484 строки
21 KiB
Plaintext
484 строки
21 KiB
Plaintext
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
|