[PATCH 24/38] vfs: Provide documentation for new mount API [ver #10]

David Howells
2018-07-27T17:34:10Z

Provide documentation for the new mount API.

Signed-off-by: David Howells <dhowells@redhat.com>
---

Documentation/filesystems/mount_api.txt | 706 +++++++++++++++++++++++++++++++
1 file changed, 706 insertions(+)
create mode 100644 Documentation/filesystems/mount_api.txt

diff --git a/Documentation/filesystems/mount_api.txt b/Documentation/filesystems/mount_api.txt
new file mode 100644
index 000000000000..5fec78eed4f4
--- /dev/null
+++ b/Documentation/filesystems/mount_api.txt
@@ -0,0 +1,706 @@
+ ====================
+ 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, struct dentry *reference);
+
+which is invoked to set up the filesystem-specific parts of a filesystem
+context, including the additional space. The reference parameter is used to
+convey a superblock and an automount point or a point to reconfigure from which
+the filesystem may draw extra information (such as namespaces) for submount
+(FS_CONTEXT_FOR_SUBMOUNT) or reconfiguration (FS_CONTEXT_FOR_RECONFIGURE)
+purposes - otherwise it will be NULL.
+
+Note that security initialisation is done *after* the filesystem is called so
+that the namespaces may be adjusted first.
+
+And the super_operations struct gains one field:
+
+ int (*reconfigure)(struct super_block *, struct fs_context *);
+
+This shadows the ->reconfigure() operation and takes a prepared filesystem
+context instead of the mount flags and data page. It may modify the sb_flags
+in the context for the caller to pick up.
+
+[NOTE] reconfigure is intended as a replacement for remount_fs.
+
+
+======================
+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;
+ 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
+
+ This holds the SB_* flags to be set 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_USER_MOUNT, -- New superblock for user-specified mount
+ FS_CONTEXT_FOR_KERNEL_MOUNT, -- New superblock for kernel-internal 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(), vfs_sb_reconfig()
+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,
+ size_t data_size);
+ int (*validate)(struct fs_context *fc);
+ int (*get_tree)(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, size_t data_size);
+
+ 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 (*validate)(struct fs_context *fc);
+
+ Called when all the options have been applied and the mount is about to
+ take place. It is should check for inconsistencies from mount options and
+ it is also allowed to do preliminary resource acquisition. For instance,
+ the core NFS module could load the NFS protocol module here.
+
+ Note that if fc->purpose == FS_CONTEXT_FOR_RECONFIGURE, some of the
+ options necessary for a new mount may not be set.
+
+ The return value is as for ->parse_option().
+
+ (*) 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.
+
+
+===========================
+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() is less likely to fail. It should return 0 or a
+ negative error code.
+
+ (*) 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.
+
+ (*) 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,
+ enum fs_context_purpose purpose);
+
+ Create a filesystem context for a given filesystem type and purpose. This
+ allocates the filesystem context, sets the 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_sb_reconfig(struct vfsmount *mnt,
+ unsigned int sb_flags);
+
+ Create a filesystem context from the same filesystem as an extant mount
+ and initialise the mount parameters from the superblock underlying that
+ mount. This is for use by superblock parameter reconfiguration.
+
+ (*) 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.
+
+ (*) 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, size_t data_len);
+
+ 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_keys;
+ u8 nr_enums;
+ bool ignore_unknown;
+ bool no_source;
+ const struct constant_table *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_keys = ARRAY_SIZE(afs_param_keys),
+ .nr_enums = ARRAY_SIZE(afs_param_enums),
+ .keys = afs_param_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_takes_no_value 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_takes_no_value },
+ [Opt_bar] = { fs_param_is_enum },
+ [Opt_dyn] = { fs_param_takes_no_value },
+ [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".
+
+ (4) const struct constant_table *keys;
+ u8 nr_keys;
+
+ Table of key name to parameter ID mappings and the number of elements in
+ the table. This is optional if ->nr_params is 0. The table is just an
+ array of { name, integer } pairs, e.g.:
+
+ static const struct constant_table afs_param_keys[] = {
+ { "autocell", Opt_autocell },
+ { "bar", Opt_bar },
+ { "dyn", Opt_dyn },
+ { "foo", Opt_foo },
+ { "source", Opt_source },
+ };
+
+ [!] NOTE that the table must be sorted for bsearch() to use strcmp() to
+ compare 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.
+
+ (5) 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.
+
+ (6) bool ignore_unknown;
+
+ If this is set, fs_parse() will not generate an error for unknown
+ parameters, but will rather return 0.
+
+ (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_parser(const char *name,
+ const struct fs_param_parser *parser);
+
+ This is validates the parameter description. 'name' is used to illuminate
+ any error messages that are logged. It returns true if the description is
+ good and false if it is not.
+
+ (*) int fs_lookup_param(struct fs_context *fc,
+ const struct fs_param_parser *parser,
+ struct fs_parameter *value,
+ struct fs_parse_result *result,
+ struct path *_path);
+
+ This takes the result of fs_parse() for string and filename type
+ parameters and attemps to do a path lookup on them. 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.