1 ==================== 2 FILESYSTEM MOUNT API 3 ==================== 4 5CONTENTS 6 7 (1) Overview. 8 9 (2) The filesystem context. 10 11 (3) The filesystem context operations. 12 13 (4) Filesystem context security. 14 15 (5) VFS filesystem context API. 16 17 (6) Superblock creation helpers. 18 19 (7) Parameter description. 20 21 (8) Parameter helper functions. 22 23 24======== 25OVERVIEW 26======== 27 28The creation of new mounts is now to be done in a multistep process: 29 30 (1) Create a filesystem context. 31 32 (2) Parse the parameters and attach them to the context. Parameters are 33 expected to be passed individually from userspace, though legacy binary 34 parameters can also be handled. 35 36 (3) Validate and pre-process the context. 37 38 (4) Get or create a superblock and mountable root. 39 40 (5) Perform the mount. 41 42 (6) Return an error message attached to the context. 43 44 (7) Destroy the context. 45 46To support this, the file_system_type struct gains two new fields: 47 48 int (*init_fs_context)(struct fs_context *fc); 49 const struct fs_parameter_description *parameters; 50 51The first is invoked to set up the filesystem-specific parts of a filesystem 52context, including the additional space, and the second points to the 53parameter description for validation at registration time and querying by a 54future system call. 55 56Note that security initialisation is done *after* the filesystem is called so 57that the namespaces may be adjusted first. 58 59 60====================== 61THE FILESYSTEM CONTEXT 62====================== 63 64The creation and reconfiguration of a superblock is governed by a filesystem 65context. This is represented by the fs_context structure: 66 67 struct fs_context { 68 const struct fs_context_operations *ops; 69 struct file_system_type *fs_type; 70 void *fs_private; 71 struct dentry *root; 72 struct user_namespace *user_ns; 73 struct net *net_ns; 74 const struct cred *cred; 75 char *source; 76 char *subtype; 77 void *security; 78 void *s_fs_info; 79 unsigned int sb_flags; 80 unsigned int sb_flags_mask; 81 unsigned int s_iflags; 82 unsigned int lsm_flags; 83 enum fs_context_purpose purpose:8; 84 ... 85 }; 86 87The fs_context fields are as follows: 88 89 (*) const struct fs_context_operations *ops 90 91 These are operations that can be done on a filesystem context (see 92 below). This must be set by the ->init_fs_context() file_system_type 93 operation. 94 95 (*) struct file_system_type *fs_type 96 97 A pointer to the file_system_type of the filesystem that is being 98 constructed or reconfigured. This retains a reference on the type owner. 99 100 (*) void *fs_private 101 102 A pointer to the file system's private data. This is where the filesystem 103 will need to store any options it parses. 104 105 (*) struct dentry *root 106 107 A pointer to the root of the mountable tree (and indirectly, the 108 superblock thereof). This is filled in by the ->get_tree() op. If this 109 is set, an active reference on root->d_sb must also be held. 110 111 (*) struct user_namespace *user_ns 112 (*) struct net *net_ns 113 114 There are a subset of the namespaces in use by the invoking process. They 115 retain references on each namespace. The subscribed namespaces may be 116 replaced by the filesystem to reflect other sources, such as the parent 117 mount superblock on an automount. 118 119 (*) const struct cred *cred 120 121 The mounter's credentials. This retains a reference on the credentials. 122 123 (*) char *source 124 125 This specifies the source. It may be a block device (e.g. /dev/sda1) or 126 something more exotic, such as the "host:/path" that NFS desires. 127 128 (*) char *subtype 129 130 This is a string to be added to the type displayed in /proc/mounts to 131 qualify it (used by FUSE). This is available for the filesystem to set if 132 desired. 133 134 (*) void *security 135 136 A place for the LSMs to hang their security data for the superblock. The 137 relevant security operations are described below. 138 139 (*) void *s_fs_info 140 141 The proposed s_fs_info for a new superblock, set in the superblock by 142 sget_fc(). This can be used to distinguish superblocks. 143 144 (*) unsigned int sb_flags 145 (*) unsigned int sb_flags_mask 146 147 Which bits SB_* flags are to be set/cleared in super_block::s_flags. 148 149 (*) unsigned int s_iflags 150 151 These will be bitwise-OR'd with s->s_iflags when a superblock is created. 152 153 (*) enum fs_context_purpose 154 155 This indicates the purpose for which the context is intended. The 156 available values are: 157 158 FS_CONTEXT_FOR_MOUNT, -- New superblock for explicit mount 159 FS_CONTEXT_FOR_SUBMOUNT -- New automatic submount of extant mount 160 FS_CONTEXT_FOR_RECONFIGURE -- Change an existing mount 161 162The mount context is created by calling vfs_new_fs_context() or 163vfs_dup_fs_context() and is destroyed with put_fs_context(). Note that the 164structure is not refcounted. 165 166VFS, security and filesystem mount options are set individually with 167vfs_parse_mount_option(). Options provided by the old mount(2) system call as 168a page of data can be parsed with generic_parse_monolithic(). 169 170When mounting, the filesystem is allowed to take data from any of the pointers 171and attach it to the superblock (or whatever), provided it clears the pointer 172in the mount context. 173 174The filesystem is also allowed to allocate resources and pin them with the 175mount context. For instance, NFS might pin the appropriate protocol version 176module. 177 178 179================================= 180THE FILESYSTEM CONTEXT OPERATIONS 181================================= 182 183The filesystem context points to a table of operations: 184 185 struct fs_context_operations { 186 void (*free)(struct fs_context *fc); 187 int (*dup)(struct fs_context *fc, struct fs_context *src_fc); 188 int (*parse_param)(struct fs_context *fc, 189 struct struct fs_parameter *param); 190 int (*parse_monolithic)(struct fs_context *fc, void *data); 191 int (*get_tree)(struct fs_context *fc); 192 int (*reconfigure)(struct fs_context *fc); 193 }; 194 195These operations are invoked by the various stages of the mount procedure to 196manage the filesystem context. They are as follows: 197 198 (*) void (*free)(struct fs_context *fc); 199 200 Called to clean up the filesystem-specific part of the filesystem context 201 when the context is destroyed. It should be aware that parts of the 202 context may have been removed and NULL'd out by ->get_tree(). 203 204 (*) int (*dup)(struct fs_context *fc, struct fs_context *src_fc); 205 206 Called when a filesystem context has been duplicated to duplicate the 207 filesystem-private data. An error may be returned to indicate failure to 208 do this. 209 210 [!] Note that even if this fails, put_fs_context() will be called 211 immediately thereafter, so ->dup() *must* make the 212 filesystem-private data safe for ->free(). 213 214 (*) int (*parse_param)(struct fs_context *fc, 215 struct struct fs_parameter *param); 216 217 Called when a parameter is being added to the filesystem context. param 218 points to the key name and maybe a value object. VFS-specific options 219 will have been weeded out and fc->sb_flags updated in the context. 220 Security options will also have been weeded out and fc->security updated. 221 222 The parameter can be parsed with fs_parse() and fs_lookup_param(). Note 223 that the source(s) are presented as parameters named "source". 224 225 If successful, 0 should be returned or a negative error code otherwise. 226 227 (*) int (*parse_monolithic)(struct fs_context *fc, void *data); 228 229 Called when the mount(2) system call is invoked to pass the entire data 230 page in one go. If this is expected to be just a list of "key[=val]" 231 items separated by commas, then this may be set to NULL. 232 233 The return value is as for ->parse_param(). 234 235 If the filesystem (e.g. NFS) needs to examine the data first and then 236 finds it's the standard key-val list then it may pass it off to 237 generic_parse_monolithic(). 238 239 (*) int (*get_tree)(struct fs_context *fc); 240 241 Called to get or create the mountable root and superblock, using the 242 information stored in the filesystem context (reconfiguration goes via a 243 different vector). It may detach any resources it desires from the 244 filesystem context and transfer them to the superblock it creates. 245 246 On success it should set fc->root to the mountable root and return 0. In 247 the case of an error, it should return a negative error code. 248 249 The phase on a userspace-driven context will be set to only allow this to 250 be called once on any particular context. 251 252 (*) int (*reconfigure)(struct fs_context *fc); 253 254 Called to effect reconfiguration of a superblock using information stored 255 in the filesystem context. It may detach any resources it desires from 256 the filesystem context and transfer them to the superblock. The 257 superblock can be found from fc->root->d_sb. 258 259 On success it should return 0. In the case of an error, it should return 260 a negative error code. 261 262 [NOTE] reconfigure is intended as a replacement for remount_fs. 263 264 265=========================== 266FILESYSTEM CONTEXT SECURITY 267=========================== 268 269The filesystem context contains a security pointer that the LSMs can use for 270building up a security context for the superblock to be mounted. There are a 271number of operations used by the new mount code for this purpose: 272 273 (*) int security_fs_context_alloc(struct fs_context *fc, 274 struct dentry *reference); 275 276 Called to initialise fc->security (which is preset to NULL) and allocate 277 any resources needed. It should return 0 on success or a negative error 278 code on failure. 279 280 reference will be non-NULL if the context is being created for superblock 281 reconfiguration (FS_CONTEXT_FOR_RECONFIGURE) in which case it indicates 282 the root dentry of the superblock to be reconfigured. It will also be 283 non-NULL in the case of a submount (FS_CONTEXT_FOR_SUBMOUNT) in which case 284 it indicates the automount point. 285 286 (*) int security_fs_context_dup(struct fs_context *fc, 287 struct fs_context *src_fc); 288 289 Called to initialise fc->security (which is preset to NULL) and allocate 290 any resources needed. The original filesystem context is pointed to by 291 src_fc and may be used for reference. It should return 0 on success or a 292 negative error code on failure. 293 294 (*) void security_fs_context_free(struct fs_context *fc); 295 296 Called to clean up anything attached to fc->security. Note that the 297 contents may have been transferred to a superblock and the pointer cleared 298 during get_tree. 299 300 (*) int security_fs_context_parse_param(struct fs_context *fc, 301 struct fs_parameter *param); 302 303 Called for each mount parameter, including the source. The arguments are 304 as for the ->parse_param() method. It should return 0 to indicate that 305 the parameter should be passed on to the filesystem, 1 to indicate that 306 the parameter should be discarded or an error to indicate that the 307 parameter should be rejected. 308 309 The value pointed to by param may be modified (if a string) or stolen 310 (provided the value pointer is NULL'd out). If it is stolen, 1 must be 311 returned to prevent it being passed to the filesystem. 312 313 (*) int security_fs_context_validate(struct fs_context *fc); 314 315 Called after all the options have been parsed to validate the collection 316 as a whole and to do any necessary allocation so that 317 security_sb_get_tree() and security_sb_reconfigure() are less likely to 318 fail. It should return 0 or a negative error code. 319 320 In the case of reconfiguration, the target superblock will be accessible 321 via fc->root. 322 323 (*) int security_sb_get_tree(struct fs_context *fc); 324 325 Called during the mount procedure to verify that the specified superblock 326 is allowed to be mounted and to transfer the security data there. It 327 should return 0 or a negative error code. 328 329 (*) void security_sb_reconfigure(struct fs_context *fc); 330 331 Called to apply any reconfiguration to an LSM's context. It must not 332 fail. Error checking and resource allocation must be done in advance by 333 the parameter parsing and validation hooks. 334 335 (*) int security_sb_mountpoint(struct fs_context *fc, struct path *mountpoint, 336 unsigned int mnt_flags); 337 338 Called during the mount procedure to verify that the root dentry attached 339 to the context is permitted to be attached to the specified mountpoint. 340 It should return 0 on success or a negative error code on failure. 341 342 343========================== 344VFS FILESYSTEM CONTEXT API 345========================== 346 347There are four operations for creating a filesystem context and one for 348destroying a context: 349 350 (*) struct fs_context *fs_context_for_mount( 351 struct file_system_type *fs_type, 352 unsigned int sb_flags); 353 354 Allocate a filesystem context for the purpose of setting up a new mount, 355 whether that be with a new superblock or sharing an existing one. This 356 sets the superblock flags, initialises the security and calls 357 fs_type->init_fs_context() to initialise the filesystem private data. 358 359 fs_type specifies the filesystem type that will manage the context and 360 sb_flags presets the superblock flags stored therein. 361 362 (*) struct fs_context *fs_context_for_reconfigure( 363 struct dentry *dentry, 364 unsigned int sb_flags, 365 unsigned int sb_flags_mask); 366 367 Allocate a filesystem context for the purpose of reconfiguring an 368 existing superblock. dentry provides a reference to the superblock to be 369 configured. sb_flags and sb_flags_mask indicate which superblock flags 370 need changing and to what. 371 372 (*) struct fs_context *fs_context_for_submount( 373 struct file_system_type *fs_type, 374 struct dentry *reference); 375 376 Allocate a filesystem context for the purpose of creating a new mount for 377 an automount point or other derived superblock. fs_type specifies the 378 filesystem type that will manage the context and the reference dentry 379 supplies the parameters. Namespaces are propagated from the reference 380 dentry's superblock also. 381 382 Note that it's not a requirement that the reference dentry be of the same 383 filesystem type as fs_type. 384 385 (*) struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc); 386 387 Duplicate a filesystem context, copying any options noted and duplicating 388 or additionally referencing any resources held therein. This is available 389 for use where a filesystem has to get a mount within a mount, such as NFS4 390 does by internally mounting the root of the target server and then doing a 391 private pathwalk to the target directory. 392 393 The purpose in the new context is inherited from the old one. 394 395 (*) void put_fs_context(struct fs_context *fc); 396 397 Destroy a filesystem context, releasing any resources it holds. This 398 calls the ->free() operation. This is intended to be called by anyone who 399 created a filesystem context. 400 401 [!] filesystem contexts are not refcounted, so this causes unconditional 402 destruction. 403 404In all the above operations, apart from the put op, the return is a mount 405context pointer or a negative error code. 406 407For the remaining operations, if an error occurs, a negative error code will be 408returned. 409 410 (*) int vfs_parse_fs_param(struct fs_context *fc, 411 struct fs_parameter *param); 412 413 Supply a single mount parameter to the filesystem context. This include 414 the specification of the source/device which is specified as the "source" 415 parameter (which may be specified multiple times if the filesystem 416 supports that). 417 418 param specifies the parameter key name and the value. The parameter is 419 first checked to see if it corresponds to a standard mount flag (in which 420 case it is used to set an SB_xxx flag and consumed) or a security option 421 (in which case the LSM consumes it) before it is passed on to the 422 filesystem. 423 424 The parameter value is typed and can be one of: 425 426 fs_value_is_flag, Parameter not given a value. 427 fs_value_is_string, Value is a string 428 fs_value_is_blob, Value is a binary blob 429 fs_value_is_filename, Value is a filename* + dirfd 430 fs_value_is_filename_empty, Value is a filename* + dirfd + AT_EMPTY_PATH 431 fs_value_is_file, Value is an open file (file*) 432 433 If there is a value, that value is stored in a union in the struct in one 434 of param->{string,blob,name,file}. Note that the function may steal and 435 clear the pointer, but then becomes responsible for disposing of the 436 object. 437 438 (*) int vfs_parse_fs_string(struct fs_context *fc, const char *key, 439 const char *value, size_t v_size); 440 441 A wrapper around vfs_parse_fs_param() that copies the value string it is 442 passed. 443 444 (*) int generic_parse_monolithic(struct fs_context *fc, void *data); 445 446 Parse a sys_mount() data page, assuming the form to be a text list 447 consisting of key[=val] options separated by commas. Each item in the 448 list is passed to vfs_mount_option(). This is the default when the 449 ->parse_monolithic() method is NULL. 450 451 (*) int vfs_get_tree(struct fs_context *fc); 452 453 Get or create the mountable root and superblock, using the parameters in 454 the filesystem context to select/configure the superblock. This invokes 455 the ->get_tree() method. 456 457 (*) struct vfsmount *vfs_create_mount(struct fs_context *fc); 458 459 Create a mount given the parameters in the specified filesystem context. 460 Note that this does not attach the mount to anything. 461 462 463=========================== 464SUPERBLOCK CREATION HELPERS 465=========================== 466 467A number of VFS helpers are available for use by filesystems for the creation 468or looking up of superblocks. 469 470 (*) struct super_block * 471 sget_fc(struct fs_context *fc, 472 int (*test)(struct super_block *sb, struct fs_context *fc), 473 int (*set)(struct super_block *sb, struct fs_context *fc)); 474 475 This is the core routine. If test is non-NULL, it searches for an 476 existing superblock matching the criteria held in the fs_context, using 477 the test function to match them. If no match is found, a new superblock 478 is created and the set function is called to set it up. 479 480 Prior to the set function being called, fc->s_fs_info will be transferred 481 to sb->s_fs_info - and fc->s_fs_info will be cleared if set returns 482 success (ie. 0). 483 484The following helpers all wrap sget_fc(): 485 486 (*) int vfs_get_super(struct fs_context *fc, 487 enum vfs_get_super_keying keying, 488 int (*fill_super)(struct super_block *sb, 489 struct fs_context *fc)) 490 491 This creates/looks up a deviceless superblock. The keying indicates how 492 many superblocks of this type may exist and in what manner they may be 493 shared: 494 495 (1) vfs_get_single_super 496 497 Only one such superblock may exist in the system. Any further 498 attempt to get a new superblock gets this one (and any parameter 499 differences are ignored). 500 501 (2) vfs_get_keyed_super 502 503 Multiple superblocks of this type may exist and they're keyed on 504 their s_fs_info pointer (for example this may refer to a 505 namespace). 506 507 (3) vfs_get_independent_super 508 509 Multiple independent superblocks of this type may exist. This 510 function never matches an existing one and always creates a new 511 one. 512 513 514===================== 515PARAMETER DESCRIPTION 516===================== 517 518Parameters are described using structures defined in linux/fs_parser.h. 519There's a core description struct that links everything together: 520 521 struct fs_parameter_description { 522 const char name[16]; 523 const struct fs_parameter_spec *specs; 524 const struct fs_parameter_enum *enums; 525 }; 526 527For example: 528 529 enum { 530 Opt_autocell, 531 Opt_bar, 532 Opt_dyn, 533 Opt_foo, 534 Opt_source, 535 }; 536 537 static const struct fs_parameter_description afs_fs_parameters = { 538 .name = "kAFS", 539 .specs = afs_param_specs, 540 .enums = afs_param_enums, 541 }; 542 543The members are as follows: 544 545 (1) const char name[16]; 546 547 The name to be used in error messages generated by the parse helper 548 functions. 549 550 (2) const struct fs_parameter_specification *specs; 551 552 Table of parameter specifications, terminated with a null entry, where the 553 entries are of type: 554 555 struct fs_parameter_spec { 556 const char *name; 557 u8 opt; 558 enum fs_parameter_type type:8; 559 unsigned short flags; 560 }; 561 562 The 'name' field is a string to match exactly to the parameter key (no 563 wildcards, patterns and no case-independence) and 'opt' is the value that 564 will be returned by the fs_parser() function in the case of a successful 565 match. 566 567 The 'type' field indicates the desired value type and must be one of: 568 569 TYPE NAME EXPECTED VALUE RESULT IN 570 ======================= ======================= ===================== 571 fs_param_is_flag No value n/a 572 fs_param_is_bool Boolean value result->boolean 573 fs_param_is_u32 32-bit unsigned int result->uint_32 574 fs_param_is_u32_octal 32-bit octal int result->uint_32 575 fs_param_is_u32_hex 32-bit hex int result->uint_32 576 fs_param_is_s32 32-bit signed int result->int_32 577 fs_param_is_u64 64-bit unsigned int result->uint_64 578 fs_param_is_enum Enum value name result->uint_32 579 fs_param_is_string Arbitrary string param->string 580 fs_param_is_blob Binary blob param->blob 581 fs_param_is_blockdev Blockdev path * Needs lookup 582 fs_param_is_path Path * Needs lookup 583 fs_param_is_fd File descriptor result->int_32 584 585 Note that if the value is of fs_param_is_bool type, fs_parse() will try 586 to match any string value against "0", "1", "no", "yes", "false", "true". 587 588 Each parameter can also be qualified with 'flags': 589 590 fs_param_v_optional The value is optional 591 fs_param_neg_with_no result->negated set if key is prefixed with "no" 592 fs_param_neg_with_empty result->negated set if value is "" 593 fs_param_deprecated The parameter is deprecated. 594 595 These are wrapped with a number of convenience wrappers: 596 597 MACRO SPECIFIES 598 ======================= =============================================== 599 fsparam_flag() fs_param_is_flag 600 fsparam_flag_no() fs_param_is_flag, fs_param_neg_with_no 601 fsparam_bool() fs_param_is_bool 602 fsparam_u32() fs_param_is_u32 603 fsparam_u32oct() fs_param_is_u32_octal 604 fsparam_u32hex() fs_param_is_u32_hex 605 fsparam_s32() fs_param_is_s32 606 fsparam_u64() fs_param_is_u64 607 fsparam_enum() fs_param_is_enum 608 fsparam_string() fs_param_is_string 609 fsparam_blob() fs_param_is_blob 610 fsparam_bdev() fs_param_is_blockdev 611 fsparam_path() fs_param_is_path 612 fsparam_fd() fs_param_is_fd 613 614 all of which take two arguments, name string and option number - for 615 example: 616 617 static const struct fs_parameter_spec afs_param_specs[] = { 618 fsparam_flag ("autocell", Opt_autocell), 619 fsparam_flag ("dyn", Opt_dyn), 620 fsparam_string ("source", Opt_source), 621 fsparam_flag_no ("foo", Opt_foo), 622 {} 623 }; 624 625 An addition macro, __fsparam() is provided that takes an additional pair 626 of arguments to specify the type and the flags for anything that doesn't 627 match one of the above macros. 628 629 (6) const struct fs_parameter_enum *enums; 630 631 Table of enum value names to integer mappings, terminated with a null 632 entry. This is of type: 633 634 struct fs_parameter_enum { 635 u8 opt; 636 char name[14]; 637 u8 value; 638 }; 639 640 Where the array is an unsorted list of { parameter ID, name }-keyed 641 elements that indicate the value to map to, e.g.: 642 643 static const struct fs_parameter_enum afs_param_enums[] = { 644 { Opt_bar, "x", 1}, 645 { Opt_bar, "y", 23}, 646 { Opt_bar, "z", 42}, 647 }; 648 649 If a parameter of type fs_param_is_enum is encountered, fs_parse() will 650 try to look the value up in the enum table and the result will be stored 651 in the parse result. 652 653The parser should be pointed to by the parser pointer in the file_system_type 654struct as this will provide validation on registration (if 655CONFIG_VALIDATE_FS_PARSER=y) and will allow the description to be queried from 656userspace using the fsinfo() syscall. 657 658 659========================== 660PARAMETER HELPER FUNCTIONS 661========================== 662 663A number of helper functions are provided to help a filesystem or an LSM 664process the parameters it is given. 665 666 (*) int lookup_constant(const struct constant_table tbl[], 667 const char *name, int not_found); 668 669 Look up a constant by name in a table of name -> integer mappings. The 670 table is an array of elements of the following type: 671 672 struct constant_table { 673 const char *name; 674 int value; 675 }; 676 677 If a match is found, the corresponding value is returned. If a match 678 isn't found, the not_found value is returned instead. 679 680 (*) bool validate_constant_table(const struct constant_table *tbl, 681 size_t tbl_size, 682 int low, int high, int special); 683 684 Validate a constant table. Checks that all the elements are appropriately 685 ordered, that there are no duplicates and that the values are between low 686 and high inclusive, though provision is made for one allowable special 687 value outside of that range. If no special value is required, special 688 should just be set to lie inside the low-to-high range. 689 690 If all is good, true is returned. If the table is invalid, errors are 691 logged to dmesg and false is returned. 692 693 (*) bool fs_validate_description(const struct fs_parameter_description *desc); 694 695 This performs some validation checks on a parameter description. It 696 returns true if the description is good and false if it is not. It will 697 log errors to dmesg if validation fails. 698 699 (*) int fs_parse(struct fs_context *fc, 700 const struct fs_parameter_description *desc, 701 struct fs_parameter *param, 702 struct fs_parse_result *result); 703 704 This is the main interpreter of parameters. It uses the parameter 705 description to look up a parameter by key name and to convert that to an 706 option number (which it returns). 707 708 If successful, and if the parameter type indicates the result is a 709 boolean, integer or enum type, the value is converted by this function and 710 the result stored in result->{boolean,int_32,uint_32,uint_64}. 711 712 If a match isn't initially made, the key is prefixed with "no" and no 713 value is present then an attempt will be made to look up the key with the 714 prefix removed. If this matches a parameter for which the type has flag 715 fs_param_neg_with_no set, then a match will be made and result->negated 716 will be set to true. 717 718 If the parameter isn't matched, -ENOPARAM will be returned; if the 719 parameter is matched, but the value is erroneous, -EINVAL will be 720 returned; otherwise the parameter's option number will be returned. 721 722 (*) int fs_lookup_param(struct fs_context *fc, 723 struct fs_parameter *value, 724 bool want_bdev, 725 struct path *_path); 726 727 This takes a parameter that carries a string or filename type and attempts 728 to do a path lookup on it. If the parameter expects a blockdev, a check 729 is made that the inode actually represents one. 730 731 Returns 0 if successful and *_path will be set; returns a negative error 732 code if not. 733