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 enum fs_context_purpose purpose:8; 83 ... 84 }; 85 86The fs_context fields are as follows: 87 88 * :: 89 90 const struct fs_context_operations *ops 91 92 These are operations that can be done on a filesystem context (see 93 below). This must be set by the ->init_fs_context() file_system_type 94 operation. 95 96 * :: 97 98 struct file_system_type *fs_type 99 100 A pointer to the file_system_type of the filesystem that is being 101 constructed or reconfigured. This retains a reference on the type owner. 102 103 * :: 104 105 void *fs_private 106 107 A pointer to the file system's private data. This is where the filesystem 108 will need to store any options it parses. 109 110 * :: 111 112 struct dentry *root 113 114 A pointer to the root of the mountable tree (and indirectly, the 115 superblock thereof). This is filled in by the ->get_tree() op. If this 116 is set, an active reference on root->d_sb must also be held. 117 118 * :: 119 120 struct user_namespace *user_ns 121 struct net *net_ns 122 123 There are a subset of the namespaces in use by the invoking process. They 124 retain references on each namespace. The subscribed namespaces may be 125 replaced by the filesystem to reflect other sources, such as the parent 126 mount superblock on an automount. 127 128 * :: 129 130 const struct cred *cred 131 132 The mounter's credentials. This retains a reference on the credentials. 133 134 * :: 135 136 char *source 137 138 This specifies the source. It may be a block device (e.g. /dev/sda1) or 139 something more exotic, such as the "host:/path" that NFS desires. 140 141 * :: 142 143 char *subtype 144 145 This is a string to be added to the type displayed in /proc/mounts to 146 qualify it (used by FUSE). This is available for the filesystem to set if 147 desired. 148 149 * :: 150 151 void *security 152 153 A place for the LSMs to hang their security data for the superblock. The 154 relevant security operations are described below. 155 156 * :: 157 158 void *s_fs_info 159 160 The proposed s_fs_info for a new superblock, set in the superblock by 161 sget_fc(). This can be used to distinguish superblocks. 162 163 * :: 164 165 unsigned int sb_flags 166 unsigned int sb_flags_mask 167 168 Which bits SB_* flags are to be set/cleared in super_block::s_flags. 169 170 * :: 171 172 unsigned int s_iflags 173 174 These will be bitwise-OR'd with s->s_iflags when a superblock is created. 175 176 * :: 177 178 enum fs_context_purpose 179 180 This indicates the purpose for which the context is intended. The 181 available values are: 182 183 ========================== ====================================== 184 FS_CONTEXT_FOR_MOUNT, New superblock for explicit mount 185 FS_CONTEXT_FOR_SUBMOUNT New automatic submount of extant mount 186 FS_CONTEXT_FOR_RECONFIGURE Change an existing mount 187 ========================== ====================================== 188 189The mount context is created by calling vfs_new_fs_context() or 190vfs_dup_fs_context() and is destroyed with put_fs_context(). Note that the 191structure is not refcounted. 192 193VFS, security and filesystem mount options are set individually with 194vfs_parse_mount_option(). Options provided by the old mount(2) system call as 195a page of data can be parsed with generic_parse_monolithic(). 196 197When mounting, the filesystem is allowed to take data from any of the pointers 198and attach it to the superblock (or whatever), provided it clears the pointer 199in the mount context. 200 201The filesystem is also allowed to allocate resources and pin them with the 202mount context. For instance, NFS might pin the appropriate protocol version 203module. 204 205 206The Filesystem Context Operations 207================================= 208 209The filesystem context points to a table of operations:: 210 211 struct fs_context_operations { 212 void (*free)(struct fs_context *fc); 213 int (*dup)(struct fs_context *fc, struct fs_context *src_fc); 214 int (*parse_param)(struct fs_context *fc, 215 struct fs_parameter *param); 216 int (*parse_monolithic)(struct fs_context *fc, void *data); 217 int (*get_tree)(struct fs_context *fc); 218 int (*reconfigure)(struct fs_context *fc); 219 }; 220 221These operations are invoked by the various stages of the mount procedure to 222manage the filesystem context. They are as follows: 223 224 * :: 225 226 void (*free)(struct fs_context *fc); 227 228 Called to clean up the filesystem-specific part of the filesystem context 229 when the context is destroyed. It should be aware that parts of the 230 context may have been removed and NULL'd out by ->get_tree(). 231 232 * :: 233 234 int (*dup)(struct fs_context *fc, struct fs_context *src_fc); 235 236 Called when a filesystem context has been duplicated to duplicate the 237 filesystem-private data. An error may be returned to indicate failure to 238 do this. 239 240 .. Warning:: 241 242 Note that even if this fails, put_fs_context() will be called 243 immediately thereafter, so ->dup() *must* make the 244 filesystem-private data safe for ->free(). 245 246 * :: 247 248 int (*parse_param)(struct fs_context *fc, 249 struct fs_parameter *param); 250 251 Called when a parameter is being added to the filesystem context. param 252 points to the key name and maybe a value object. VFS-specific options 253 will have been weeded out and fc->sb_flags updated in the context. 254 Security options will also have been weeded out and fc->security updated. 255 256 The parameter can be parsed with fs_parse() and fs_lookup_param(). Note 257 that the source(s) are presented as parameters named "source". 258 259 If successful, 0 should be returned or a negative error code otherwise. 260 261 * :: 262 263 int (*parse_monolithic)(struct fs_context *fc, void *data); 264 265 Called when the mount(2) system call is invoked to pass the entire data 266 page in one go. If this is expected to be just a list of "key[=val]" 267 items separated by commas, then this may be set to NULL. 268 269 The return value is as for ->parse_param(). 270 271 If the filesystem (e.g. NFS) needs to examine the data first and then 272 finds it's the standard key-val list then it may pass it off to 273 generic_parse_monolithic(). 274 275 * :: 276 277 int (*get_tree)(struct fs_context *fc); 278 279 Called to get or create the mountable root and superblock, using the 280 information stored in the filesystem context (reconfiguration goes via a 281 different vector). It may detach any resources it desires from the 282 filesystem context and transfer them to the superblock it creates. 283 284 On success it should set fc->root to the mountable root and return 0. In 285 the case of an error, it should return a negative error code. 286 287 The phase on a userspace-driven context will be set to only allow this to 288 be called once on any particular context. 289 290 * :: 291 292 int (*reconfigure)(struct fs_context *fc); 293 294 Called to effect reconfiguration of a superblock using information stored 295 in the filesystem context. It may detach any resources it desires from 296 the filesystem context and transfer them to the superblock. The 297 superblock can be found from fc->root->d_sb. 298 299 On success it should return 0. In the case of an error, it should return 300 a negative error code. 301 302 .. Note:: reconfigure is intended as a replacement for remount_fs. 303 304 305Filesystem context Security 306=========================== 307 308The filesystem context contains a security pointer that the LSMs can use for 309building up a security context for the superblock to be mounted. There are a 310number of operations used by the new mount code for this purpose: 311 312 * :: 313 314 int security_fs_context_alloc(struct fs_context *fc, 315 struct dentry *reference); 316 317 Called to initialise fc->security (which is preset to NULL) and allocate 318 any resources needed. It should return 0 on success or a negative error 319 code on failure. 320 321 reference will be non-NULL if the context is being created for superblock 322 reconfiguration (FS_CONTEXT_FOR_RECONFIGURE) in which case it indicates 323 the root dentry of the superblock to be reconfigured. It will also be 324 non-NULL in the case of a submount (FS_CONTEXT_FOR_SUBMOUNT) in which case 325 it indicates the automount point. 326 327 * :: 328 329 int security_fs_context_dup(struct fs_context *fc, 330 struct fs_context *src_fc); 331 332 Called to initialise fc->security (which is preset to NULL) and allocate 333 any resources needed. The original filesystem context is pointed to by 334 src_fc and may be used for reference. It should return 0 on success or a 335 negative error code on failure. 336 337 * :: 338 339 void security_fs_context_free(struct fs_context *fc); 340 341 Called to clean up anything attached to fc->security. Note that the 342 contents may have been transferred to a superblock and the pointer cleared 343 during get_tree. 344 345 * :: 346 347 int security_fs_context_parse_param(struct fs_context *fc, 348 struct fs_parameter *param); 349 350 Called for each mount parameter, including the source. The arguments are 351 as for the ->parse_param() method. It should return 0 to indicate that 352 the parameter should be passed on to the filesystem, 1 to indicate that 353 the parameter should be discarded or an error to indicate that the 354 parameter should be rejected. 355 356 The value pointed to by param may be modified (if a string) or stolen 357 (provided the value pointer is NULL'd out). If it is stolen, 1 must be 358 returned to prevent it being passed to the filesystem. 359 360 * :: 361 362 int security_fs_context_validate(struct fs_context *fc); 363 364 Called after all the options have been parsed to validate the collection 365 as a whole and to do any necessary allocation so that 366 security_sb_get_tree() and security_sb_reconfigure() are less likely to 367 fail. It should return 0 or a negative error code. 368 369 In the case of reconfiguration, the target superblock will be accessible 370 via fc->root. 371 372 * :: 373 374 int security_sb_get_tree(struct fs_context *fc); 375 376 Called during the mount procedure to verify that the specified superblock 377 is allowed to be mounted and to transfer the security data there. It 378 should return 0 or a negative error code. 379 380 * :: 381 382 void security_sb_reconfigure(struct fs_context *fc); 383 384 Called to apply any reconfiguration to an LSM's context. It must not 385 fail. Error checking and resource allocation must be done in advance by 386 the parameter parsing and validation hooks. 387 388 * :: 389 390 int security_sb_mountpoint(struct fs_context *fc, 391 struct path *mountpoint, 392 unsigned int mnt_flags); 393 394 Called during the mount procedure to verify that the root dentry attached 395 to the context is permitted to be attached to the specified mountpoint. 396 It should return 0 on success or a negative error code on failure. 397 398 399VFS Filesystem context API 400========================== 401 402There are four operations for creating a filesystem context and one for 403destroying a context: 404 405 * :: 406 407 struct fs_context *fs_context_for_mount(struct file_system_type *fs_type, 408 unsigned int sb_flags); 409 410 Allocate a filesystem context for the purpose of setting up a new mount, 411 whether that be with a new superblock or sharing an existing one. This 412 sets the superblock flags, initialises the security and calls 413 fs_type->init_fs_context() to initialise the filesystem private data. 414 415 fs_type specifies the filesystem type that will manage the context and 416 sb_flags presets the superblock flags stored therein. 417 418 * :: 419 420 struct fs_context *fs_context_for_reconfigure( 421 struct dentry *dentry, 422 unsigned int sb_flags, 423 unsigned int sb_flags_mask); 424 425 Allocate a filesystem context for the purpose of reconfiguring an 426 existing superblock. dentry provides a reference to the superblock to be 427 configured. sb_flags and sb_flags_mask indicate which superblock flags 428 need changing and to what. 429 430 * :: 431 432 struct fs_context *fs_context_for_submount( 433 struct file_system_type *fs_type, 434 struct dentry *reference); 435 436 Allocate a filesystem context for the purpose of creating a new mount for 437 an automount point or other derived superblock. fs_type specifies the 438 filesystem type that will manage the context and the reference dentry 439 supplies the parameters. Namespaces are propagated from the reference 440 dentry's superblock also. 441 442 Note that it's not a requirement that the reference dentry be of the same 443 filesystem type as fs_type. 444 445 * :: 446 447 struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc); 448 449 Duplicate a filesystem context, copying any options noted and duplicating 450 or additionally referencing any resources held therein. This is available 451 for use where a filesystem has to get a mount within a mount, such as NFS4 452 does by internally mounting the root of the target server and then doing a 453 private pathwalk to the target directory. 454 455 The purpose in the new context is inherited from the old one. 456 457 * :: 458 459 void put_fs_context(struct fs_context *fc); 460 461 Destroy a filesystem context, releasing any resources it holds. This 462 calls the ->free() operation. This is intended to be called by anyone who 463 created a filesystem context. 464 465 .. Warning:: 466 467 filesystem contexts are not refcounted, so this causes unconditional 468 destruction. 469 470In all the above operations, apart from the put op, the return is a mount 471context pointer or a negative error code. 472 473For the remaining operations, if an error occurs, a negative error code will be 474returned. 475 476 * :: 477 478 int vfs_parse_fs_param(struct fs_context *fc, 479 struct fs_parameter *param); 480 481 Supply a single mount parameter to the filesystem context. This includes 482 the specification of the source/device which is specified as the "source" 483 parameter (which may be specified multiple times if the filesystem 484 supports that). 485 486 param specifies the parameter key name and the value. The parameter is 487 first checked to see if it corresponds to a standard mount flag (in which 488 case it is used to set an SB_xxx flag and consumed) or a security option 489 (in which case the LSM consumes it) before it is passed on to the 490 filesystem. 491 492 The parameter value is typed and can be one of: 493 494 ==================== ============================= 495 fs_value_is_flag Parameter not given a value 496 fs_value_is_string Value is a string 497 fs_value_is_blob Value is a binary blob 498 fs_value_is_filename Value is a filename* + dirfd 499 fs_value_is_file Value is an open file (file*) 500 ==================== ============================= 501 502 If there is a value, that value is stored in a union in the struct in one 503 of param->{string,blob,name,file}. Note that the function may steal and 504 clear the pointer, but then becomes responsible for disposing of the 505 object. 506 507 * :: 508 509 int vfs_parse_fs_qstr(struct fs_context *fc, const char *key, 510 const struct qstr *value); 511 512 A wrapper around vfs_parse_fs_param() that copies the value string it is 513 passed. 514 515 * :: 516 517 int vfs_parse_fs_string(struct fs_context *fc, const char *key, 518 const char *value); 519 520 A wrapper around vfs_parse_fs_param() that copies the value string it is 521 passed. 522 523 * :: 524 525 int generic_parse_monolithic(struct fs_context *fc, void *data); 526 527 Parse a sys_mount() data page, assuming the form to be a text list 528 consisting of key[=val] options separated by commas. Each item in the 529 list is passed to vfs_mount_option(). This is the default when the 530 ->parse_monolithic() method is NULL. 531 532 * :: 533 534 int vfs_get_tree(struct fs_context *fc); 535 536 Get or create the mountable root and superblock, using the parameters in 537 the filesystem context to select/configure the superblock. This invokes 538 the ->get_tree() method. 539 540 * :: 541 542 struct vfsmount *vfs_create_mount(struct fs_context *fc); 543 544 Create a mount given the parameters in the specified filesystem context. 545 Note that this does not attach the mount to anything. 546 547 548Superblock Creation Helpers 549=========================== 550 551A number of VFS helpers are available for use by filesystems for the creation 552or looking up of superblocks. 553 554 * :: 555 556 struct super_block * 557 sget_fc(struct fs_context *fc, 558 int (*test)(struct super_block *sb, struct fs_context *fc), 559 int (*set)(struct super_block *sb, struct fs_context *fc)); 560 561 This is the core routine. If test is non-NULL, it searches for an 562 existing superblock matching the criteria held in the fs_context, using 563 the test function to match them. If no match is found, a new superblock 564 is created and the set function is called to set it up. 565 566 Prior to the set function being called, fc->s_fs_info will be transferred 567 to sb->s_fs_info - and fc->s_fs_info will be cleared if set returns 568 success (ie. 0). 569 570The following helpers all wrap sget_fc(): 571 572 (1) vfs_get_single_super 573 574 Only one such superblock may exist in the system. Any further 575 attempt to get a new superblock gets this one (and any parameter 576 differences are ignored). 577 578 (2) vfs_get_keyed_super 579 580 Multiple superblocks of this type may exist and they're keyed on 581 their s_fs_info pointer (for example this may refer to a 582 namespace). 583 584 (3) vfs_get_independent_super 585 586 Multiple independent superblocks of this type may exist. This 587 function never matches an existing one and always creates a new 588 one. 589 590 591Parameter Description 592===================== 593 594Parameters are described using structures defined in linux/fs_parser.h. 595There's a core description struct that links everything together:: 596 597 struct fs_parameter_description { 598 const struct fs_parameter_spec *specs; 599 const struct fs_parameter_enum *enums; 600 }; 601 602For example:: 603 604 enum { 605 Opt_autocell, 606 Opt_bar, 607 Opt_dyn, 608 Opt_foo, 609 Opt_source, 610 }; 611 612 static const struct fs_parameter_description afs_fs_parameters = { 613 .specs = afs_param_specs, 614 .enums = afs_param_enums, 615 }; 616 617The members are as follows: 618 619 (1) :: 620 621 const struct fs_parameter_specification *specs; 622 623 Table of parameter specifications, terminated with a null entry, where the 624 entries are of type:: 625 626 struct fs_parameter_spec { 627 const char *name; 628 u8 opt; 629 enum fs_parameter_type type:8; 630 unsigned short flags; 631 }; 632 633 The 'name' field is a string to match exactly to the parameter key (no 634 wildcards, patterns and no case-independence) and 'opt' is the value that 635 will be returned by the fs_parser() function in the case of a successful 636 match. 637 638 The 'type' field indicates the desired value type and must be one of: 639 640 ======================= ======================= ===================== 641 TYPE NAME EXPECTED VALUE RESULT IN 642 ======================= ======================= ===================== 643 fs_param_is_flag No value n/a 644 fs_param_is_bool Boolean value result->boolean 645 fs_param_is_u32 32-bit unsigned int result->uint_32 646 fs_param_is_u32_octal 32-bit octal int result->uint_32 647 fs_param_is_u32_hex 32-bit hex int result->uint_32 648 fs_param_is_s32 32-bit signed int result->int_32 649 fs_param_is_u64 64-bit unsigned int result->uint_64 650 fs_param_is_enum Enum value name result->uint_32 651 fs_param_is_string Arbitrary string param->string 652 fs_param_is_blob Binary blob param->blob 653 fs_param_is_blockdev Blockdev path * Needs lookup 654 fs_param_is_path Path * Needs lookup 655 fs_param_is_fd File descriptor result->int_32 656 fs_param_is_uid User ID (u32) result->uid 657 fs_param_is_gid Group ID (u32) result->gid 658 ======================= ======================= ===================== 659 660 Note that if the value is of fs_param_is_bool type, fs_parse() will try 661 to match any string value against "0", "1", "no", "yes", "false", "true". 662 663 Each parameter can also be qualified with 'flags': 664 665 ======================= ================================================ 666 fs_param_v_optional The value is optional 667 fs_param_neg_with_no result->negated set if key is prefixed with "no" 668 fs_param_neg_with_empty result->negated set if value is "" 669 fs_param_deprecated The parameter is deprecated. 670 ======================= ================================================ 671 672 These are wrapped with a number of convenience wrappers: 673 674 ======================= =============================================== 675 MACRO SPECIFIES 676 ======================= =============================================== 677 fsparam_flag() fs_param_is_flag 678 fsparam_flag_no() fs_param_is_flag, fs_param_neg_with_no 679 fsparam_bool() fs_param_is_bool 680 fsparam_u32() fs_param_is_u32 681 fsparam_u32oct() fs_param_is_u32_octal 682 fsparam_s32() fs_param_is_s32 683 fsparam_u64() fs_param_is_u64 684 fsparam_enum() fs_param_is_enum 685 fsparam_string() fs_param_is_string 686 fsparam_blob() fs_param_is_blob 687 fsparam_bdev() fs_param_is_blockdev 688 fsparam_path() fs_param_is_path 689 fsparam_fd() fs_param_is_fd 690 fsparam_uid() fs_param_is_uid 691 fsparam_gid() fs_param_is_gid 692 ======================= =============================================== 693 694 all of which take two arguments, name string and option number - for 695 example:: 696 697 static const struct fs_parameter_spec afs_param_specs[] = { 698 fsparam_flag ("autocell", Opt_autocell), 699 fsparam_flag ("dyn", Opt_dyn), 700 fsparam_string ("source", Opt_source), 701 fsparam_flag_no ("foo", Opt_foo), 702 {} 703 }; 704 705 An addition macro, __fsparam() is provided that takes an additional pair 706 of arguments to specify the type and the flags for anything that doesn't 707 match one of the above macros. 708 709 (2) :: 710 711 const struct fs_parameter_enum *enums; 712 713 Table of enum value names to integer mappings, terminated with a null 714 entry. This is of type:: 715 716 struct fs_parameter_enum { 717 u8 opt; 718 char name[14]; 719 u8 value; 720 }; 721 722 Where the array is an unsorted list of { parameter ID, name }-keyed 723 elements that indicate the value to map to, e.g.:: 724 725 static const struct fs_parameter_enum afs_param_enums[] = { 726 { Opt_bar, "x", 1}, 727 { Opt_bar, "y", 23}, 728 { Opt_bar, "z", 42}, 729 }; 730 731 If a parameter of type fs_param_is_enum is encountered, fs_parse() will 732 try to look the value up in the enum table and the result will be stored 733 in the parse result. 734 735The parser should be pointed to by the parser pointer in the file_system_type 736struct as this will provide validation on registration (if 737CONFIG_VALIDATE_FS_PARSER=y) and will allow the description to be queried from 738userspace using the fsinfo() syscall. 739 740 741Parameter Helper Functions 742========================== 743 744A number of helper functions are provided to help a filesystem or an LSM 745process the parameters it is given. 746 747 * :: 748 749 int lookup_constant(const struct constant_table tbl[], 750 const char *name, int not_found); 751 752 Look up a constant by name in a table of name -> integer mappings. The 753 table is an array of elements of the following type:: 754 755 struct constant_table { 756 const char *name; 757 int value; 758 }; 759 760 If a match is found, the corresponding value is returned. If a match 761 isn't found, the not_found value is returned instead. 762 763 * :: 764 765 bool fs_validate_description(const char *name, 766 const struct fs_parameter_description *desc); 767 768 This performs some validation checks on a parameter description. It 769 returns true if the description is good and false if it is not. It will 770 log errors to the kernel log buffer if validation fails. 771 772 * :: 773 774 int fs_parse(struct fs_context *fc, 775 const struct fs_parameter_description *desc, 776 struct fs_parameter *param, 777 struct fs_parse_result *result); 778 779 This is the main interpreter of parameters. It uses the parameter 780 description to look up a parameter by key name and to convert that to an 781 option number (which it returns). 782 783 If successful, and if the parameter type indicates the result is a 784 boolean, integer, enum, uid, or gid type, the value is converted by this 785 function and the result stored in 786 result->{boolean,int_32,uint_32,uint_64,uid,gid}. 787 788 If a match isn't initially made, the key is prefixed with "no" and no 789 value is present then an attempt will be made to look up the key with the 790 prefix removed. If this matches a parameter for which the type has flag 791 fs_param_neg_with_no set, then a match will be made and result->negated 792 will be set to true. 793 794 If the parameter isn't matched, -ENOPARAM will be returned; if the 795 parameter is matched, but the value is erroneous, -EINVAL will be 796 returned; otherwise the parameter's option number will be returned. 797 798 * :: 799 800 int fs_lookup_param(struct fs_context *fc, 801 struct fs_parameter *value, 802 bool want_bdev, 803 unsigned int flags, 804 struct path *_path); 805 806 This takes a parameter that carries a string or filename type and attempts 807 to do a path lookup on it. If the parameter expects a blockdev, a check 808 is made that the inode actually represents one. 809 810 Returns 0 if successful and ``*_path`` will be set; returns a negative 811 error code if not. 812