[PATCH 14/17] fsinfo: Add API documentation [ver #17]

[David Howells]

Add API documentation for fsinfo.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 Documentation/filesystems/fsinfo.rst |  491 ++++++++++++++++++++++++++++++++++
 1 file changed, 491 insertions(+)
 create mode 100644 Documentation/filesystems/fsinfo.rst

diff --git a/Documentation/filesystems/fsinfo.rst b/Documentation/filesystems/fsinfo.rst
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+============================
+Filesystem Information Query
+============================
+
+The fsinfo() system call allows the querying of filesystem and filesystem
+security information beyond what stat(), statx() and statfs() can obtain.  It
+does not require a file to be opened as does ioctl().
+
+fsinfo() may be called with a path, with open file descriptor or a with a mount
+object identifier.
+
+The fsinfo() system call needs to be configured on by enabling:
+
+	"File systems"/"Enable the fsinfo() system call" (CONFIG_FSINFO)
+
+This document has the following sections:
+
+.. contents:: :local:
+
+
+Overview
+========
+
+The fsinfo() system call retrieves one of a number of attributes, the IDs of
+which can be found in include/uapi/linux/fsinfo.h::
+
+	FSINFO_ATTR_STATFS	- statfs()-style state
+	FSINFO_ATTR_IDS		- Filesystem IDs
+	FSINFO_ATTR_LIMITS	- Filesystem limits
+	...
+	FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO - Information about an attribute
+	FSINFO_ATTR_FSINFO_ATTRIBUTES - List of available attributes
+	...
+	FSINFO_ATTR_MOUNT_INFO	- Information about the mount topology
+	...
+
+Each attribute can have zero or more values, which can be of one of the
+following types:
+
+ * ``VStruct``.  This is a structure with a version-dependent length.  New
+   versions of the kernel may append more fields, though they are not
+   permitted to remove or replace old ones.
+
+   Older applications, expecting an older version of the field, can ask for a
+   shorter struct and will only get the fields they requested; newer
+   applications running on an older kernel will get the extra fields they
+   requested filled with zeros.  Either way, the system call returns the size
+   of the internal struct, regardless of how much data it returned.
+
+   This allows for struct-type fields to be extended in future.
+
+ * ``String``.  This is a variable-length string of up to 4096 characters (no
+   NUL character is included).  The returned string will be truncated if the
+   output buffer is too small.  The total size of the string is returned,
+   regardless of any truncation.
+
+ * ``Opaque``.  This is a variable-length blob of indeterminate structure.  It
+   may be up to INT_MAX bytes in size.
+
+ * ``List``.  This is a variable-length list of fixed-size structures.  The
+   element size may not vary over time, so the element format must be designed
+   with care.  The maximum length is INT_MAX bytes, though this depends on the
+   kernel being able to allocate an internal buffer large enough.
+
+Value type is an inherent propery of an attribute and all the values of an
+attribute must be of that type.  Each attribute can have a single value, a
+sequence of values or a sequence-of-sequences of values.
+
+
+Filesystem API
+==============
+
+If the filesystem wishes to provide a list of queryable attributes, it should
+set the table pointer in the superblock::
+
+	const struct fsinfo_attribute *fsinfo_attributes;
+
+terminating it with a blank entry.  Each entry is a ``struct fsinfo_attribute``
+and these can be created with a set of helper macros::
+
+	FSINFO_VSTRUCT(A,G)
+	FSINFO_VSTRUCT_N(A,G)
+	FSINFO_VSTRUCT_NM(A,G)
+	FSINFO_STRING(A,G)
+	FSINFO_STRING_N(A,G)
+	FSINFO_STRING_NM(A,G)
+	FSINFO_OPAQUE(A,G)
+	FSINFO_LIST(A,G)
+	FSINFO_LIST_N(A,G)
+
+The names of the macro are a combination of type (vstruct, string, opaque and
+list) and an optional qualifier, if the attribute has N values or N lots of M
+values.  ``A`` is the name of the attribute and ``G`` is a function to get a
+value for that attribute.
+
+For vstruct- and list-type attributes, it is expected that there is a macro
+defined with the name ``A##__STRUCT`` that indicates the structure or element
+type.
+
+The get function needs to match the following type::
+
+	int (*get)(struct path *path, struct fsinfo_context *ctx);
+
+where "path" indicates the object to be queried and ctx is a context describing
+the parameters and the output buffer.  The function should return the total
+size of the data it would like to produce or an error.
+
+The parameter struct looks like::
+
+	struct fsinfo_context {
+		__u32		requested_attr;
+		__u32		Nth;
+		__u32		Mth;
+		bool		want_size_only;
+		unsigned int	buf_size;
+		unsigned int	usage;
+		void		*buffer;
+		...
+	};
+
+The fields relevant to the filesystem are as follows:
+
+ * ``requested_attr``
+
+   Which attribute is being requested.  EOPNOTSUPP should be returned if the
+   attribute is not supported by the filesystem or the LSM.
+
+ * ``Nth`` and ``Mth``
+
+   Which value of an attribute is being requested.
+
+   For a single-value attribute Nth and Mth will both be 0.
+
+   For a "1D" attribute, Nth will indicate which value and Mth will always
+   be 0.  Take, for example, FSINFO_ATTR_SERVER_NAME - for a network
+   filesystem, the superblock will be backed by a number of servers.  This will
+   return the name of the Nth server.  ENODATA will be returned if Nth goes
+   beyond the end of the array.
+
+   For a "2D" attribute, Mth will indicate the index in the Nth set of values.
+   Take, for example, an attribute for a network filesystems that returns
+   server addresses - each server may have one or more addresses.  This could
+   return the Mth address of the Nth server.  ENODATA should be returned if the
+   Nth set doesn't exist or the Mth element of the Nth set doesn't exist.
+
+ * ``want_size_only``
+
+   Is set to true if the caller only wants the size of the value so that the
+   get function doesn't have to make expensive calculations or calls to
+   retrieve the value.
+
+ * ``buf_size``
+
+   This indicates the current size of the buffer.  For the list type and the
+   opaque type this will be increased if the current buffer won't hold the
+   value and the filesystem will be called again.
+
+ * ``usage``
+
+   This indicates how much of the buffer has been used so far for an list or
+   opaque type attribute.  This is updated by the fsinfo_note_param*()
+   functions.
+
+ * ``buffer``
+
+   This points to the output buffer.  For struct- and string-type attributes it
+   will always be big enough; for list- and opaque-type, it will be buf_size in
+   size and will be resized if the returned size is larger than this.
+
+To simplify filesystem code, there will always be at least a minimal buffer
+available if the ->fsinfo() method gets called - and the filesystem should
+always write what it can into the buffer.  It's possible that the fsinfo()
+system call will then throw the contents away and just return the length.
+
+
+Helper Functions
+================
+
+The API includes a number of helper functions:
+
+ * ``void fsinfo_set_feature(struct fsinfo_features *ft,
+			     enum fsinfo_feature feature);``
+
+   This function sets a feature flag.
+
+ * ``void fsinfo_clear_feature(struct fsinfo_features *ft,
+			       enum fsinfo_feature feature);``
+
+   This function clears a feature flag.
+
+ * ``void fsinfo_set_unix_features(struct fsinfo_features *ft);``
+
+   Set feature flags appropriate to the features of a standard UNIX filesystem,
+   such as having numeric UIDS and GIDS; allowing the creation of directories,
+   symbolic links, hard links, device files, FIFO and socket files; permitting
+   sparse files; and having access, change and modification times.
+
+
+Attribute Summary
+=================
+
+To summarise the attributes that are defined::
+
+  Symbolic name				Type
+  =====================================	===============
+  FSINFO_ATTR_STATFS			vstruct
+  FSINFO_ATTR_IDS			vstruct
+  FSINFO_ATTR_LIMITS			vstruct
+  FSINFO_ATTR_SUPPORTS			vstruct
+  FSINFO_ATTR_FEATURES			vstruct
+  FSINFO_ATTR_TIMESTAMP_INFO		vstruct
+  FSINFO_ATTR_VOLUME_ID			string
+  FSINFO_ATTR_VOLUME_UUID		vstruct
+  FSINFO_ATTR_VOLUME_NAME		string
+  FSINFO_ATTR_NAME_ENCODING		string
+  FSINFO_ATTR_NAME_CODEPAGE		string
+  FSINFO_ATTR_FSINFO			vstruct
+  FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO	vstruct
+  FSINFO_ATTR_FSINFO_ATTRIBUTES		list
+  FSINFO_ATTR_MOUNT_INFO		vstruct
+  FSINFO_ATTR_MOUNT_DEVNAME		string
+  FSINFO_ATTR_MOUNT_POINT		string
+  FSINFO_ATTR_MOUNT_CHILDREN		list
+  FSINFO_ATTR_AFS_CELL_NAME		string
+  FSINFO_ATTR_AFS_SERVER_NAME		N Ã string
+  FSINFO_ATTR_AFS_SERVER_ADDRESS	N Ã struct
+
+
+Attribute Catalogue
+===================
+
+A number of the attributes convey information about a filesystem superblock:
+
+ *  ``FSINFO_ATTR_STATFS``
+
+    This struct-type attribute gives most of the equivalent data to statfs(),
+    but with all the fields as unconditional 64-bit or 128-bit integers.  Note
+    that static data like IDs that don't change are retrieved with
+    FSINFO_ATTR_IDS instead.
+
+    Further, superblock flags (such as MS_RDONLY) are not exposed by this
+    attribute; rather the parameters must be listed and the attributes picked
+    out from that.
+
+ *  ``FSINFO_ATTR_IDS``
+
+    This struct-type attribute conveys various identifiers used by the target
+    filesystem.  This includes the filesystem name, the NFS filesystem ID, the
+    superblock ID used in notifications, the filesystem magic type number and
+    the primary device ID.
+
+ *  ``FSINFO_ATTR_LIMITS``
+
+    This struct-type attribute conveys the limits on various aspects of a
+    filesystem, such as maximum file, symlink and xattr sizes, maxiumm filename
+    and xattr name length, maximum number of symlinks, maximum device major and
+    minor numbers and maximum UID, GID and project ID numbers.
+
+ *  ``FSINFO_ATTR_SUPPORTS``
+
+    This struct-type attribute conveys information about the support the
+    filesystem has for various UAPI features of a filesystem.  This includes
+    information about which bits are supported in various masks employed by the
+    statx system call, what FS_IOC_* flags are supported by ioctls and what
+    DOS/Windows file attribute flags are supported.
+
+ *  ``FSINFO_ATTR_TIMESTAMP_INFO``
+
+    This struct-type attribute conveys information about the resolution and
+    range of the timestamps available in a filesystem.  The resolutions are
+    given as a mantissa and exponent (resolution = mantissa * 10^exponent
+    seconds), where the exponent can be negative to indicate a sub-second
+    resolution (-9 being nanoseconds, for example).
+
+ *  ``FSINFO_ATTR_VOLUME_ID``
+
+    This is a string-type attribute that conveys the superblock identifier for
+    the volume.  By default it will be filled in from the contents of s_id from
+    the superblock.  For a block-based filesystem, for example, this might be
+    the name of the primary block device.
+
+ *  ``FSINFO_ATTR_VOLUME_UUID``
+
+    This is a struct-type attribute that conveys the UUID identifier for the
+    volume.  By default it will be filled in from the contents of s_uuid from
+    the superblock.  If this doesn't exist, it will be an entirely zeros.
+
+ *  ``FSINFO_ATTR_VOLUME_NAME``
+
+    This is a string-type attribute that conveys the name of the volume.  By
+    default it will return EOPNOTSUPP.  For a disk-based filesystem, it might
+    convey the partition label; for a network-based filesystem, it might convey
+    the name of the remote volume.
+
+ *  ``FSINFO_ATTR_FEATURES``
+
+    This is a special attribute, being a set of single-bit feature flags,
+    formatted as struct-type attribute.  The meanings of the feature bits are
+    listed below - see the "Feature Bit Catalogue" section.  The feature bits
+    are grouped numerically into bytes, such that features 0-7 are in byte 0,
+    8-15 are in byte 1, 16-23 in byte 2 and so on.
+
+    Any feature bit that's not supported by the kernel will be set to false if
+    asked for.  The highest supported feature can be obtained from attribute
+    "FSINFO_ATTR_FSINFO".
+
+
+Some attributes give information about fsinfo itself:
+
+ *  ``FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO``
+
+    This struct-type attribute gives metadata about the attribute with the ID
+    specified by the Nth parameter, including its type, default size and
+    element size.
+
+ *  ``FSINFO_ATTR_FSINFO_ATTRIBUTES``
+
+    This list-type attribute gives a list of the attribute IDs available at the
+    point of reference.  FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO can then be used to
+    query each attribute.
+
+ *  ``FSINFO_ATTR_FSINFO``
+
+    This struct-type attribute gives information about the fsinfo() system call
+    itself, including the maximum number of feature bits supported.
+
+
+Then there are filesystem-specific attributes, e.g.:
+
+ *  ``FSINFO_ATTR_AFS_CELL_NAME``
+
+    This is a string-type attribute that retrieves the AFS cell name of the
+    target object.
+
+ *  ``FSINFO_ATTR_AFS_SERVER_NAME``
+
+    This is a string-type attribute that conveys the name of the Nth server
+    backing a network-filesystem superblock.
+
+ *  ``FSINFO_ATTR_AFS_SERVER_ADDRESSES``
+
+    This is a list-type attribute that conveys the Mth address of the Nth
+    server, as returned by FSINFO_ATTR_SERVER_NAME.
+
+
+Feature Bit Catalogue
+=====================
+
+The feature bits convey single true/false assertions about a specific instance
+of a filesystem (ie. a specific superblock).  They are accessed using the
+"FSINFO_ATTR_FEATURE" attribute:
+
+ *  ``FSINFO_FEAT_IS_KERNEL_FS``
+ *  ``FSINFO_FEAT_IS_BLOCK_FS``
+ *  ``FSINFO_FEAT_IS_FLASH_FS``
+ *  ``FSINFO_FEAT_IS_NETWORK_FS``
+ *  ``FSINFO_FEAT_IS_AUTOMOUNTER_FS``
+ *  ``FSINFO_FEAT_IS_MEMORY_FS``
+
+    These indicate what kind of filesystem the target is: kernel API (proc),
+    block-based (ext4), flash/nvm-based (jffs2), remote over the network (NFS),
+    local quasi-filesystem that acts as a tray of mountpoints (autofs), plain
+    in-memory filesystem (shmem).
+
+ *  ``FSINFO_FEAT_AUTOMOUNTS``
+
+    This indicate if a filesystem may have objects that are automount points.
+
+ *  ``FSINFO_FEAT_ADV_LOCKS``
+ *  ``FSINFO_FEAT_MAND_LOCKS``
+ *  ``FSINFO_FEAT_LEASES``
+
+    These indicate if a filesystem supports advisory locks, mandatory locks or
+    leases.
+
+ *  ``FSINFO_FEAT_UIDS``
+ *  ``FSINFO_FEAT_GIDS``
+ *  ``FSINFO_FEAT_PROJIDS``
+
+    These indicate if a filesystem supports/stores/transports numeric user IDs,
+    group IDs or project IDs.  The "FSINFO_ATTR_LIMITS" attribute can be used
+    to find out the upper limits on the IDs values.
+
+ *  ``FSINFO_FEAT_STRING_USER_IDS``
+
+    This indicates if a filesystem supports/stores/transports string user
+    identifiers.
+
+ *  ``FSINFO_FEAT_GUID_USER_IDS``
+
+    This indicates if a filesystem supports/stores/transports Windows GUIDs as
+    user identifiers (eg. ntfs).
+
+ *  ``FSINFO_FEAT_WINDOWS_ATTRS``
+
+    This indicates if a filesystem supports Windows FILE_* attribute bits
+    (eg. cifs, jfs).  The "FSINFO_ATTR_SUPPORTS" attribute can be used to find
+    out which windows file attributes are supported by the filesystem.
+
+ *  ``FSINFO_FEAT_USER_QUOTAS``
+ *  ``FSINFO_FEAT_GROUP_QUOTAS``
+ *  ``FSINFO_FEAT_PROJECT_QUOTAS``
+
+    These indicate if a filesystem supports quotas for users, groups or
+    projects.
+
+ *  ``FSINFO_FEAT_XATTRS``
+
+    These indicate if a filesystem supports extended attributes.  The
+    "FSINFO_ATTR_LIMITS" attribute can be used to find out the upper limits on
+    the supported name and body lengths.
+
+ *  ``FSINFO_FEAT_JOURNAL``
+ *  ``FSINFO_FEAT_DATA_IS_JOURNALLED``
+
+    These indicate whether the filesystem has a journal and whether data
+    changes are logged to it.
+
+ *  ``FSINFO_FEAT_O_SYNC``
+ *  ``FSINFO_FEAT_O_DIRECT``
+
+    These indicate whether the filesystem supports the O_SYNC and O_DIRECT
+    flags.
+
+ *  ``FSINFO_FEAT_VOLUME_ID``
+ *  ``FSINFO_FEAT_VOLUME_UUID``
+ *  ``FSINFO_FEAT_VOLUME_NAME``
+ *  ``FSINFO_FEAT_VOLUME_FSID``
+
+    These indicate whether ID, UUID, name and FSID identifiers actually exist
+    in the filesystem and thus might be considered persistent.
+
+ *  ``FSINFO_FEAT_IVER_ALL_CHANGE``
+ *  ``FSINFO_FEAT_IVER_DATA_CHANGE``
+ *  ``FSINFO_FEAT_IVER_MONO_INCR``
+
+    These indicate whether i_version in the inode is supported and, if so, what
+    mode it operates in.  The first two indicate if it's changed for any data
+    or metadata change, or whether it's only changed for any data changes; the
+    last indicates whether or not it's monotonically increasing for each such
+    change.
+
+ *  ``FSINFO_FEAT_HARD_LINKS``
+ *  ``FSINFO_FEAT_HARD_LINKS_1DIR``
+
+    These indicate whether the filesystem can have hard links made in it, and
+    whether they can be made between directory or only within the same
+    directory.
+
+ *  ``FSINFO_FEAT_DIRECTORIES``
+ *  ``FSINFO_FEAT_SYMLINKS``
+ *  ``FSINFO_FEAT_DEVICE_FILES``
+ *  ``FSINFO_FEAT_UNIX_SPECIALS``
+
+    These indicate whether directories; symbolic links; device files; or pipes
+    and sockets can be made within the filesystem.
+
+ *  ``FSINFO_FEAT_RESOURCE_FORKS``
+
+    This indicates if the filesystem supports resource forks.
+
+ *  ``FSINFO_FEAT_NAME_CASE_INDEP``
+ *  ``FSINFO_FEAT_NAME_NON_UTF8``
+ *  ``FSINFO_FEAT_NAME_HAS_CODEPAGE``
+
+    These indicate if the filesystem supports case-independent file names,
+    whether the filenames are non-utf8 (see the "FSINFO_ATTR_NAME_ENCODING"
+    attribute) and whether a codepage is in use to transliterate them (see
+    the "FSINFO_ATTR_NAME_CODEPAGE" attribute).
+
+ *  ``FSINFO_FEAT_SPARSE``
+
+    This indicates if a filesystem supports sparse files.
+
+ *  ``FSINFO_FEAT_NOT_PERSISTENT``
+
+    This indicates if a filesystem is not persistent.
+
+ *  ``FSINFO_FEAT_NO_UNIX_MODE``
+
+    This indicates if a filesystem doesn't support UNIX mode bits (though they
+    may be manufactured from other bits, such as Windows file attribute flags).
+
+ *  ``FSINFO_FEAT_HAS_ATIME``
+ *  ``FSINFO_FEAT_HAS_BTIME``
+ *  ``FSINFO_FEAT_HAS_CTIME``
+ *  ``FSINFO_FEAT_HAS_MTIME``
+
+    These indicate which timestamps a filesystem supports (access, birth,
+    change, modify).  The range and resolutions can be queried with the
+    "FSINFO_ATTR_TIMESTAMPS" attribute).