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ovl: Add documentation on nesting of overlayfs mounts

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Signed-off-by: Alexander Larsson <alexl@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
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Alexander Larsson 2023-08-16 12:57:41 +02:00 committed by Amir Goldstein
parent bc8df7a3dc
commit bb7055a734
1 changed files with 23 additions and 0 deletions
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Documentation/filesystems/overlayfs.rst
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@ -504,6 +504,29 @@ 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.
Nesting overlayfs mounts
------------------------
It is possible to use a lower directory that is stored on an overlayfs
mount. For regular files this does not need any special care. However, files
that have overlayfs attributes, such as whiteouts or "overlay.*" xattrs will be
interpreted by the underlying overlayfs mount and stripped out. In order to
allow the second overlayfs mount to see the attributes they must be escaped.
Overlayfs specific xattrs are escaped by using a special prefix of
"overlay.overlay.". So, a file with a "trusted.overlay.overlay.metacopy" xattr
in the lower dir will be exposed as a regular file with a
"trusted.overlay.metacopy" xattr in the overlayfs mount. This can be nested by
repeating the prefix multiple time, as each instance only removes one prefix.
A lower dir with a regular whiteout will always be handled by the overlayfs
mount, so to support storing an effective whiteout file in an overlayfs mount an
alternative form of whiteout is supported. This form is a regular, zero-size
file with the "overlay.whiteout" xattr set, inside a directory with the
"overlay.whiteouts" xattr set. Such whiteouts are never created by overlayfs,
but can be used by userspace tools (like containers) that generate lower layers.
These alternative whiteouts can be escaped using the standard xattr escape
mechanism in order to properly nest to any depth.
Non-standard behavior
---------------------