diff options
Diffstat (limited to 'Documentation/filesystems/overlayfs.rst')
| -rw-r--r-- | Documentation/filesystems/overlayfs.rst | 52 |
1 files changed, 48 insertions, 4 deletions
diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index 5b93268e4..b28e5e3c2 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -145,7 +145,9 @@ 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. +A whiteout is created as a character device with 0/0 device number or +as a zero-size regular file with the xattr "trusted.overlay.whiteout". + 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. @@ -154,6 +156,13 @@ 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. +An opaque directory should not conntain any whiteouts, because they do not +serve any purpose. A merge directory containing regular files with the xattr +"trusted.overlay.whiteout", should be additionally marked by setting the xattr +"trusted.overlay.opaque" to "x" on the merge directory itself. +This is needed to avoid the overhead of checking the "trusted.overlay.whiteout" +on all entries during readdir in the common case. + readdir ------- @@ -344,10 +353,11 @@ escaping the colons with a single backslash. For example: mount -t overlay overlay -olowerdir=/a\:lower\:\:dir /merged -Since kernel version v6.5, directory names containing colons can also -be provided as lower layer using the fsconfig syscall from new mount api: +Since kernel version v6.8, directory names containing colons can also +be configured as lower layer using the "lowerdir+" mount options and the +fsconfig syscall from new mount api. For example: - fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir", "/a:lower::dir", 0); + fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir+", "/a:lower::dir", 0); In the latter case, colons in lower layer directory names will be escaped as an octal characters (\072) when displayed in /proc/self/mountinfo. @@ -416,6 +426,16 @@ Only the data of the files in the "data-only" lower layers may be visible when a "metacopy" file in one of the lower layers above it, has a "redirect" to the absolute path of the "lower data" file in the "data-only" lower layer. +Since kernel version v6.8, "data-only" lower layers can also be added using +the "datadir+" mount options and the fsconfig syscall from new mount api. +For example: + + fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir+", "/l1", 0); + fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir+", "/l2", 0); + fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir+", "/l3", 0); + fsconfig(fs_fd, FSCONFIG_SET_STRING, "datadir+", "/do1", 0); + fsconfig(fs_fd, FSCONFIG_SET_STRING, "datadir+", "/do2", 0); + fs-verity support ---------------------- @@ -504,6 +524,30 @@ 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.opaque" xattr set to "x" (see `whiteouts and opaque directories`_). +These alternative 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 --------------------- |