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.

The result will inevitably fail to look exactly like a normal
filesystem for various technical reasons.  The expectation is that
many use cases will be able to ignore these differences.


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.


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
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.

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
---------------------

The copy_up operation essentially creates a new, identical file and
moves it over to the old name.  Any open files referring to this inode
will access the old data.

The new file may be on a different filesystem, so both st_dev and st_ino
of the real file may change.  The values of st_dev and st_ino returned by
stat(2) on an overlay object are often not the same as the real file
stat(2) values to prevent the values from changing on copy_up.

Unless "xino" feature is enabled, when overlay layers are not all on the
same underlying filesystem, the value of st_dev may be different for two
non-directory objects in the same overlay filesystem and the value of
st_ino for directory objects may be non persistent and could change even
while the overlay filesystem is still mounted.

Unless "inode index" feature is enabled, if 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.

Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged
directory will fail with EXDEV.


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