1.. SPDX-License-Identifier: GPL-2.0 2 3==================== 4Filesystem Mount API 5==================== 6 7.. CONTENTS 8 9 (1) Overview. 10 11 (2) The filesystem context. 12 13 (3) The filesystem context operations. 14 15 (4) Filesystem context security. 16 17 (5) VFS filesystem context API. 18 19 (6) Superblock creation helpers. 20 21 (7) Parameter description. 22 23 (8) Parameter helper functions. 24 25 26Overview 27======== 28 29The creation of new mounts is now to be done in a multistep process: 30 31 (1) Create a filesystem context. 32 33 (2) Parse the parameters and attach them to the context. Parameters are 34 expected to be passed individually from userspace, though legacy binary 35 parameters can also be handled. 36 37 (3) Validate and pre-process the context. 38 39 (4) Get or create a superblock and mountable root. 40 41 (5) Perform the mount. 42 43 (6) Return an error message attached to the context. 44 45 (7) Destroy the context. 46 47To support this, the file_system_type struct gains two new fields:: 48 49 int (*init_fs_context)(struct fs_context *fc); 50 const struct fs_parameter_description *parameters; 51 52The first is invoked to set up the filesystem-specific parts of a filesystem 53context, including the additional space, and the second points to the 54parameter description for validation at registration time and querying by a 55future system call. 56 57Note that security initialisation is done *after* the filesystem is called so 58that the namespaces may be adjusted first. 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 * :: 90 91 const struct fs_context_operations *ops 92 93 These are operations that can be done on a filesystem context (see 94 below). This must be set by the ->init_fs_context() file_system_type 95 operation. 96 97 * :: 98 99 struct file_system_type *fs_type 100 101 A pointer to the file_system_type of the filesystem that is being 102 constructed or reconfigured. This retains a reference on the type owner. 103 104 * :: 105 106 void *fs_private 107 108 A pointer to the file system's private data. This is where the filesystem 109 will need to store any options it parses. 110 111 * :: 112 113 struct dentry *root 114 115 A pointer to the root of the mountable tree (and indirectly, the 116 superblock thereof). This is filled in by the ->get_tree() op. If this 117 is set, an active reference on root->d_sb must also be held. 118 119 * :: 120 121 struct user_namespace *user_ns 122 struct net *net_ns 123 124 There are a subset of the namespaces in use by the invoking process. They 125 retain references on each namespace. The subscribed namespaces may be 126 replaced by the filesystem to reflect other sources, such as the parent 127 mount superblock on an automount. 128 129 * :: 130 131 const struct cred *cred 132 133 The mounter's credentials. This retains a reference on the credentials. 134 135 * :: 136 137 char *source 138 139 This specifies the source. It may be a block device (e.g. /dev/sda1) or 140 something more exotic, such as the "host:/path" that NFS desires. 141 142 * :: 143 144 char *subtype 145 146 This is a string to be added to the type displayed in /proc/mounts to 147 qualify it (used by FUSE). This is available for the filesystem to set if 148 desired. 149 150 * :: 151 152 void *security 153 154 A place for the LSMs to hang their security data for the superblock. The 155 relevant security operations are described below. 156 157 * :: 158 159 void *s_fs_info 160 161 The proposed s_fs_info for a new superblock, set in the superblock by 162 sget_fc(). This can be used to distinguish superblocks. 163 164 * :: 165 166 unsigned int sb_flags 167 unsigned int sb_flags_mask 168 169 Which bits SB_* flags are to be set/cleared in super_block::s_flags. 170 171 * :: 172 173 unsigned int s_iflags 174 175 These will be bitwise-OR'd with s->s_iflags when a superblock is created. 176 177 * :: 178 179 enum fs_context_purpose 180 181 This indicates the purpose for which the context is intended. The 182 available values are: 183 184 ========================== ====================================== 185 FS_CONTEXT_FOR_MOUNT, New superblock for explicit mount 186 FS_CONTEXT_FOR_SUBMOUNT New automatic submount of extant mount 187 FS_CONTEXT_FOR_RECONFIGURE Change an existing mount 188 ========================== ====================================== 189 190The mount context is created by calling vfs_new_fs_context() or 191vfs_dup_fs_context() and is destroyed with put_fs_context(). Note that the 192structure is not refcounted. 193 194VFS, security and filesystem mount options are set individually with 195vfs_parse_mount_option(). Options provided by the old mount(2) system call as 196a page of data can be parsed with generic_parse_monolithic(). 197 198When mounting, the filesystem is allowed to take data from any of the pointers 199and attach it to the superblock (or whatever), provided it clears the pointer 200in the mount context. 201 202The filesystem is also allowed to allocate resources and pin them with the 203mount context. For instance, NFS might pin the appropriate protocol version 204module. 205 206 207The Filesystem Context Operations 208================================= 209 210The filesystem context points to a table of operations:: 211 212 struct fs_context_operations { 213 void (*free)(struct fs_context *fc); 214 int (*dup)(struct fs_context *fc, struct fs_context *src_fc); 215 int (*parse_param)(struct fs_context *fc, 216 struct fs_parameter *param); 217 int (*parse_monolithic)(struct fs_context *fc, void *data); 218 int (*get_tree)(struct fs_context *fc); 219 int (*reconfigure)(struct fs_context *fc); 220 }; 221 222These operations are invoked by the various stages of the mount procedure to 223manage the filesystem context. They are as follows: 224 225 * :: 226 227 void (*free)(struct fs_context *fc); 228 229 Called to clean up the filesystem-specific part of the filesystem context 230 when the context is destroyed. It should be aware that parts of the 231 context may have been removed and NULL'd out by ->get_tree(). 232 233 * :: 234 235 int (*dup)(struct fs_context *fc, struct fs_context *src_fc); 236 237 Called when a filesystem context has been duplicated to duplicate the 238 filesystem-private data. An error may be returned to indicate failure to 239 do this. 240 241 .. Warning:: 242 243 Note that even if this fails, put_fs_context() will be called 244 immediately thereafter, so ->dup() *must* make the 245 filesystem-private data safe for ->free(). 246 247 * :: 248 249 int (*parse_param)(struct fs_context *fc, 250 struct fs_parameter *param); 251 252 Called when a parameter is being added to the filesystem context. param 253 points to the key name and maybe a value object. VFS-specific options 254 will have been weeded out and fc->sb_flags updated in the context. 255 Security options will also have been weeded out and fc->security updated. 256 257 The parameter can be parsed with fs_parse() and fs_lookup_param(). Note 258 that the source(s) are presented as parameters named "source". 259 260 If successful, 0 should be returned or a negative error code otherwise. 261 262 * :: 263 264 int (*parse_monolithic)(struct fs_context *fc, void *data); 265 266 Called when the mount(2) system call is invoked to pass the entire data 267 page in one go. If this is expected to be just a list of "key[=val]" 268 items separated by commas, then this may be set to NULL. 269 270 The return value is as for ->parse_param(). 271 272 If the filesystem (e.g. NFS) needs to examine the data first and then 273 finds it's the standard key-val list then it may pass it off to 274 generic_parse_monolithic(). 275 276 * :: 277 278 int (*get_tree)(struct fs_context *fc); 279 280 Called to get or create the mountable root and superblock, using the 281 information stored in the filesystem context (reconfiguration goes via a 282 different vector). It may detach any resources it desires from the 283 filesystem context and transfer them to the superblock it creates. 284 285 On success it should set fc->root to the mountable root and return 0. In 286 the case of an error, it should return a negative error code. 287 288 The phase on a userspace-driven context will be set to only allow this to 289 be called once on any particular context. 290 291 * :: 292 293 int (*reconfigure)(struct fs_context *fc); 294 295 Called to effect reconfiguration of a superblock using information stored 296 in the filesystem context. It may detach any resources it desires from 297 the filesystem context and transfer them to the superblock. The 298 superblock can be found from fc->root->d_sb. 299 300 On success it should return 0. In the case of an error, it should return 301 a negative error code. 302 303 .. Note:: reconfigure is intended as a replacement for remount_fs. 304 305 306Filesystem context Security 307=========================== 308 309The filesystem context contains a security pointer that the LSMs can use for 310building up a security context for the superblock to be mounted. There are a 311number of operations used by the new mount code for this purpose: 312 313 * :: 314 315 int security_fs_context_alloc(struct fs_context *fc, 316 struct dentry *reference); 317 318 Called to initialise fc->security (which is preset to NULL) and allocate 319 any resources needed. It should return 0 on success or a negative error 320 code on failure. 321 322 reference will be non-NULL if the context is being created for superblock 323 reconfiguration (FS_CONTEXT_FOR_RECONFIGURE) in which case it indicates 324 the root dentry of the superblock to be reconfigured. It will also be 325 non-NULL in the case of a submount (FS_CONTEXT_FOR_SUBMOUNT) in which case 326 it indicates the automount point. 327 328 * :: 329 330 int security_fs_context_dup(struct fs_context *fc, 331 struct fs_context *src_fc); 332 333 Called to initialise fc->security (which is preset to NULL) and allocate 334 any resources needed. The original filesystem context is pointed to by 335 src_fc and may be used for reference. It should return 0 on success or a 336 negative error code on failure. 337 338 * :: 339 340 void security_fs_context_free(struct fs_context *fc); 341 342 Called to clean up anything attached to fc->security. Note that the 343 contents may have been transferred to a superblock and the pointer cleared 344 during get_tree. 345 346 * :: 347 348 int security_fs_context_parse_param(struct fs_context *fc, 349 struct fs_parameter *param); 350 351 Called for each mount parameter, including the source. The arguments are 352 as for the ->parse_param() method. It should return 0 to indicate that 353 the parameter should be passed on to the filesystem, 1 to indicate that 354 the parameter should be discarded or an error to indicate that the 355 parameter should be rejected. 356 357 The value pointed to by param may be modified (if a string) or stolen 358 (provided the value pointer is NULL'd out). If it is stolen, 1 must be 359 returned to prevent it being passed to the filesystem. 360 361 * :: 362 363 int security_fs_context_validate(struct fs_context *fc); 364 365 Called after all the options have been parsed to validate the collection 366 as a whole and to do any necessary allocation so that 367 security_sb_get_tree() and security_sb_reconfigure() are less likely to 368 fail. It should return 0 or a negative error code. 369 370 In the case of reconfiguration, the target superblock will be accessible 371 via fc->root. 372 373 * :: 374 375 int security_sb_get_tree(struct fs_context *fc); 376 377 Called during the mount procedure to verify that the specified superblock 378 is allowed to be mounted and to transfer the security data there. It 379 should return 0 or a negative error code. 380 381 * :: 382 383 void security_sb_reconfigure(struct fs_context *fc); 384 385 Called to apply any reconfiguration to an LSM's context. It must not 386 fail. Error checking and resource allocation must be done in advance by 387 the parameter parsing and validation hooks. 388 389 * :: 390 391 int security_sb_mountpoint(struct fs_context *fc, 392 struct path *mountpoint, 393 unsigned int mnt_flags); 394 395 Called during the mount procedure to verify that the root dentry attached 396 to the context is permitted to be attached to the specified mountpoint. 397 It should return 0 on success or a negative error code on failure. 398 399 400VFS Filesystem context API 401========================== 402 403There are four operations for creating a filesystem context and one for 404destroying a context: 405 406 * :: 407 408 struct fs_context *fs_context_for_mount(struct file_system_type *fs_type, 409 unsigned int sb_flags); 410 411 Allocate a filesystem context for the purpose of setting up a new mount, 412 whether that be with a new superblock or sharing an existing one. This 413 sets the superblock flags, initialises the security and calls 414 fs_type->init_fs_context() to initialise the filesystem private data. 415 416 fs_type specifies the filesystem type that will manage the context and 417 sb_flags presets the superblock flags stored therein. 418 419 * :: 420 421 struct fs_context *fs_context_for_reconfigure( 422 struct dentry *dentry, 423 unsigned int sb_flags, 424 unsigned int sb_flags_mask); 425 426 Allocate a filesystem context for the purpose of reconfiguring an 427 existing superblock. dentry provides a reference to the superblock to be 428 configured. sb_flags and sb_flags_mask indicate which superblock flags 429 need changing and to what. 430 431 * :: 432 433 struct fs_context *fs_context_for_submount( 434 struct file_system_type *fs_type, 435 struct dentry *reference); 436 437 Allocate a filesystem context for the purpose of creating a new mount for 438 an automount point or other derived superblock. fs_type specifies the 439 filesystem type that will manage the context and the reference dentry 440 supplies the parameters. Namespaces are propagated from the reference 441 dentry's superblock also. 442 443 Note that it's not a requirement that the reference dentry be of the same 444 filesystem type as fs_type. 445 446 * :: 447 448 struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc); 449 450 Duplicate a filesystem context, copying any options noted and duplicating 451 or additionally referencing any resources held therein. This is available 452 for use where a filesystem has to get a mount within a mount, such as NFS4 453 does by internally mounting the root of the target server and then doing a 454 private pathwalk to the target directory. 455 456 The purpose in the new context is inherited from the old one. 457 458 * :: 459 460 void put_fs_context(struct fs_context *fc); 461 462 Destroy a filesystem context, releasing any resources it holds. This 463 calls the ->free() operation. This is intended to be called by anyone who 464 created a filesystem context. 465 466 .. Warning:: 467 468 filesystem contexts are not refcounted, so this causes unconditional 469 destruction. 470 471In all the above operations, apart from the put op, the return is a mount 472context pointer or a negative error code. 473 474For the remaining operations, if an error occurs, a negative error code will be 475returned. 476 477 * :: 478 479 int vfs_parse_fs_param(struct fs_context *fc, 480 struct fs_parameter *param); 481 482 Supply a single mount parameter to the filesystem context. This includes 483 the specification of the source/device which is specified as the "source" 484 parameter (which may be specified multiple times if the filesystem 485 supports that). 486 487 param specifies the parameter key name and the value. The parameter is 488 first checked to see if it corresponds to a standard mount flag (in which 489 case it is used to set an SB_xxx flag and consumed) or a security option 490 (in which case the LSM consumes it) before it is passed on to the 491 filesystem. 492 493 The parameter value is typed and can be one of: 494 495 ==================== ============================= 496 fs_value_is_flag Parameter not given a value 497 fs_value_is_string Value is a string 498 fs_value_is_blob Value is a binary blob 499 fs_value_is_filename Value is a filename* + dirfd 500 fs_value_is_file Value is an open file (file*) 501 ==================== ============================= 502 503 If there is a value, that value is stored in a union in the struct in one 504 of param->{string,blob,name,file}. Note that the function may steal and 505 clear the pointer, but then becomes responsible for disposing of the 506 object. 507 508 * :: 509 510 int vfs_parse_fs_string(struct fs_context *fc, const char *key, 511 const char *value, size_t v_size); 512 513 A wrapper around vfs_parse_fs_param() that copies the value string it is 514 passed. 515 516 * :: 517 518 int generic_parse_monolithic(struct fs_context *fc, void *data); 519 520 Parse a sys_mount() data page, assuming the form to be a text list 521 consisting of key[=val] options separated by commas. Each item in the 522 list is passed to vfs_mount_option(). This is the default when the 523 ->parse_monolithic() method is NULL. 524 525 * :: 526 527 int vfs_get_tree(struct fs_context *fc); 528 529 Get or create the mountable root and superblock, using the parameters in 530 the filesystem context to select/configure the superblock. This invokes 531 the ->get_tree() method. 532 533 * :: 534 535 struct vfsmount *vfs_create_mount(struct fs_context *fc); 536 537 Create a mount given the parameters in the specified filesystem context. 538 Note that this does not attach the mount to anything. 539 540 541Superblock Creation Helpers 542=========================== 543 544A number of VFS helpers are available for use by filesystems for the creation 545or looking up of superblocks. 546 547 * :: 548 549 struct super_block * 550 sget_fc(struct fs_context *fc, 551 int (*test)(struct super_block *sb, struct fs_context *fc), 552 int (*set)(struct super_block *sb, struct fs_context *fc)); 553 554 This is the core routine. If test is non-NULL, it searches for an 555 existing superblock matching the criteria held in the fs_context, using 556 the test function to match them. If no match is found, a new superblock 557 is created and the set function is called to set it up. 558 559 Prior to the set function being called, fc->s_fs_info will be transferred 560 to sb->s_fs_info - and fc->s_fs_info will be cleared if set returns 561 success (ie. 0). 562 563The following helpers all wrap sget_fc(): 564 565 (1) vfs_get_single_super 566 567 Only one such superblock may exist in the system. Any further 568 attempt to get a new superblock gets this one (and any parameter 569 differences are ignored). 570 571 (2) vfs_get_keyed_super 572 573 Multiple superblocks of this type may exist and they're keyed on 574 their s_fs_info pointer (for example this may refer to a 575 namespace). 576 577 (3) vfs_get_independent_super 578 579 Multiple independent superblocks of this type may exist. This 580 function never matches an existing one and always creates a new 581 one. 582 583 584Parameter Description 585===================== 586 587Parameters are described using structures defined in linux/fs_parser.h. 588There's a core description struct that links everything together:: 589 590 struct fs_parameter_description { 591 const struct fs_parameter_spec *specs; 592 const struct fs_parameter_enum *enums; 593 }; 594 595For example:: 596 597 enum { 598 Opt_autocell, 599 Opt_bar, 600 Opt_dyn, 601 Opt_foo, 602 Opt_source, 603 }; 604 605 static const struct fs_parameter_description afs_fs_parameters = { 606 .specs = afs_param_specs, 607 .enums = afs_param_enums, 608 }; 609 610The members are as follows: 611 612 (1) :: 613 614 const struct fs_parameter_specification *specs; 615 616 Table of parameter specifications, terminated with a null entry, where the 617 entries are of type:: 618 619 struct fs_parameter_spec { 620 const char *name; 621 u8 opt; 622 enum fs_parameter_type type:8; 623 unsigned short flags; 624 }; 625 626 The 'name' field is a string to match exactly to the parameter key (no 627 wildcards, patterns and no case-independence) and 'opt' is the value that 628 will be returned by the fs_parser() function in the case of a successful 629 match. 630 631 The 'type' field indicates the desired value type and must be one of: 632 633 ======================= ======================= ===================== 634 TYPE NAME EXPECTED VALUE RESULT IN 635 ======================= ======================= ===================== 636 fs_param_is_flag No value n/a 637 fs_param_is_bool Boolean value result->boolean 638 fs_param_is_u32 32-bit unsigned int result->uint_32 639 fs_param_is_u32_octal 32-bit octal int result->uint_32 640 fs_param_is_u32_hex 32-bit hex int result->uint_32 641 fs_param_is_s32 32-bit signed int result->int_32 642 fs_param_is_u64 64-bit unsigned int result->uint_64 643 fs_param_is_enum Enum value name result->uint_32 644 fs_param_is_string Arbitrary string param->string 645 fs_param_is_blob Binary blob param->blob 646 fs_param_is_blockdev Blockdev path * Needs lookup 647 fs_param_is_path Path * Needs lookup 648 fs_param_is_fd File descriptor result->int_32 649 ======================= ======================= ===================== 650 651 Note that if the value is of fs_param_is_bool type, fs_parse() will try 652 to match any string value against "0", "1", "no", "yes", "false", "true". 653 654 Each parameter can also be qualified with 'flags': 655 656 ======================= ================================================ 657 fs_param_v_optional The value is optional 658 fs_param_neg_with_no result->negated set if key is prefixed with "no" 659 fs_param_neg_with_empty result->negated set if value is "" 660 fs_param_deprecated The parameter is deprecated. 661 ======================= ================================================ 662 663 These are wrapped with a number of convenience wrappers: 664 665 ======================= =============================================== 666 MACRO SPECIFIES 667 ======================= =============================================== 668 fsparam_flag() fs_param_is_flag 669 fsparam_flag_no() fs_param_is_flag, fs_param_neg_with_no 670 fsparam_bool() fs_param_is_bool 671 fsparam_u32() fs_param_is_u32 672 fsparam_u32oct() fs_param_is_u32_octal 673 fsparam_u32hex() fs_param_is_u32_hex 674 fsparam_s32() fs_param_is_s32 675 fsparam_u64() fs_param_is_u64 676 fsparam_enum() fs_param_is_enum 677 fsparam_string() fs_param_is_string 678 fsparam_blob() fs_param_is_blob 679 fsparam_bdev() fs_param_is_blockdev 680 fsparam_path() fs_param_is_path 681 fsparam_fd() fs_param_is_fd 682 ======================= =============================================== 683 684 all of which take two arguments, name string and option number - for 685 example:: 686 687 static const struct fs_parameter_spec afs_param_specs[] = { 688 fsparam_flag ("autocell", Opt_autocell), 689 fsparam_flag ("dyn", Opt_dyn), 690 fsparam_string ("source", Opt_source), 691 fsparam_flag_no ("foo", Opt_foo), 692 {} 693 }; 694 695 An addition macro, __fsparam() is provided that takes an additional pair 696 of arguments to specify the type and the flags for anything that doesn't 697 match one of the above macros. 698 699 (2) :: 700 701 const struct fs_parameter_enum *enums; 702 703 Table of enum value names to integer mappings, terminated with a null 704 entry. This is of type:: 705 706 struct fs_parameter_enum { 707 u8 opt; 708 char name[14]; 709 u8 value; 710 }; 711 712 Where the array is an unsorted list of { parameter ID, name }-keyed 713 elements that indicate the value to map to, e.g.:: 714 715 static const struct fs_parameter_enum afs_param_enums[] = { 716 { Opt_bar, "x", 1}, 717 { Opt_bar, "y", 23}, 718 { Opt_bar, "z", 42}, 719 }; 720 721 If a parameter of type fs_param_is_enum is encountered, fs_parse() will 722 try to look the value up in the enum table and the result will be stored 723 in the parse result. 724 725The parser should be pointed to by the parser pointer in the file_system_type 726struct as this will provide validation on registration (if 727CONFIG_VALIDATE_FS_PARSER=y) and will allow the description to be queried from 728userspace using the fsinfo() syscall. 729 730 731Parameter Helper Functions 732========================== 733 734A number of helper functions are provided to help a filesystem or an LSM 735process the parameters it is given. 736 737 * :: 738 739 int lookup_constant(const struct constant_table tbl[], 740 const char *name, int not_found); 741 742 Look up a constant by name in a table of name -> integer mappings. The 743 table is an array of elements of the following type:: 744 745 struct constant_table { 746 const char *name; 747 int value; 748 }; 749 750 If a match is found, the corresponding value is returned. If a match 751 isn't found, the not_found value is returned instead. 752 753 * :: 754 755 bool validate_constant_table(const struct constant_table *tbl, 756 size_t tbl_size, 757 int low, int high, int special); 758 759 Validate a constant table. Checks that all the elements are appropriately 760 ordered, that there are no duplicates and that the values are between low 761 and high inclusive, though provision is made for one allowable special 762 value outside of that range. If no special value is required, special 763 should just be set to lie inside the low-to-high range. 764 765 If all is good, true is returned. If the table is invalid, errors are 766 logged to the kernel log buffer and false is returned. 767 768 * :: 769 770 bool fs_validate_description(const struct fs_parameter_description *desc); 771 772 This performs some validation checks on a parameter description. It 773 returns true if the description is good and false if it is not. It will 774 log errors to the kernel log buffer if validation fails. 775 776 * :: 777 778 int fs_parse(struct fs_context *fc, 779 const struct fs_parameter_description *desc, 780 struct fs_parameter *param, 781 struct fs_parse_result *result); 782 783 This is the main interpreter of parameters. It uses the parameter 784 description to look up a parameter by key name and to convert that to an 785 option number (which it returns). 786 787 If successful, and if the parameter type indicates the result is a 788 boolean, integer or enum type, the value is converted by this function and 789 the result stored in result->{boolean,int_32,uint_32,uint_64}. 790 791 If a match isn't initially made, the key is prefixed with "no" and no 792 value is present then an attempt will be made to look up the key with the 793 prefix removed. If this matches a parameter for which the type has flag 794 fs_param_neg_with_no set, then a match will be made and result->negated 795 will be set to true. 796 797 If the parameter isn't matched, -ENOPARAM will be returned; if the 798 parameter is matched, but the value is erroneous, -EINVAL will be 799 returned; otherwise the parameter's option number will be returned. 800 801 * :: 802 803 int fs_lookup_param(struct fs_context *fc, 804 struct fs_parameter *value, 805 bool want_bdev, 806 unsigned int flags, 807 struct path *_path); 808 809 This takes a parameter that carries a string or filename type and attempts 810 to do a path lookup on it. If the parameter expects a blockdev, a check 811 is made that the inode actually represents one. 812 813 Returns 0 if successful and ``*_path`` will be set; returns a negative 814 error code if not. 815