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.. SPDX-License-Identifier: GPL-2.0 |
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Written by: Neil Brown |
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Please see MAINTAINERS file for where to send questions. |
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Overlay Filesystem |
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================== |
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This document describes a prototype for a new approach to providing |
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overlay-filesystem functionality in Linux (sometimes referred to as |
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union-filesystems). An overlay-filesystem tries to present a |
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filesystem which is the result over overlaying one filesystem on top |
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of the other. |
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Overlay objects |
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--------------- |
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The overlay filesystem approach is 'hybrid', because the objects that |
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appear in the filesystem do not always appear to belong to that filesystem. |
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In many cases, an object accessed in the union will be indistinguishable |
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from accessing the corresponding object from the original filesystem. |
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This is most obvious from the 'st_dev' field returned by stat(2). |
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While directories will report an st_dev from the overlay-filesystem, |
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non-directory objects may report an st_dev from the lower filesystem or |
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upper filesystem that is providing the object. Similarly st_ino will |
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only be unique when combined with st_dev, and both of these can change |
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over the lifetime of a non-directory object. Many applications and |
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tools ignore these values and will not be affected. |
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In the special case of all overlay layers on the same underlying |
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filesystem, all objects will report an st_dev from the overlay |
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filesystem and st_ino from the underlying filesystem. This will |
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make the overlay mount more compliant with filesystem scanners and |
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overlay objects will be distinguishable from the corresponding |
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objects in the original filesystem. |
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On 64bit systems, even if all overlay layers are not on the same |
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underlying filesystem, the same compliant behavior could be achieved |
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with the "xino" feature. The "xino" feature composes a unique object |
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identifier from the real object st_ino and an underlying fsid index. |
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The "xino" feature uses the high inode number bits for fsid, because the |
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underlying filesystems rarely use the high inode number bits. In case |
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the underlying inode number does overflow into the high xino bits, overlay |
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filesystem will fall back to the non xino behavior for that inode. |
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The "xino" feature can be enabled with the "-o xino=on" overlay mount option. |
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If all underlying filesystems support NFS file handles, the value of st_ino |
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for overlay filesystem objects is not only unique, but also persistent over |
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the lifetime of the filesystem. The "-o xino=auto" overlay mount option |
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enables the "xino" feature only if the persistent st_ino requirement is met. |
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The following table summarizes what can be expected in different overlay |
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configurations. |
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Inode properties |
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```````````````` |
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+--------------+------------+------------+-----------------+----------------+ |
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|Configuration | Persistent | Uniform | st_ino == d_ino | d_ino == i_ino | |
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| | st_ino | st_dev | | [*] | |
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+==============+=====+======+=====+======+========+========+========+=======+ |
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| | dir | !dir | dir | !dir | dir + !dir | dir | !dir | |
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+--------------+-----+------+-----+------+--------+--------+--------+-------+ |
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| All layers | Y | Y | Y | Y | Y | Y | Y | Y | |
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| on same fs | | | | | | | | | |
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+--------------+-----+------+-----+------+--------+--------+--------+-------+ |
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| Layers not | N | N | Y | N | N | Y | N | Y | |
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| on same fs, | | | | | | | | | |
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| xino=off | | | | | | | | | |
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+--------------+-----+------+-----+------+--------+--------+--------+-------+ |
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| xino=on/auto | Y | Y | Y | Y | Y | Y | Y | Y | |
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+--------------+-----+------+-----+------+--------+--------+--------+-------+ |
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| xino=on/auto,| N | N | Y | N | N | Y | N | Y | |
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| ino overflow | | | | | | | | | |
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+--------------+-----+------+-----+------+--------+--------+--------+-------+ |
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[*] nfsd v3 readdirplus verifies d_ino == i_ino. i_ino is exposed via several |
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/proc files, such as /proc/locks and /proc/self/fdinfo/<fd> of an inotify |
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file descriptor. |
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Upper and Lower |
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--------------- |
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An overlay filesystem combines two filesystems - an 'upper' filesystem |
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and a 'lower' filesystem. When a name exists in both filesystems, the |
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object in the 'upper' filesystem is visible while the object in the |
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'lower' filesystem is either hidden or, in the case of directories, |
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merged with the 'upper' object. |
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It would be more correct to refer to an upper and lower 'directory |
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tree' rather than 'filesystem' as it is quite possible for both |
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directory trees to be in the same filesystem and there is no |
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requirement that the root of a filesystem be given for either upper or |
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lower. |
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A wide range of filesystems supported by Linux can be the lower filesystem, |
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but not all filesystems that are mountable by Linux have the features |
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needed for OverlayFS to work. The lower filesystem does not need to be |
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writable. The lower filesystem can even be another overlayfs. The upper |
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filesystem will normally be writable and if it is it must support the |
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creation of trusted.* and/or user.* extended attributes, and must provide |
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valid d_type in readdir responses, so NFS is not suitable. |
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A read-only overlay of two read-only filesystems may use any |
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filesystem type. |
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Directories |
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----------- |
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Overlaying mainly involves directories. If a given name appears in both |
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upper and lower filesystems and refers to a non-directory in either, |
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then the lower object is hidden - the name refers only to the upper |
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object. |
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Where both upper and lower objects are directories, a merged directory |
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is formed. |
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At mount time, the two directories given as mount options "lowerdir" and |
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"upperdir" are combined into a merged directory: |
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mount -t overlay overlay -olowerdir=/lower,upperdir=/upper,\ |
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workdir=/work /merged |
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The "workdir" needs to be an empty directory on the same filesystem |
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as upperdir. |
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Then whenever a lookup is requested in such a merged directory, the |
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lookup is performed in each actual directory and the combined result |
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is cached in the dentry belonging to the overlay filesystem. If both |
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actual lookups find directories, both are stored and a merged |
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directory is created, otherwise only one is stored: the upper if it |
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exists, else the lower. |
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Only the lists of names from directories are merged. Other content |
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such as metadata and extended attributes are reported for the upper |
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directory only. These attributes of the lower directory are hidden. |
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whiteouts and opaque directories |
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-------------------------------- |
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In order to support rm and rmdir without changing the lower |
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filesystem, an overlay filesystem needs to record in the upper filesystem |
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that files have been removed. This is done using whiteouts and opaque |
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directories (non-directories are always opaque). |
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A whiteout is created as a character device with 0/0 device number. |
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When a whiteout is found in the upper level of a merged directory, any |
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matching name in the lower level is ignored, and the whiteout itself |
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is also hidden. |
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A directory is made opaque by setting the xattr "trusted.overlay.opaque" |
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to "y". Where the upper filesystem contains an opaque directory, any |
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directory in the lower filesystem with the same name is ignored. |
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readdir |
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------- |
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When a 'readdir' request is made on a merged directory, the upper and |
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lower directories are each read and the name lists merged in the |
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obvious way (upper is read first, then lower - entries that already |
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exist are not re-added). This merged name list is cached in the |
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'struct file' and so remains as long as the file is kept open. If the |
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directory is opened and read by two processes at the same time, they |
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will each have separate caches. A seekdir to the start of the |
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directory (offset 0) followed by a readdir will cause the cache to be |
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discarded and rebuilt. |
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This means that changes to the merged directory do not appear while a |
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directory is being read. This is unlikely to be noticed by many |
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programs. |
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seek offsets are assigned sequentially when the directories are read. |
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Thus if |
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- read part of a directory |
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- remember an offset, and close the directory |
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- re-open the directory some time later |
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- seek to the remembered offset |
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there may be little correlation between the old and new locations in |
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the list of filenames, particularly if anything has changed in the |
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directory. |
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Readdir on directories that are not merged is simply handled by the |
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underlying directory (upper or lower). |
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renaming directories |
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-------------------- |
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When renaming a directory that is on the lower layer or merged (i.e. the |
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directory was not created on the upper layer to start with) overlayfs can |
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handle it in two different ways: |
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1. return EXDEV error: this error is returned by rename(2) when trying to |
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move a file or directory across filesystem boundaries. Hence |
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applications are usually prepared to hande this error (mv(1) for example |
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recursively copies the directory tree). This is the default behavior. |
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2. If the "redirect_dir" feature is enabled, then the directory will be |
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copied up (but not the contents). Then the "trusted.overlay.redirect" |
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extended attribute is set to the path of the original location from the |
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root of the overlay. Finally the directory is moved to the new |
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location. |
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There are several ways to tune the "redirect_dir" feature. |
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Kernel config options: |
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- OVERLAY_FS_REDIRECT_DIR: |
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If this is enabled, then redirect_dir is turned on by default. |
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- OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW: |
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If this is enabled, then redirects are always followed by default. Enabling |
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this results in a less secure configuration. Enable this option only when |
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worried about backward compatibility with kernels that have the redirect_dir |
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feature and follow redirects even if turned off. |
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Module options (can also be changed through /sys/module/overlay/parameters/): |
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- "redirect_dir=BOOL": |
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See OVERLAY_FS_REDIRECT_DIR kernel config option above. |
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- "redirect_always_follow=BOOL": |
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See OVERLAY_FS_REDIRECT_ALWAYS_FOLLOW kernel config option above. |
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- "redirect_max=NUM": |
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The maximum number of bytes in an absolute redirect (default is 256). |
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Mount options: |
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- "redirect_dir=on": |
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Redirects are enabled. |
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- "redirect_dir=follow": |
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Redirects are not created, but followed. |
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- "redirect_dir=off": |
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Redirects are not created and only followed if "redirect_always_follow" |
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feature is enabled in the kernel/module config. |
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- "redirect_dir=nofollow": |
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Redirects are not created and not followed (equivalent to "redirect_dir=off" |
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if "redirect_always_follow" feature is not enabled). |
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When the NFS export feature is enabled, every copied up directory is |
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indexed by the file handle of the lower inode and a file handle of the |
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upper directory is stored in a "trusted.overlay.upper" extended attribute |
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on the index entry. On lookup of a merged directory, if the upper |
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directory does not match the file handle stores in the index, that is an |
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indication that multiple upper directories may be redirected to the same |
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lower directory. In that case, lookup returns an error and warns about |
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a possible inconsistency. |
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Because lower layer redirects cannot be verified with the index, enabling |
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NFS export support on an overlay filesystem with no upper layer requires |
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turning off redirect follow (e.g. "redirect_dir=nofollow"). |
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Non-directories |
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--------------- |
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Objects that are not directories (files, symlinks, device-special |
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files etc.) are presented either from the upper or lower filesystem as |
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appropriate. When a file in the lower filesystem is accessed in a way |
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the requires write-access, such as opening for write access, changing |
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some metadata etc., the file is first copied from the lower filesystem |
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to the upper filesystem (copy_up). Note that creating a hard-link |
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also requires copy_up, though of course creation of a symlink does |
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not. |
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The copy_up may turn out to be unnecessary, for example if the file is |
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opened for read-write but the data is not modified. |
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The copy_up process first makes sure that the containing directory |
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exists in the upper filesystem - creating it and any parents as |
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necessary. It then creates the object with the same metadata (owner, |
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mode, mtime, symlink-target etc.) and then if the object is a file, the |
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data is copied from the lower to the upper filesystem. Finally any |
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extended attributes are copied up. |
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Once the copy_up is complete, the overlay filesystem simply |
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provides direct access to the newly created file in the upper |
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filesystem - future operations on the file are barely noticed by the |
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overlay filesystem (though an operation on the name of the file such as |
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rename or unlink will of course be noticed and handled). |
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Permission model |
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---------------- |
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Permission checking in the overlay filesystem follows these principles: |
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1) permission check SHOULD return the same result before and after copy up |
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2) task creating the overlay mount MUST NOT gain additional privileges |
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3) non-mounting task MAY gain additional privileges through the overlay, |
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compared to direct access on underlying lower or upper filesystems |
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This is achieved by performing two permission checks on each access |
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a) check if current task is allowed access based on local DAC (owner, |
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group, mode and posix acl), as well as MAC checks |
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b) check if mounting task would be allowed real operation on lower or |
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upper layer based on underlying filesystem permissions, again including |
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MAC checks |
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Check (a) ensures consistency (1) since owner, group, mode and posix acls |
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are copied up. On the other hand it can result in server enforced |
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permissions (used by NFS, for example) being ignored (3). |
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Check (b) ensures that no task gains permissions to underlying layers that |
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the mounting task does not have (2). This also means that it is possible |
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to create setups where the consistency rule (1) does not hold; normally, |
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however, the mounting task will have sufficient privileges to perform all |
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operations. |
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Another way to demonstrate this model is drawing parallels between |
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mount -t overlay overlay -olowerdir=/lower,upperdir=/upper,... /merged |
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and |
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cp -a /lower /upper |
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mount --bind /upper /merged |
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The resulting access permissions should be the same. The difference is in |
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the time of copy (on-demand vs. up-front). |
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Multiple lower layers |
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--------------------- |
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Multiple lower layers can now be given using the colon (":") as a |
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separator character between the directory names. For example: |
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mount -t overlay overlay -olowerdir=/lower1:/lower2:/lower3 /merged |
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As the example shows, "upperdir=" and "workdir=" may be omitted. In |
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that case the overlay will be read-only. |
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The specified lower directories will be stacked beginning from the |
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rightmost one and going left. In the above example lower1 will be the |
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top, lower2 the middle and lower3 the bottom layer. |
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Metadata only copy up |
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--------------------- |
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When metadata only copy up feature is enabled, overlayfs will only copy |
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up metadata (as opposed to whole file), when a metadata specific operation |
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like chown/chmod is performed. Full file will be copied up later when |
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file is opened for WRITE operation. |
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In other words, this is delayed data copy up operation and data is copied |
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up when there is a need to actually modify data. |
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There are multiple ways to enable/disable this feature. A config option |
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CONFIG_OVERLAY_FS_METACOPY can be set/unset to enable/disable this feature |
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by default. Or one can enable/disable it at module load time with module |
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parameter metacopy=on/off. Lastly, there is also a per mount option |
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metacopy=on/off to enable/disable this feature per mount. |
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Do not use metacopy=on with untrusted upper/lower directories. Otherwise |
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it is possible that an attacker can create a handcrafted file with |
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appropriate REDIRECT and METACOPY xattrs, and gain access to file on lower |
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pointed by REDIRECT. This should not be possible on local system as setting |
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"trusted." xattrs will require CAP_SYS_ADMIN. But it should be possible |
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for untrusted layers like from a pen drive. |
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Note: redirect_dir={off|nofollow|follow[*]} and nfs_export=on mount options |
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conflict with metacopy=on, and will result in an error. |
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[*] redirect_dir=follow only conflicts with metacopy=on if upperdir=... is |
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given. |
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Sharing and copying layers |
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-------------------------- |
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Lower layers may be shared among several overlay mounts and that is indeed |
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a very common practice. An overlay mount may use the same lower layer |
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path as another overlay mount and it may use a lower layer path that is |
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beneath or above the path of another overlay lower layer path. |
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Using an upper layer path and/or a workdir path that are already used by |
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another overlay mount is not allowed and may fail with EBUSY. Using |
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partially overlapping paths is not allowed and may fail with EBUSY. |
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If files are accessed from two overlayfs mounts which share or overlap the |
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upper layer and/or workdir path the behavior of the overlay is undefined, |
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though it will not result in a crash or deadlock. |
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Mounting an overlay using an upper layer path, where the upper layer path |
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was previously used by another mounted overlay in combination with a |
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different lower layer path, is allowed, unless the "inodes index" feature |
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or "metadata only copy up" feature is enabled. |
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With the "inodes index" feature, on the first time mount, an NFS file |
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handle of the lower layer root directory, along with the UUID of the lower |
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filesystem, are encoded and stored in the "trusted.overlay.origin" extended |
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attribute on the upper layer root directory. On subsequent mount attempts, |
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the lower root directory file handle and lower filesystem UUID are compared |
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to the stored origin in upper root directory. On failure to verify the |
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lower root origin, mount will fail with ESTALE. An overlayfs mount with |
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"inodes index" enabled will fail with EOPNOTSUPP if the lower filesystem |
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does not support NFS export, lower filesystem does not have a valid UUID or |
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if the upper filesystem does not support extended attributes. |
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For "metadata only copy up" feature there is no verification mechanism at |
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mount time. So if same upper is mounted with different set of lower, mount |
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probably will succeed but expect the unexpected later on. So don't do it. |
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It is quite a common practice to copy overlay layers to a different |
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directory tree on the same or different underlying filesystem, and even |
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to a different machine. With the "inodes index" feature, trying to mount |
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the copied layers will fail the verification of the lower root file handle. |
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Non-standard behavior |
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--------------------- |
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Current version of overlayfs can act as a mostly POSIX compliant |
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filesystem. |
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This is the list of cases that overlayfs doesn't currently handle: |
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a) POSIX mandates updating st_atime for reads. This is currently not |
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done in the case when the file resides on a lower layer. |
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b) If a file residing on a lower layer is opened for read-only and then |
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memory mapped with MAP_SHARED, then subsequent changes to the file are not |
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reflected in the memory mapping. |
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c) If a file residing on a lower layer is being executed, then opening that |
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file for write or truncating the file will not be denied with ETXTBSY. |
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The following options allow overlayfs to act more like a standards |
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compliant filesystem: |
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1) "redirect_dir" |
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Enabled with the mount option or module option: "redirect_dir=on" or with |
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the kernel config option CONFIG_OVERLAY_FS_REDIRECT_DIR=y. |
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If this feature is disabled, then rename(2) on a lower or merged directory |
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will fail with EXDEV ("Invalid cross-device link"). |
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2) "inode index" |
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Enabled with the mount option or module option "index=on" or with the |
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kernel config option CONFIG_OVERLAY_FS_INDEX=y. |
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If this feature is disabled and a file with multiple hard links is copied |
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up, then this will "break" the link. Changes will not be propagated to |
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other names referring to the same inode. |
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3) "xino" |
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Enabled with the mount option "xino=auto" or "xino=on", with the module |
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option "xino_auto=on" or with the kernel config option |
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CONFIG_OVERLAY_FS_XINO_AUTO=y. Also implicitly enabled by using the same |
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underlying filesystem for all layers making up the overlay. |
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If this feature is disabled or the underlying filesystem doesn't have |
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enough free bits in the inode number, then overlayfs will not be able to |
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guarantee that the values of st_ino and st_dev returned by stat(2) and the |
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value of d_ino returned by readdir(3) will act like on a normal filesystem. |
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E.g. the value of st_dev may be different for two objects in the same |
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overlay filesystem and the value of st_ino for filesystem objects may not be |
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persistent and could change even while the overlay filesystem is mounted, as |
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summarized in the `Inode properties`_ table above. |
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Changes to underlying filesystems |
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--------------------------------- |
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Changes to the underlying filesystems while part of a mounted overlay |
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filesystem are not allowed. If the underlying filesystem is changed, |
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the behavior of the overlay is undefined, though it will not result in |
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a crash or deadlock. |
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Offline changes, when the overlay is not mounted, are allowed to the |
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upper tree. Offline changes to the lower tree are only allowed if the |
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"metadata only copy up", "inode index", "xino" and "redirect_dir" features |
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have not been used. If the lower tree is modified and any of these |
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features has been used, the behavior of the overlay is undefined, |
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though it will not result in a crash or deadlock. |
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When the overlay NFS export feature is enabled, overlay filesystems |
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behavior on offline changes of the underlying lower layer is different |
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than the behavior when NFS export is disabled. |
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On every copy_up, an NFS file handle of the lower inode, along with the |
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UUID of the lower filesystem, are encoded and stored in an extended |
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attribute "trusted.overlay.origin" on the upper inode. |
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When the NFS export feature is enabled, a lookup of a merged directory, |
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that found a lower directory at the lookup path or at the path pointed |
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to by the "trusted.overlay.redirect" extended attribute, will verify |
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that the found lower directory file handle and lower filesystem UUID |
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match the origin file handle that was stored at copy_up time. If a |
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found lower directory does not match the stored origin, that directory |
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will not be merged with the upper directory. |
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NFS export |
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---------- |
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When the underlying filesystems supports NFS export and the "nfs_export" |
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feature is enabled, an overlay filesystem may be exported to NFS. |
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With the "nfs_export" feature, on copy_up of any lower object, an index |
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entry is created under the index directory. The index entry name is the |
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hexadecimal representation of the copy up origin file handle. For a |
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non-directory object, the index entry is a hard link to the upper inode. |
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For a directory object, the index entry has an extended attribute |
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"trusted.overlay.upper" with an encoded file handle of the upper |
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directory inode. |
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When encoding a file handle from an overlay filesystem object, the |
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following rules apply: |
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1. For a non-upper object, encode a lower file handle from lower inode |
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2. For an indexed object, encode a lower file handle from copy_up origin |
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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. |
|
|
|
Note: the mount options index=off,nfs_export=on are conflicting for a |
|
read-write mount and will result in an error. |
|
|
|
Note: the mount option uuid=off can be used to replace UUID of the underlying |
|
filesystem in file handles with null, and effectively disable UUID checks. This |
|
can be useful in case the underlying disk is copied and the UUID of this copy |
|
is changed. This is only applicable if all lower/upper/work directories are on |
|
the same filesystem, otherwise it will fallback to normal behaviour. |
|
|
|
Volatile mount |
|
-------------- |
|
|
|
This is enabled with the "volatile" mount option. Volatile mounts are not |
|
guaranteed to survive a crash. It is strongly recommended that volatile |
|
mounts are only used if data written to the overlay can be recreated |
|
without significant effort. |
|
|
|
The advantage of mounting with the "volatile" option is that all forms of |
|
sync calls to the upper filesystem are omitted. |
|
|
|
In order to avoid a giving a false sense of safety, the syncfs (and fsync) |
|
semantics of volatile mounts are slightly different than that of the rest of |
|
VFS. If any writeback error occurs on the upperdir's filesystem after a |
|
volatile mount takes place, all sync functions will return an error. Once this |
|
condition is reached, the filesystem will not recover, and every subsequent sync |
|
call will return an error, even if the upperdir has not experience a new error |
|
since the last sync call. |
|
|
|
When overlay is mounted with "volatile" option, the directory |
|
"$workdir/work/incompat/volatile" is created. During next mount, overlay |
|
checks for this directory and refuses to mount if present. This is a strong |
|
indicator that user should throw away upper and work directories and create |
|
fresh one. In very limited cases where the user knows that the system has |
|
not crashed and contents of upperdir are intact, The "volatile" directory |
|
can be removed. |
|
|
|
|
|
User xattr |
|
---------- |
|
|
|
The "-o userxattr" mount option forces overlayfs to use the |
|
"user.overlay." xattr namespace instead of "trusted.overlay.". This is |
|
useful for unprivileged mounting of overlayfs. |
|
|
|
|
|
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
|
|
|