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overlayfs.rst: fix ReST formatting
Fix some indentation issues and fix missing newlines in quoted text by converting quoted text to code blocks. Reported-by: Christian Brauner <brauner@kernel.org> Suggested-by: Bagas Sanjaya <bagasdotme@gmail.com> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> Reviewed-by: Akira Yokosawa <akiyks@gmail.com> Signed-off-by: Amir Goldstein <amir73il@gmail.com>
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Amir Goldstein
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@ -118,7 +118,7 @@ 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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"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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@ -172,12 +172,12 @@ 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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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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- 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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@ -290,9 +290,9 @@ Permission checking in the overlay filesystem follows these principles:
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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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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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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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@ -311,11 +311,11 @@ 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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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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and::
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cp -a /lower /upper
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mount --bind /upper /merged
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@ -328,7 +328,7 @@ 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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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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@ -340,13 +340,13 @@ 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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Note: directory names containing colons can be provided as lower layer by
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escaping the colons with a single backslash. For example:
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escaping the colons with a single backslash. For example::
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mount -t overlay overlay -olowerdir=/a\:lower\:\:dir /merged
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Since kernel version v6.8, directory names containing colons can also
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be configured as lower layer using the "lowerdir+" mount options and the
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fsconfig syscall from new mount api. For example:
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fsconfig syscall from new mount api. For example::
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fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir+", "/a:lower::dir", 0);
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@ -405,7 +405,7 @@ A normal lower layer is not allowed to be below a data-only layer, so single
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colon separators are not allowed to the right of double colon ("::") separators.
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For example:
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For example::
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mount -t overlay overlay -olowerdir=/l1:/l2:/l3::/do1::/do2 /merged
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@ -419,7 +419,7 @@ to the absolute path of the "lower data" file in the "data-only" lower layer.
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Since kernel version v6.8, "data-only" lower layers can also be added using
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the "datadir+" mount options and the fsconfig syscall from new mount api.
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For example:
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For example::
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fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir+", "/l1", 0);
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fsconfig(fs_fd, FSCONFIG_SET_STRING, "lowerdir+", "/l2", 0);
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@ -429,7 +429,7 @@ For example:
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fs-verity support
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----------------------
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-----------------
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During metadata copy up of a lower file, if the source file has
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fs-verity enabled and overlay verity support is enabled, then the
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@ -547,15 +547,15 @@ 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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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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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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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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@ -647,12 +647,13 @@ 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,
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encode an upper file handle from upper inode
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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,
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encode an upper file handle from upper inode
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The encoded overlay file handle includes:
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- Header including path type information (e.g. lower/upper)
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- UUID of the underlying filesystem
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- Underlying filesystem encoding of underlying inode
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@ -662,15 +663,15 @@ are stored in extended attribute "trusted.overlay.origin".
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When decoding an overlay file handle, the following steps are followed:
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1. Find underlying layer by UUID and path type information.
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2. Decode the underlying filesystem file handle to underlying dentry.
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3. For a lower file handle, lookup the handle in index directory by name.
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4. If a whiteout is found in index, return ESTALE. This represents an
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overlay object that was deleted after its file handle was encoded.
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5. For a non-directory, instantiate a disconnected overlay dentry from the
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decoded underlying dentry, the path type and index inode, if found.
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6. For a directory, use the connected underlying decoded dentry, path type
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and index, to lookup a connected overlay dentry.
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1. Find underlying layer by UUID and path type information.
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2. Decode the underlying filesystem file handle to underlying dentry.
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3. For a lower file handle, lookup the handle in index directory by name.
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4. If a whiteout is found in index, return ESTALE. This represents an
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overlay object that was deleted after its file handle was encoded.
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5. For a non-directory, instantiate a disconnected overlay dentry from the
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decoded underlying dentry, the path type and index inode, if found.
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6. For a directory, use the connected underlying decoded dentry, path type
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and index, to lookup a connected overlay dentry.
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Decoding a non-directory file handle may return a disconnected dentry.
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copy_up of that disconnected dentry will create an upper index entry with
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@ -773,9 +774,9 @@ Testsuite
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There's a testsuite originally developed by David Howells and currently
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maintained by Amir Goldstein at:
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https://github.com/amir73il/unionmount-testsuite.git
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https://github.com/amir73il/unionmount-testsuite.git
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Run as root:
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Run as root::
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# cd unionmount-testsuite
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# ./run --ov --verify
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