vfs: Provide documentation for new mount API

Provide documentation for the new mount API.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/filesystems/mount_api.txt

+			     ====================
+			     FILESYSTEM MOUNT API
+			     ====================
+
+CONTENTS
+
+ (1) Overview.
+
+ (2) The filesystem context.
+
+ (3) The filesystem context operations.
+
+ (4) Filesystem context security.
+
+ (5) VFS filesystem context operations.
+
+ (6) Parameter description.
+
+ (7) Parameter helper functions.
+
+
+========
+OVERVIEW
+========
+
+The creation of new mounts is now to be done in a multistep process:
+
+ (1) Create a filesystem context.
+
+ (2) Parse the parameters and attach them to the context.  Parameters are
+     expected to be passed individually from userspace, though legacy binary
+     parameters can also be handled.
+
+ (3) Validate and pre-process the context.
+
+ (4) Get or create a superblock and mountable root.
+
+ (5) Perform the mount.
+
+ (6) Return an error message attached to the context.
+
+ (7) Destroy the context.
+
+To support this, the file_system_type struct gains a new field:
+
+	int (*init_fs_context)(struct fs_context *fc);
+
+which is invoked to set up the filesystem-specific parts of a filesystem
+context, including the additional space.
+
+Note that security initialisation is done *after* the filesystem is called so
+that the namespaces may be adjusted first.
+
+
+======================
+THE FILESYSTEM CONTEXT
+======================
+
+The creation and reconfiguration of a superblock is governed by a filesystem
+context.  This is represented by the fs_context structure:
+
+	struct fs_context {
+		const struct fs_context_operations *ops;
+		struct file_system_type *fs_type;
+		void			*fs_private;
+		struct dentry		*root;
+		struct user_namespace	*user_ns;
+		struct net		*net_ns;
+		const struct cred	*cred;
+		char			*source;
+		char			*subtype;
+		void			*security;
+		void			*s_fs_info;
+		unsigned int		sb_flags;
+		unsigned int		sb_flags_mask;
+		enum fs_context_purpose	purpose:8;
+		bool			sloppy:1;
+		bool			silent:1;
+		...
+	};
+
+The fs_context fields are as follows:
+
+ (*) const struct fs_context_operations *ops
+
+     These are operations that can be done on a filesystem context (see
+     below).  This must be set by the ->init_fs_context() file_system_type
+     operation.
+
+ (*) struct file_system_type *fs_type
+
+     A pointer to the file_system_type of the filesystem that is being
+     constructed or reconfigured.  This retains a reference on the type owner.
+
+ (*) void *fs_private
+
+     A pointer to the file system's private data.  This is where the filesystem
+     will need to store any options it parses.
+
+ (*) struct dentry *root
+
+     A pointer to the root of the mountable tree (and indirectly, the
+     superblock thereof).  This is filled in by the ->get_tree() op.  If this
+     is set, an active reference on root->d_sb must also be held.
+
+ (*) struct user_namespace *user_ns
+ (*) struct net *net_ns
+
+     There are a subset of the namespaces in use by the invoking process.  They
+     retain references on each namespace.  The subscribed namespaces may be
+     replaced by the filesystem to reflect other sources, such as the parent
+     mount superblock on an automount.
+
+ (*) const struct cred *cred
+
+     The mounter's credentials.  This retains a reference on the credentials.
+
+ (*) char *source
+
+     This specifies the source.  It may be a block device (e.g. /dev/sda1) or
+     something more exotic, such as the "host:/path" that NFS desires.
+
+ (*) char *subtype
+
+     This is a string to be added to the type displayed in /proc/mounts to
+     qualify it (used by FUSE).  This is available for the filesystem to set if
+     desired.
+
+ (*) void *security
+
+     A place for the LSMs to hang their security data for the superblock.  The
+     relevant security operations are described below.
+
+ (*) void *s_fs_info
+
+     The proposed s_fs_info for a new superblock, set in the superblock by
+     sget_fc().  This can be used to distinguish superblocks.
+
+ (*) unsigned int sb_flags
+ (*) unsigned int sb_flags_mask
+
+     Which bits SB_* flags are to be set/cleared in super_block::s_flags.
+
+ (*) enum fs_context_purpose
+
+     This indicates the purpose for which the context is intended.  The
+     available values are:
+
+	FS_CONTEXT_FOR_MOUNT,		-- New superblock for explicit mount
+	FS_CONTEXT_FOR_SUBMOUNT		-- New automatic submount of extant mount
+	FS_CONTEXT_FOR_RECONFIGURE	-- Change an existing mount
+
+ (*) bool sloppy
+ (*) bool silent
+
+     These are set if the sloppy or silent mount options are given.
+
+     [NOTE] sloppy is probably unnecessary when userspace passes over one
+     option at a time since the error can just be ignored if userspace deems it
+     to be unimportant.
+
+     [NOTE] silent is probably redundant with sb_flags & SB_SILENT.
+
+The mount context is created by calling vfs_new_fs_context() or
+vfs_dup_fs_context() and is destroyed with put_fs_context().  Note that the
+structure is not refcounted.
+
+VFS, security and filesystem mount options are set individually with
+vfs_parse_mount_option().  Options provided by the old mount(2) system call as
+a page of data can be parsed with generic_parse_monolithic().
+
+When mounting, the filesystem is allowed to take data from any of the pointers
+and attach it to the superblock (or whatever), provided it clears the pointer
+in the mount context.
+
+The filesystem is also allowed to allocate resources and pin them with the
+mount context.  For instance, NFS might pin the appropriate protocol version
+module.
+
+
+=================================
+THE FILESYSTEM CONTEXT OPERATIONS
+=================================
+
+The filesystem context points to a table of operations:
+
+	struct fs_context_operations {
+		void (*free)(struct fs_context *fc);
+		int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
+		int (*parse_param)(struct fs_context *fc,
+				   struct struct fs_parameter *param);
+		int (*parse_monolithic)(struct fs_context *fc, void *data);
+		int (*get_tree)(struct fs_context *fc);
+		int (*reconfigure)(struct fs_context *fc);
+	};
+
+These operations are invoked by the various stages of the mount procedure to
+manage the filesystem context.  They are as follows:
+
+ (*) void (*free)(struct fs_context *fc);
+
+     Called to clean up the filesystem-specific part of the filesystem context
+     when the context is destroyed.  It should be aware that parts of the
+     context may have been removed and NULL'd out by ->get_tree().
+
+ (*) int (*dup)(struct fs_context *fc, struct fs_context *src_fc);
+
+     Called when a filesystem context has been duplicated to duplicate the
+     filesystem-private data.  An error may be returned to indicate failure to
+     do this.
+
+     [!] Note that even if this fails, put_fs_context() will be called
+	 immediately thereafter, so ->dup() *must* make the
+	 filesystem-private data safe for ->free().
+
+ (*) int (*parse_param)(struct fs_context *fc,
+			struct struct fs_parameter *param);
+
+     Called when a parameter is being added to the filesystem context.  param
+     points to the key name and maybe a value object.  VFS-specific options
+     will have been weeded out and fc->sb_flags updated in the context.
+     Security options will also have been weeded out and fc->security updated.
+
+     The parameter can be parsed with fs_parse() and fs_lookup_param().  Note
+     that the source(s) are presented as parameters named "source".
+
+     If successful, 0 should be returned or a negative error code otherwise.
+
+ (*) int (*parse_monolithic)(struct fs_context *fc, void *data);
+
+     Called when the mount(2) system call is invoked to pass the entire data
+     page in one go.  If this is expected to be just a list of "key[=val]"
+     items separated by commas, then this may be set to NULL.
+
+     The return value is as for ->parse_param().
+
+     If the filesystem (e.g. NFS) needs to examine the data first and then
+     finds it's the standard key-val list then it may pass it off to
+     generic_parse_monolithic().
+
+ (*) int (*get_tree)(struct fs_context *fc);
+
+     Called to get or create the mountable root and superblock, using the
+     information stored in the filesystem context (reconfiguration goes via a
+     different vector).  It may detach any resources it desires from the
+     filesystem context and transfer them to the superblock it creates.
+
+     On success it should set fc->root to the mountable root and return 0.  In
+     the case of an error, it should return a negative error code.
+
+     The phase on a userspace-driven context will be set to only allow this to
+     be called once on any particular context.
+
+ (*) int (*reconfigure)(struct fs_context *fc);
+
+     Called to effect reconfiguration of a superblock using information stored
+     in the filesystem context.  It may detach any resources it desires from
+     the filesystem context and transfer them to the superblock.  The
+     superblock can be found from fc->root->d_sb.
+
+     On success it should return 0.  In the case of an error, it should return
+     a negative error code.
+
+     [NOTE] reconfigure is intended as a replacement for remount_fs.
+
+
+===========================
+FILESYSTEM CONTEXT SECURITY
+===========================
+
+The filesystem context contains a security pointer that the LSMs can use for
+building up a security context for the superblock to be mounted.  There are a
+number of operations used by the new mount code for this purpose:
+
+ (*) int security_fs_context_alloc(struct fs_context *fc,
+				   struct dentry *reference);
+
+     Called to initialise fc->security (which is preset to NULL) and allocate
+     any resources needed.  It should return 0 on success or a negative error
+     code on failure.
+
+     reference will be non-NULL if the context is being created for superblock
+     reconfiguration (FS_CONTEXT_FOR_RECONFIGURE) in which case it indicates
+     the root dentry of the superblock to be reconfigured.  It will also be
+     non-NULL in the case of a submount (FS_CONTEXT_FOR_SUBMOUNT) in which case
+     it indicates the automount point.
+
+ (*) int security_fs_context_dup(struct fs_context *fc,
+				 struct fs_context *src_fc);
+
+     Called to initialise fc->security (which is preset to NULL) and allocate
+     any resources needed.  The original filesystem context is pointed to by
+     src_fc and may be used for reference.  It should return 0 on success or a
+     negative error code on failure.
+
+ (*) void security_fs_context_free(struct fs_context *fc);
+
+     Called to clean up anything attached to fc->security.  Note that the
+     contents may have been transferred to a superblock and the pointer cleared
+     during get_tree.
+
+ (*) int security_fs_context_parse_param(struct fs_context *fc,
+					 struct fs_parameter *param);
+
+     Called for each mount parameter, including the source.  The arguments are
+     as for the ->parse_param() method.  It should return 0 to indicate that
+     the parameter should be passed on to the filesystem, 1 to indicate that
+     the parameter should be discarded or an error to indicate that the
+     parameter should be rejected.
+
+     The value pointed to by param may be modified (if a string) or stolen
+     (provided the value pointer is NULL'd out).  If it is stolen, 1 must be
+     returned to prevent it being passed to the filesystem.
+
+ (*) int security_fs_context_validate(struct fs_context *fc);
+
+     Called after all the options have been parsed to validate the collection
+     as a whole and to do any necessary allocation so that
+     security_sb_get_tree() and security_sb_reconfigure() are less likely to
+     fail.  It should return 0 or a negative error code.
+
+     In the case of reconfiguration, the target superblock will be accessible
+     via fc->root.
+
+ (*) int security_sb_get_tree(struct fs_context *fc);
+
+     Called during the mount procedure to verify that the specified superblock
+     is allowed to be mounted and to transfer the security data there.  It
+     should return 0 or a negative error code.
+
+ (*) void security_sb_reconfigure(struct fs_context *fc);
+
+     Called to apply any reconfiguration to an LSM's context.  It must not
+     fail.  Error checking and resource allocation must be done in advance by
+     the parameter parsing and validation hooks.
+
+ (*) int security_sb_mountpoint(struct fs_context *fc, struct path *mountpoint,
+				unsigned int mnt_flags);
+
+     Called during the mount procedure to verify that the root dentry attached
+     to the context is permitted to be attached to the specified mountpoint.
+     It should return 0 on success or a negative error code on failure.
+
+
+=================================
+VFS FILESYSTEM CONTEXT OPERATIONS
+=================================
+
+There are four operations for creating a filesystem context and
+one for destroying a context:
+
+ (*) struct fs_context *vfs_new_fs_context(struct file_system_type *fs_type,
+					   struct dentry *reference,
+					   unsigned int sb_flags,
+					   unsigned int sb_flags_mask,
+					   enum fs_context_purpose purpose);
+
+     Create a filesystem context for a given filesystem type and purpose.  This
+     allocates the filesystem context, sets the superblock flags, initialises
+     the security and calls fs_type->init_fs_context() to initialise the
+     filesystem private data.
+
+     reference can be NULL or it may indicate the root dentry of a superblock
+     that is going to be reconfigured (FS_CONTEXT_FOR_RECONFIGURE) or
+     the automount point that triggered a submount (FS_CONTEXT_FOR_SUBMOUNT).
+     This is provided as a source of namespace information.
+
+ (*) struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc);
+
+     Duplicate a filesystem context, copying any options noted and duplicating
+     or additionally referencing any resources held therein.  This is available
+     for use where a filesystem has to get a mount within a mount, such as NFS4
+     does by internally mounting the root of the target server and then doing a
+     private pathwalk to the target directory.
+
+     The purpose in the new context is inherited from the old one.
+
+ (*) void put_fs_context(struct fs_context *fc);
+
+     Destroy a filesystem context, releasing any resources it holds.  This
+     calls the ->free() operation.  This is intended to be called by anyone who
+     created a filesystem context.
+
+     [!] filesystem contexts are not refcounted, so this causes unconditional
+	 destruction.
+
+In all the above operations, apart from the put op, the return is a mount
+context pointer or a negative error code.
+
+For the remaining operations, if an error occurs, a negative error code will be
+returned.
+
+ (*) int vfs_get_tree(struct fs_context *fc);
+
+     Get or create the mountable root and superblock, using the parameters in
+     the filesystem context to select/configure the superblock.  This invokes
+     the ->validate() op and then the ->get_tree() op.
+
+     [NOTE] ->validate() could perhaps be rolled into ->get_tree() and
+     ->reconfigure().
+
+ (*) struct vfsmount *vfs_create_mount(struct fs_context *fc);
+
+     Create a mount given the parameters in the specified filesystem context.
+     Note that this does not attach the mount to anything.
+
+ (*) int vfs_parse_fs_param(struct fs_context *fc,
+			    struct fs_parameter *param);
+
+     Supply a single mount parameter to the filesystem context.  This include
+     the specification of the source/device which is specified as the "source"
+     parameter (which may be specified multiple times if the filesystem
+     supports that).
+
+     param specifies the parameter key name and the value.  The parameter is
+     first checked to see if it corresponds to a standard mount flag (in which
+     case it is used to set an SB_xxx flag and consumed) or a security option
+     (in which case the LSM consumes it) before it is passed on to the
+     filesystem.
+
+     The parameter value is typed and can be one of:
+
+	fs_value_is_flag,		Parameter not given a value.
+	fs_value_is_string,		Value is a string
+	fs_value_is_blob,		Value is a binary blob
+	fs_value_is_filename,		Value is a filename* + dirfd
+	fs_value_is_filename_empty,	Value is a filename* + dirfd + AT_EMPTY_PATH
+	fs_value_is_file,		Value is an open file (file*)
+
+     If there is a value, that value is stored in a union in the struct in one
+     of param->{string,blob,name,file}.  Note that the function may steal and
+     clear the pointer, but then becomes responsible for disposing of the
+     object.
+
+ (*) int vfs_parse_fs_string(struct fs_context *fc, char *key,
+			     const char *value, size_t v_size);
+
+     A wrapper around vfs_parse_fs_param() that just passes a constant string.
+
+ (*) int generic_parse_monolithic(struct fs_context *fc, void *data);
+
+     Parse a sys_mount() data page, assuming the form to be a text list
+     consisting of key[=val] options separated by commas.  Each item in the
+     list is passed to vfs_mount_option().  This is the default when the
+     ->parse_monolithic() operation is NULL.
+
+
+=====================
+PARAMETER DESCRIPTION
+=====================
+
+Parameters are described using structures defined in linux/fs_parser.h.
+There's a core description struct that links everything together:
+
+	struct fs_parameter_description {
+		const char	name[16];
+		u8		nr_params;
+		u8		nr_alt_keys;
+		u8		nr_enums;
+		bool		ignore_unknown;
+		bool		no_source;
+		const char *const *keys;
+		const struct constant_table *alt_keys;
+		const struct fs_parameter_spec *specs;
+		const struct fs_parameter_enum *enums;
+	};
+
+For example:
+
+	enum afs_param {
+		Opt_autocell,
+		Opt_bar,
+		Opt_dyn,
+		Opt_foo,
+		Opt_source,
+		nr__afs_params
+	};
+
+	static const struct fs_parameter_description afs_fs_parameters = {
+		.name		= "kAFS",
+		.nr_params	= nr__afs_params,
+		.nr_alt_keys	= ARRAY_SIZE(afs_param_alt_keys),
+		.nr_enums	= ARRAY_SIZE(afs_param_enums),
+		.keys		= afs_param_keys,
+		.alt_keys	= afs_param_alt_keys,
+		.specs		= afs_param_specs,
+		.enums		= afs_param_enums,
+	};
+
+The members are as follows:
+
+ (1) const char name[16];
+
+     The name to be used in error messages generated by the parse helper
+     functions.
+
+ (2) u8 nr_params;
+
+     The number of discrete parameter identifiers.  This indicates the number
+     of elements in the ->types[] array and also limits the values that may be
+     used in the values that the ->keys[] array maps to.
+
+     It is expected that, for example, two parameters that are related, say
+     "acl" and "noacl" with have the same ID, but will be flagged to indicate
+     that one is the inverse of the other.  The value can then be picked out
+     from the parse result.
+
+ (3) const struct fs_parameter_specification *specs;
+
+     Table of parameter specifications, where the entries are of type:
+
+	struct fs_parameter_type {
+		enum fs_parameter_spec	type:8;
+		u8			flags;
+	};
+
+     and the parameter identifier is the index to the array.  'type' indicates
+     the desired value type and must be one of:
+
+	TYPE NAME		EXPECTED VALUE		RESULT IN
+	=======================	=======================	=====================
+	fs_param_is_flag	No value		n/a
+	fs_param_is_bool	Boolean value		result->boolean
+	fs_param_is_u32		32-bit unsigned int	result->uint_32
+	fs_param_is_u32_octal	32-bit octal int	result->uint_32
+	fs_param_is_u32_hex	32-bit hex int		result->uint_32
+	fs_param_is_s32		32-bit signed int	result->int_32
+	fs_param_is_enum	Enum value name 	result->uint_32
+	fs_param_is_string	Arbitrary string	param->string
+	fs_param_is_blob	Binary blob		param->blob
+	fs_param_is_blockdev	Blockdev path		* Needs lookup
+	fs_param_is_path	Path			* Needs lookup
+	fs_param_is_fd		File descriptor		param->file
+
+     And each parameter can be qualified with 'flags':
+
+     	fs_param_v_optional	The value is optional
+	fs_param_neg_with_no	If key name is prefixed with "no", it is false
+	fs_param_neg_with_empty	If value is "", it is false
+	fs_param_deprecated	The parameter is deprecated.
+
+     For example:
+
+	static const struct fs_parameter_spec afs_param_specs[nr__afs_params] = {
+		[Opt_autocell]	= { fs_param_is flag },
+		[Opt_bar]	= { fs_param_is_enum },
+		[Opt_dyn]	= { fs_param_is flag },
+		[Opt_foo]	= { fs_param_is_bool, fs_param_neg_with_no },
+		[Opt_source]	= { fs_param_is_string },
+	};
+
+     Note that if the value is of fs_param_is_bool type, fs_parse() will try
+     to match any string value against "0", "1", "no", "yes", "false", "true".
+
+     [!] NOTE that the table must be sorted according to primary key name so
+     	 that ->keys[] is also sorted.
+
+ (4) const char *const *keys;
+
+     Table of primary key names for the parameters.  There must be one entry
+     per defined parameter.  The table is optional if ->nr_params is 0.  The
+     table is just an array of names e.g.:
+
+	static const char *const afs_param_keys[nr__afs_params] = {
+		[Opt_autocell]	= "autocell",
+		[Opt_bar]	= "bar",
+		[Opt_dyn]	= "dyn",
+		[Opt_foo]	= "foo",
+		[Opt_source]	= "source",
+	};
+
+     [!] NOTE that the table must be sorted such that the table can be searched
+     	 with bsearch() using strcmp().  This means that the Opt_* values must
+     	 correspond to the entries in this table.
+
+ (5) const struct constant_table *alt_keys;
+     u8 nr_alt_keys;
+
+     Table of additional key names and their mappings to parameter ID plus the
+     number of elements in the table.  This is optional.  The table is just an
+     array of { name, integer } pairs, e.g.:
+
+	static const struct constant_table afs_param_keys[] = {
+		{ "baz",	Opt_bar },
+		{ "dynamic",	Opt_dyn },
+	};
+
+     [!] NOTE that the table must be sorted such that strcmp() can be used with
+     	 bsearch() to search the entries.
+
+     The parameter ID can also be fs_param_key_removed to indicate that a
+     deprecated parameter has been removed and that an error will be given.
+     This differs from fs_param_deprecated where the parameter may still have
+     an effect.
+
+     Further, the behaviour of the parameter may differ when an alternate name
+     is used (for instance with NFS, "v3", "v4.2", etc. are alternate names).
+
+ (6) const struct fs_parameter_enum *enums;
+     u8 nr_enums;
+
+     Table of enum value names to integer mappings and the number of elements
+     stored therein.  This is of type:
+
+	struct fs_parameter_enum {
+		u8		param_id;
+		char		name[14];
+		u8		value;
+	};
+
+     Where the array is an unsorted list of { parameter ID, name }-keyed
+     elements that indicate the value to map to, e.g.:
+
+	static const struct fs_parameter_enum afs_param_enums[] = {
+		{ Opt_bar,   "x",      1},
+		{ Opt_bar,   "y",      23},
+		{ Opt_bar,   "z",      42},
+	};
+
+     If a parameter of type fs_param_is_enum is encountered, fs_parse() will
+     try to look the value up in the enum table and the result will be stored
+     in the parse result.
+
+ (7) bool no_source;
+
+     If this is set, fs_parse() will ignore any "source" parameter and not
+     pass it to the filesystem.
+
+The parser should be pointed to by the parser pointer in the file_system_type
+struct as this will provide validation on registration (if
+CONFIG_VALIDATE_FS_PARSER=y) and will allow the description to be queried from
+userspace using the fsinfo() syscall.
+
+
+==========================
+PARAMETER HELPER FUNCTIONS
+==========================
+
+A number of helper functions are provided to help a filesystem or an LSM
+process the parameters it is given.
+
+ (*) int lookup_constant(const struct constant_table tbl[],
+			 const char *name, int not_found);
+
+     Look up a constant by name in a table of name -> integer mappings.  The
+     table is an array of elements of the following type:
+
+	struct constant_table {
+		const char	*name;
+		int		value;
+	};
+
+     and it must be sorted such that it can be searched using bsearch() using
+     strcmp().  If a match is found, the corresponding value is returned.  If a
+     match isn't found, the not_found value is returned instead.
+
+ (*) bool validate_constant_table(const struct constant_table *tbl,
+				  size_t tbl_size,
+				  int low, int high, int special);
+
+     Validate a constant table.  Checks that all the elements are appropriately
+     ordered, that there are no duplicates and that the values are between low
+     and high inclusive, though provision is made for one allowable special
+     value outside of that range.  If no special value is required, special
+     should just be set to lie inside the low-to-high range.
+
+     If all is good, true is returned.  If the table is invalid, errors are
+     logged to dmesg, the stack is dumped and false is returned.
+
+ (*) int fs_parse(struct fs_context *fc,
+		  const struct fs_param_parser *parser,
+		  struct fs_parameter *param,
+		  struct fs_param_parse_result *result);
+
+     This is the main interpreter of parameters.  It uses the parameter
+     description (parser) to look up the name of the parameter to use and to
+     convert that to a parameter ID (stored in result->key).
+
+     If successful, and if the parameter type indicates the result is a
+     boolean, integer or enum type, the value is converted by this function and
+     the result stored in result->{boolean,int_32,uint_32}.
+
+     If a match isn't initially made, the key is prefixed with "no" and no
+     value is present then an attempt will be made to look up the key with the
+     prefix removed.  If this matches a parameter for which the type has flag
+     fs_param_neg_with_no set, then a match will be made and the value will be
+     set to false/0/NULL.
+
+     If the parameter is successfully matched and, optionally, parsed
+     correctly, 1 is returned.  If the parameter isn't matched and
+     parser->ignore_unknown is set, then 0 is returned.  Otherwise -EINVAL is
+     returned.
+
+ (*) bool fs_validate_description(const struct fs_parameter_description *desc);
+
+     This is validates the parameter description.  It returns true if the
+     description is good and false if it is not.
+
+ (*) int fs_lookup_param(struct fs_context *fc,
+			 struct fs_parameter *value,
+			 bool want_bdev,
+			 struct path *_path);
+
+     This takes a parameter that carries a string or filename type and attempts
+     to do a path lookup on it.  If the parameter expects a blockdev, a check
+     is made that the inode actually represents one.
+
+     Returns 0 if successful and *_path will be set; returns a negative error
+     code if not.