Add API documentation for fsinfo.
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
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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 - Information about fsinfo() itself
+ ...
+ 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 × string
+ FSINFO_ATTR_AFS_SERVER_ADDRESS N × 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).
