix.Rmdir(mergedDir); rmErr != nil && !os.IsNotExist(rmErr) {
logrus.WithField("storage-driver", "overlay").Warnf("Failed to remove %s: %v: %v", id, rmErr, err)
}
}
}
}()
lowerID, err := ioutil.ReadFile(path.Join(dir, "lower-id"))
if err != nil {
return nil, err
}
rootUID, rootGID, err := idtools.GetRootUIDGID(d.uidMaps, d.gidMaps)
if err != nil {
return nil, err
}
if err := idtools.MkdirAndChown(mergedDir, 0700, idtools.Identity{UID: rootUID, GID: rootGID}); err != nil {
return nil, err
}
var (
lowerDir = path.Join(d.dir(string(lowerID)), "root")
upperDir = path.Join(dir, "upper")
workDir = path.Join(dir, "work")
opts = fmt.Sprintf("lowerdir=%s,upperdir=%s,workdir=%s", lowerDir, upperDir, workDir)
)
if err := unix.Mount("overlay", mergedDir, "overlay", 0, label.FormatMountLabel(opts, mountLabel)); err != nil {
return nil, fmt.Errorf("error creating overlay mount to %s: %v", mergedDir, err)
}
// chown "workdir/work" to the remapped root UID/GID. Overlay fs inside a
// user namespace requires this to move a directory from lower to upper.
if err := os.Chown(path.Join(workDir, "work"), rootUID, rootGID); err != nil {
return nil, err
}
return containerfs.NewLocalContainerFS(mergedDir), nil
}
// Put unmounts the mount path created for the give id.
// It also removes the 'merged' directory to force the kernel to unmount the
// overlay mount in other namespaces.
func (d *Driver) Put(id string) error {
d.locker.Lock(id)
defer d.locker.Unlock(id)
// If id has a root, just return
if _, err := os.Stat(path.Join(d.dir(id), "root")); err == nil {
return nil
}
mountpoint := path.Join(d.dir(id), "merged")
logger := logrus.WithField("storage-driver", "overlay")
if count := d.ctr.Decrement(mountpoint); count > 0 {
return nil
}
if err := unix.Unmount(mountpoint, unix.MNT_DETACH); err != nil {
logger.Debugf("Failed to unmount %s overlay: %v", id, err)
}
// Remove the mountpoint here. Removing the mountpoint (in newer kernels)
// will cause all other instances of this mount in other mount namespaces
// to be unmounted. This is necessary to avoid cases where an overlay mount
// that is present in another namespace will cause subsequent mounts
// operations to fail with ebusy. We ignore any errors here because this may
// fail on older kernels which don't have
// torvalds/linux@8ed936b5671bfb33d89bc60bdcc7cf0470ba52fe applied.
if err := unix.Rmdir(mountpoint); err != nil {
logger.Debugf("Failed to remove %s overlay: %v", id, err)
}
return nil
}
// ApplyDiff applies the new layer on top of the root, if parent does not exist with will return an ErrApplyDiffFallback error.
func (d *Driver) ApplyDiff(id string, parent string, diff io.Reader) (size int64, err error) {
dir := d.dir(id)
if parent == "" {
return 0, ErrApplyDiffFallback
}
parentRootDir := path.Join(d.dir(parent), "root")
if _, err := os.Stat(parentRootDir); err != nil {
return 0, ErrApplyDiffFallback
}
// We now know there is a parent, and it has a "root" directory containing
// the full root filesystem. We can just hardlink it and apply the
// layer. This relies on two things:
// 1) ApplyDiff is only run once on a clean (no writes to upper layer) container
// 2) ApplyDiff doesn't do any in-place writes to files (would break hardlinks)
// These are all currently true and are not expected to break
tmpRootDir, err := ioutil.TempDir(dir, "tmproot")
if err != nil {
return 0, err
}
defer func() {
if err != nil {
os.RemoveAll(tmpRootDir)
} else {
os.RemoveAll(path.Join(dir, "upper"))
os.RemoveAll(path.Join(dir, "work"))
os.RemoveAll(path.Join(dir, "merged"))
os.RemoveAll(path.Join(dir, "lower-id"))
}
}()
if err = copy.DirCopy(parentRootDir, tmpRootDir, copy.Hardlink, true); err != nil {
return 0, err
}
options := &archive.TarOptions{UIDMaps: d.uidMaps, GIDMaps: d.gidMaps}
if size, err = graphdriver.ApplyUncompressedLayer(tmpRootDir, diff, options); err != nil {
return 0, err
}
rootDir := path.Join(dir, "root")
if err := os.Rename(tmpRootDir, rootDir); err != nil {
return 0, err
}
return
}
// Exists checks to see if the id is already mounted.
func (d *Driver) Exists(id string) bool {
_, err := os.Stat(d.dir(id))
return err == nil
}