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Diffstat (limited to 'Documentation/filesystems/overlayfs.txt')
| -rw-r--r-- | Documentation/filesystems/overlayfs.txt | 81 |
1 files changed, 59 insertions, 22 deletions
diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt index 72615a2c0752..51c136c821bf 100644 --- a/Documentation/filesystems/overlayfs.txt +++ b/Documentation/filesystems/overlayfs.txt @@ -10,10 +10,6 @@ union-filesystems). An overlay-filesystem tries to present a filesystem which is the result over overlaying one filesystem on top of the other. -The result will inevitably fail to look exactly like a normal -filesystem for various technical reasons. The expectation is that -many use cases will be able to ignore these differences. - Overlay objects --------------- @@ -266,6 +262,30 @@ 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. + Sharing and copying layers -------------------------- @@ -284,7 +304,7 @@ 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 -is enabled. +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 @@ -297,6 +317,10 @@ lower root origin, mount will fail with ESTALE. An overlayfs mount with 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 @@ -306,27 +330,40 @@ the copied layers will fail the verification of the lower root file handle. Non-standard behavior --------------------- -The copy_up operation essentially creates a new, identical file and -moves it over to the old name. Any open files referring to this inode -will access the old data. +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. -The new file may be on a different filesystem, so both st_dev and st_ino -of the real file may change. The values of st_dev and st_ino returned by -stat(2) on an overlay object are often not the same as the real file -stat(2) values to prevent the values from changing on copy_up. +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. -Unless "xino" feature is enabled, when overlay layers are not all on the -same underlying filesystem, the value of st_dev may be different for two -non-directory objects in the same overlay filesystem and the value of -st_ino for directory objects may be non persistent and could change even -while the overlay filesystem is still mounted. +3) "xino" -Unless "inode index" feature is enabled, if 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. +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. -Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged -directory will fail with EXDEV. +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 |
