Written by: Neil Brown
  Please see MAINTAINERS file for where to send questions.

  
  Overlay Filesystem
  ==================
  
  This document describes a prototype for a new approach to providing
  overlay-filesystem functionality in Linux (sometimes referred to as
  union-filesystems).  An overlay-filesystem tries to present a
  filesystem which is the result over overlaying one filesystem on top
  of the other.

  
  Overlay objects
  ---------------
  
  The overlay filesystem approach is 'hybrid', because the objects that
  appear in the filesystem do not always appear to belong to that filesystem.
  In many cases, an object accessed in the union will be indistinguishable

  from accessing the corresponding object from the original filesystem.
  This is most obvious from the 'st_dev' field returned by stat(2).
  
  While directories will report an st_dev from the overlay-filesystem,

  non-directory objects may report an st_dev from the lower filesystem or

  upper filesystem that is providing the object.  Similarly st_ino will
  only be unique when combined with st_dev, and both of these can change
  over the lifetime of a non-directory object.  Many applications and
  tools ignore these values and will not be affected.

  In the special case of all overlay layers on the same underlying
  filesystem, all objects will report an st_dev from the overlay
  filesystem and st_ino from the underlying filesystem.  This will
  make the overlay mount more compliant with filesystem scanners and
  overlay objects will be distinguishable from the corresponding
  objects in the original filesystem.

  On 64bit systems, even if all overlay layers are not on the same
  underlying filesystem, the same compliant behavior could be achieved
  with the "xino" feature.  The "xino" feature composes a unique object
  identifier from the real object st_ino and an underlying fsid index.
  If all underlying filesystems support NFS file handles and export file
  handles with 32bit inode number encoding (e.g. ext4), overlay filesystem
  will use the high inode number bits for fsid.  Even when the underlying
  filesystem uses 64bit inode numbers, users can still enable the "xino"
  feature with the "-o xino=on" overlay mount option.  That is useful for the
  case of underlying filesystems like xfs and tmpfs, which use 64bit inode
  numbers, but are very unlikely to use the high inode number bit.

  Upper and Lower
  ---------------
  
  An overlay filesystem combines two filesystems - an 'upper' filesystem
  and a 'lower' filesystem.  When a name exists in both filesystems, the
  object in the 'upper' filesystem is visible while the object in the
  'lower' filesystem is either hidden or, in the case of directories,
  merged with the 'upper' object.
  
  It would be more correct to refer to an upper and lower 'directory
  tree' rather than 'filesystem' as it is quite possible for both
  directory trees to be in the same filesystem and there is no
  requirement that the root of a filesystem be given for either upper or
  lower.
  
  The lower filesystem can be any filesystem supported by Linux and does
  not need to be writable.  The lower filesystem can even be another
  overlayfs.  The upper filesystem will normally be writable and if it
  is it must support the creation of trusted.* extended attributes, and
  must provide valid d_type in readdir responses, so NFS is not suitable.
  
  A read-only overlay of two read-only filesystems may use any
  filesystem type.
  
  Directories
  -----------
  
  Overlaying mainly involves directories.  If a given name appears in both
  upper and lower filesystems and refers to a non-directory in either,
  then the lower object is hidden - the name refers only to the upper
  object.
  
  Where both upper and lower objects are directories, a merged directory
  is formed.
  
  At mount time, the two directories given as mount options "lowerdir" and
  "upperdir" are combined into a merged directory:

    mount -t overlay overlay -olowerdir=/lower,upperdir=/upper,\

    workdir=/work /merged

  
  The "workdir" needs to be an empty directory on the same filesystem
  as upperdir.
  
  Then whenever a lookup is requested in such a merged directory, the
  lookup is performed in each actual directory and the combined result
  is cached in the dentry belonging to the overlay filesystem.  If both
  actual lookups find directories, both are stored and a merged
  directory is created, otherwise only one is stored: the upper if it
  exists, else the lower.
  
  Only the lists of names from directories are merged.  Other content
  such as metadata and extended attributes are reported for the upper
  directory only.  These attributes of the lower directory are hidden.
  
  whiteouts and opaque directories
  --------------------------------
  
  In order to support rm and rmdir without changing the lower
  filesystem, an overlay filesystem needs to record in the upper filesystem
  that files have been removed.  This is done using whiteouts and opaque
  directories (non-directories are always opaque).
  
  A whiteout is created as a character device with 0/0 device number.
  When a whiteout is found in the upper level of a merged directory, any
  matching name in the lower level is ignored, and the whiteout itself
  is also hidden.
  
  A directory is made opaque by setting the xattr "trusted.overlay.opaque"
  to "y".  Where the upper filesystem contains an opaque directory, any
  directory in the lower filesystem with the same name is ignored.
  
  readdir
  -------
  
  When a 'readdir' request is made on a merged directory, the upper and
  lower directories are each read and the name lists merged in the
  obvious way (upper is read first, then lower - entries that already
  exist are not re-added).  This merged name list is cached in the
  'struct file' and so remains as long as the file is kept open.  If the
  directory is opened and read by two processes at the same time, they
  will each have separate caches.  A seekdir to the start of the
  directory (offset 0) followed by a readdir will cause the cache to be
  discarded and rebuilt.
  
  This means that changes to the merged directory do not appear while a
  directory is being read.  This is unlikely to be noticed by many
  programs.
  
  seek offsets are assigned sequentially when the directories are read.
  Thus if

    - read part of a directory
    - remember an offset, and close the directory
    - re-open the directory some time later
    - seek to the remembered offset
  
  there may be little correlation between the old and new locations in
  the list of filenames, particularly if anything has changed in the
  directory.
  
  Readdir on directories that are not merged is simply handled by the
  underlying directory (upper or lower).

  renaming directories
  --------------------
  
  When renaming a directory that is on the lower layer or merged (i.e. the
  directory was not created on the upper layer to start with) overlayfs can
  handle it in two different ways:

  1. return EXDEV error: this error is returned by rename(2) when trying to

     move a file or directory across filesystem boundaries.  Hence
     applications are usually prepared to hande this error (mv(1) for example
     recursively copies the directory tree).  This is the default behavior.

  2. If the "redirect_dir" feature is enabled, then the directory will be

     copied up (but not the contents).  Then the "trusted.overlay.redirect"
     extended attribute is set to the path of the original location from the
     root of the overlay.  Finally the directory is moved to the new
     location.

  There are several ways to tune the "redirect_dir" feature.
  
  Kernel config options:
  
  - OVERLAY_FS_REDIRECT_DIR:
      If this is enabled, then redirect_dir is turned on by  default.
  - OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW:
      If this is enabled, then redirects are always followed by default. Enabling
      this results in a less secure configuration.  Enable this option only when
      worried about backward compatibility with kernels that have the redirect_dir
      feature and follow redirects even if turned off.
  
  Module options (can also be changed through /sys/module/overlay/parameters/*):
  
  - "redirect_dir=BOOL":
      See OVERLAY_FS_REDIRECT_DIR kernel config option above.
  - "redirect_always_follow=BOOL":
      See OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW kernel config option above.
  - "redirect_max=NUM":
      The maximum number of bytes in an absolute redirect (default is 256).
  
  Mount options:
  
  - "redirect_dir=on":
      Redirects are enabled.
  - "redirect_dir=follow":
      Redirects are not created, but followed.
  - "redirect_dir=off":
      Redirects are not created and only followed if "redirect_always_follow"
      feature is enabled in the kernel/module config.
  - "redirect_dir=nofollow":
      Redirects are not created and not followed (equivalent to "redirect_dir=off"
      if "redirect_always_follow" feature is not enabled).

  When the NFS export feature is enabled, every copied up directory is
  indexed by the file handle of the lower inode and a file handle of the
  upper directory is stored in a "trusted.overlay.upper" extended attribute
  on the index entry.  On lookup of a merged directory, if the upper
  directory does not match the file handle stores in the index, that is an
  indication that multiple upper directories may be redirected to the same
  lower directory.  In that case, lookup returns an error and warns about
  a possible inconsistency.
  
  Because lower layer redirects cannot be verified with the index, enabling
  NFS export support on an overlay filesystem with no upper layer requires
  turning off redirect follow (e.g. "redirect_dir=nofollow").

  Non-directories
  ---------------
  
  Objects that are not directories (files, symlinks, device-special
  files etc.) are presented either from the upper or lower filesystem as
  appropriate.  When a file in the lower filesystem is accessed in a way
  the requires write-access, such as opening for write access, changing
  some metadata etc., the file is first copied from the lower filesystem
  to the upper filesystem (copy_up).  Note that creating a hard-link
  also requires copy_up, though of course creation of a symlink does
  not.
  
  The copy_up may turn out to be unnecessary, for example if the file is
  opened for read-write but the data is not modified.
  
  The copy_up process first makes sure that the containing directory
  exists in the upper filesystem - creating it and any parents as
  necessary.  It then creates the object with the same metadata (owner,
  mode, mtime, symlink-target etc.) and then if the object is a file, the
  data is copied from the lower to the upper filesystem.  Finally any
  extended attributes are copied up.
  
  Once the copy_up is complete, the overlay filesystem simply
  provides direct access to the newly created file in the upper
  filesystem - future operations on the file are barely noticed by the
  overlay filesystem (though an operation on the name of the file such as
  rename or unlink will of course be noticed and handled).

  Multiple lower layers
  ---------------------
  
  Multiple lower layers can now be given using the the colon (":") as a
  separator character between the directory names.  For example:
  
    mount -t overlay overlay -olowerdir=/lower1:/lower2:/lower3 /merged

  As the example shows, "upperdir=" and "workdir=" may be omitted.  In
  that case the overlay will be read-only.
  
  The specified lower directories will be stacked beginning from the
  rightmost one and going left.  In the above example lower1 will be the
  top, lower2 the middle and lower3 the bottom layer.

  Metadata only copy up
  --------------------
  
  When metadata only copy up feature is enabled, overlayfs will only copy
  up metadata (as opposed to whole file), when a metadata specific operation
  like chown/chmod is performed. Full file will be copied up later when
  file is opened for WRITE operation.
  
  In other words, this is delayed data copy up operation and data is copied
  up when there is a need to actually modify data.
  
  There are multiple ways to enable/disable this feature. A config option
  CONFIG_OVERLAY_FS_METACOPY can be set/unset to enable/disable this feature
  by default. Or one can enable/disable it at module load time with module
  parameter metacopy=on/off. Lastly, there is also a per mount option
  metacopy=on/off to enable/disable this feature per mount.
  
  Do not use metacopy=on with untrusted upper/lower directories. Otherwise
  it is possible that an attacker can create a handcrafted file with
  appropriate REDIRECT and METACOPY xattrs, and gain access to file on lower
  pointed by REDIRECT. This should not be possible on local system as setting
  "trusted." xattrs will require CAP_SYS_ADMIN. But it should be possible
  for untrusted layers like from a pen drive.

  Note: redirect_dir={off|nofollow|follow(*)} conflicts with metacopy=on, and
  results in an error.
  
  (*) redirect_dir=follow only conflicts with metacopy=on if upperdir=... is
  given.

  Sharing and copying layers
  --------------------------
  
  Lower layers may be shared among several overlay mounts and that is indeed
  a very common practice.  An overlay mount may use the same lower layer
  path as another overlay mount and it may use a lower layer path that is
  beneath or above the path of another overlay lower layer path.
  
  Using an upper layer path and/or a workdir path that are already used by

  another overlay mount is not allowed and may fail with EBUSY.  Using

  partially overlapping paths is not allowed but will not fail with EBUSY.

  If files are accessed from two overlayfs mounts which share or overlap the
  upper layer and/or workdir path the behavior of the overlay is undefined,
  though it will not result in a crash or deadlock.

  
  Mounting an overlay using an upper layer path, where the upper layer path
  was previously used by another mounted overlay in combination with a
  different lower layer path, is allowed, unless the "inodes index" feature

  or "metadata only copy up" feature is enabled.

  
  With the "inodes index" feature, on the first time mount, an NFS file
  handle of the lower layer root directory, along with the UUID of the lower
  filesystem, are encoded and stored in the "trusted.overlay.origin" extended
  attribute on the upper layer root directory.  On subsequent mount attempts,
  the lower root directory file handle and lower filesystem UUID are compared
  to the stored origin in upper root directory.  On failure to verify the
  lower root origin, mount will fail with ESTALE.  An overlayfs mount with
  "inodes index" enabled will fail with EOPNOTSUPP if the lower filesystem
  does not support NFS export, lower filesystem does not have a valid UUID or
  if the upper filesystem does not support extended attributes.

  For "metadata only copy up" feature there is no verification mechanism at
  mount time. So if same upper is mounted with different set of lower, mount
  probably will succeed but expect the unexpected later on. So don't do it.

  It is quite a common practice to copy overlay layers to a different
  directory tree on the same or different underlying filesystem, and even
  to a different machine.  With the "inodes index" feature, trying to mount
  the copied layers will fail the verification of the lower root file handle.

  Non-standard behavior
  ---------------------

  Overlayfs can now act as a POSIX compliant filesystem with the following
  features turned on:
  
  1) "redirect_dir"
  
  Enabled with the mount option or module option: "redirect_dir=on" or with
  the kernel config option CONFIG_OVERLAY_FS_REDIRECT_DIR=y.
  
  If this feature is disabled, then rename(2) on a lower or merged directory
  will fail with EXDEV ("Invalid cross-device link").
  
  2) "inode index"
  
  Enabled with the mount option or module option "index=on" or with the
  kernel config option CONFIG_OVERLAY_FS_INDEX=y.

  If this feature is disabled and a file with multiple hard links is copied
  up, then this will "break" the link.  Changes will not be propagated to
  other names referring to the same inode.

  3) "xino"

  Enabled with the mount option "xino=auto" or "xino=on", with the module
  option "xino_auto=on" or with the kernel config option
  CONFIG_OVERLAY_FS_XINO_AUTO=y.  Also implicitly enabled by using the same
  underlying filesystem for all layers making up the overlay.

  If this feature is disabled or the underlying filesystem doesn't have
  enough free bits in the inode number, then overlayfs will not be able to
  guarantee that the values of st_ino and st_dev returned by stat(2) and the
  value of d_ino returned by readdir(3) will act like on a normal filesystem.
  E.g. the value of st_dev may be different for two objects in the same
  overlay filesystem and the value of st_ino for directory objects may not be
  persistent and could change even while the overlay filesystem is mounted.

  Changes to underlying filesystems
  ---------------------------------
  
  Offline changes, when the overlay is not mounted, are allowed to either
  the upper or the lower trees.
  
  Changes to the underlying filesystems while part of a mounted overlay
  filesystem are not allowed.  If the underlying filesystem is changed,
  the behavior of the overlay is undefined, though it will not result in
  a crash or deadlock.

  When the overlay NFS export feature is enabled, overlay filesystems
  behavior on offline changes of the underlying lower layer is different
  than the behavior when NFS export is disabled.
  
  On every copy_up, an NFS file handle of the lower inode, along with the
  UUID of the lower filesystem, are encoded and stored in an extended
  attribute "trusted.overlay.origin" on the upper inode.
  
  When the NFS export feature is enabled, a lookup of a merged directory,
  that found a lower directory at the lookup path or at the path pointed
  to by the "trusted.overlay.redirect" extended attribute, will verify
  that the found lower directory file handle and lower filesystem UUID
  match the origin file handle that was stored at copy_up time.  If a
  found lower directory does not match the stored origin, that directory
  will not be merged with the upper directory.

  
  NFS export
  ----------
  
  When the underlying filesystems supports NFS export and the "nfs_export"
  feature is enabled, an overlay filesystem may be exported to NFS.
  
  With the "nfs_export" feature, on copy_up of any lower object, an index
  entry is created under the index directory.  The index entry name is the
  hexadecimal representation of the copy up origin file handle.  For a
  non-directory object, the index entry is a hard link to the upper inode.
  For a directory object, the index entry has an extended attribute
  "trusted.overlay.upper" with an encoded file handle of the upper
  directory inode.
  
  When encoding a file handle from an overlay filesystem object, the
  following rules apply:
  
  1. For a non-upper object, encode a lower file handle from lower inode
  2. For an indexed object, encode a lower file handle from copy_up origin
  3. For a pure-upper object and for an existing non-indexed upper object,
     encode an upper file handle from upper inode
  
  The encoded overlay file handle includes:
   - Header including path type information (e.g. lower/upper)
   - UUID of the underlying filesystem
   - Underlying filesystem encoding of underlying inode
  
  This encoding format is identical to the encoding format file handles that
  are stored in extended attribute "trusted.overlay.origin".
  
  When decoding an overlay file handle, the following steps are followed:
  
  1. Find underlying layer by UUID and path type information.
  2. Decode the underlying filesystem file handle to underlying dentry.
  3. For a lower file handle, lookup the handle in index directory by name.
  4. If a whiteout is found in index, return ESTALE. This represents an
     overlay object that was deleted after its file handle was encoded.
  5. For a non-directory, instantiate a disconnected overlay dentry from the
     decoded underlying dentry, the path type and index inode, if found.
  6. For a directory, use the connected underlying decoded dentry, path type
     and index, to lookup a connected overlay dentry.
  
  Decoding a non-directory file handle may return a disconnected dentry.
  copy_up of that disconnected dentry will create an upper index entry with
  no upper alias.
  
  When overlay filesystem has multiple lower layers, a middle layer
  directory may have a "redirect" to lower directory.  Because middle layer
  "redirects" are not indexed, a lower file handle that was encoded from the
  "redirect" origin directory, cannot be used to find the middle or upper
  layer directory.  Similarly, a lower file handle that was encoded from a
  descendant of the "redirect" origin directory, cannot be used to
  reconstruct a connected overlay path.  To mitigate the cases of
  directories that cannot be decoded from a lower file handle, these
  directories are copied up on encode and encoded as an upper file handle.
  On an overlay filesystem with no upper layer this mitigation cannot be
  used NFS export in this setup requires turning off redirect follow (e.g.
  "redirect_dir=nofollow").
  
  The overlay filesystem does not support non-directory connectable file
  handles, so exporting with the 'subtree_check' exportfs configuration will
  cause failures to lookup files over NFS.
  
  When the NFS export feature is enabled, all directory index entries are
  verified on mount time to check that upper file handles are not stale.
  This verification may cause significant overhead in some cases.

  Testsuite
  ---------

  There's a testsuite originally developed by David Howells and currently
  maintained by Amir Goldstein at:

    https://github.com/amir73il/unionmount-testsuite.git

  
  Run as root:
  
    # cd unionmount-testsuite

    # ./run --ov --verify