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diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst new file mode 100644 index 000000000000..e443be7928db --- /dev/null +++ b/Documentation/filesystems/overlayfs.rst @@ -0,0 +1,497 @@ +.. SPDX-License-Identifier: GPL-2.0 + +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 and may 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 +--------------------- + +Current version of overlayfs can act as a mostly POSIX compliant +filesystem. + +This is the list of cases that overlayfs doesn't currently handle: + +a) POSIX mandates updating st_atime for reads. This is currently not +done in the case when the file resides on a lower layer. + +b) If a file residing on a lower layer is opened for read-only and then +memory mapped with MAP_SHARED, then subsequent changes to the file are not +reflected in the memory mapping. + +The following options allow overlayfs to act more like a standards +compliant filesystem: + +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 |