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 303Filesystem context Security 304=========================== 305 306The filesystem context contains a security pointer that the LSMs can use for 307building up a security context for the superblock to be mounted. There are a 308number of operations used by the new mount code for this purpose: 309 310 * :: 311 312 int security_fs_context_alloc(struct fs_context *fc, 313 struct dentry *reference); 314 315 Called to initialise fc->security (which is preset to NULL) and allocate 316 any resources needed. It should return 0 on success or a negative error 317 code on failure. 318 319 reference will be non-NULL if the context is being created for superblock 320 reconfiguration (FS_CONTEXT_FOR_RECONFIGURE) in which case it indicates 321 the root dentry of the superblock to be reconfigured. It will also be 322 non-NULL in the case of a submount (FS_CONTEXT_FOR_SUBMOUNT) in which case 323 it indicates the automount point. 324 325 * :: 326 327 int security_fs_context_dup(struct fs_context *fc, 328 struct fs_context *src_fc); 329 330 Called to initialise fc->security (which is preset to NULL) and allocate 331 any resources needed. The original filesystem context is pointed to by 332 src_fc and may be used for reference. It should return 0 on success or a 333 negative error code on failure. 334 335 * :: 336 337 void security_fs_context_free(struct fs_context *fc); 338 339 Called to clean up anything attached to fc->security. Note that the 340 contents may have been transferred to a superblock and the pointer cleared 341 during get_tree. 342 343 * :: 344 345 int security_fs_context_parse_param(struct fs_context *fc, 346 struct fs_parameter *param); 347 348 Called for each mount parameter, including the source. The arguments are 349 as for the ->parse_param() method. It should return 0 to indicate that 350 the parameter should be passed on to the filesystem, 1 to indicate that 351 the parameter should be discarded or an error to indicate that the 352 parameter should be rejected. 353 354 The value pointed to by param may be modified (if a string) or stolen 355 (provided the value pointer is NULL'd out). If it is stolen, 1 must be 356 returned to prevent it being passed to the filesystem. 357 358 * :: 359 360 int security_fs_context_validate(struct fs_context *fc); 361 362 Called after all the options have been parsed to validate the collection 363 as a whole and to do any necessary allocation so that 364 security_sb_get_tree() and security_sb_reconfigure() are less likely to 365 fail. It should return 0 or a negative error code. 366 367 In the case of reconfiguration, the target superblock will be accessible 368 via fc->root. 369 370 * :: 371 372 int security_sb_get_tree(struct fs_context *fc); 373 374 Called during the mount procedure to verify that the specified superblock 375 is allowed to be mounted and to transfer the security data there. It 376 should return 0 or a negative error code. 377 378 * :: 379 380 void security_sb_reconfigure(struct fs_context *fc); 381 382 Called to apply any reconfiguration to an LSM's context. It must not 383 fail. Error checking and resource allocation must be done in advance by 384 the parameter parsing and validation hooks. 385 386 * :: 387 388 int security_sb_mountpoint(struct fs_context *fc, 389 struct path *mountpoint, 390 unsigned int mnt_flags); 391 392 Called during the mount procedure to verify that the root dentry attached 393 to the context is permitted to be attached to the specified mountpoint. 394 It should return 0 on success or a negative error code on failure. 395 396 397VFS Filesystem context API 398========================== 399 400There are four operations for creating a filesystem context and one for 401destroying a context: 402 403 * :: 404 405 struct fs_context *fs_context_for_mount(struct file_system_type *fs_type, 406 unsigned int sb_flags); 407 408 Allocate a filesystem context for the purpose of setting up a new mount, 409 whether that be with a new superblock or sharing an existing one. This 410 sets the superblock flags, initialises the security and calls 411 fs_type->init_fs_context() to initialise the filesystem private data. 412 413 fs_type specifies the filesystem type that will manage the context and 414 sb_flags presets the superblock flags stored therein. 415 416 * :: 417 418 struct fs_context *fs_context_for_reconfigure( 419 struct dentry *dentry, 420 unsigned int sb_flags, 421 unsigned int sb_flags_mask); 422 423 Allocate a filesystem context for the purpose of reconfiguring an 424 existing superblock. dentry provides a reference to the superblock to be 425 configured. sb_flags and sb_flags_mask indicate which superblock flags 426 need changing and to what. 427 428 * :: 429 430 struct fs_context *fs_context_for_submount( 431 struct file_system_type *fs_type, 432 struct dentry *reference); 433 434 Allocate a filesystem context for the purpose of creating a new mount for 435 an automount point or other derived superblock. fs_type specifies the 436 filesystem type that will manage the context and the reference dentry 437 supplies the parameters. Namespaces are propagated from the reference 438 dentry's superblock also. 439 440 Note that it's not a requirement that the reference dentry be of the same 441 filesystem type as fs_type. 442 443 * :: 444 445 struct fs_context *vfs_dup_fs_context(struct fs_context *src_fc); 446 447 Duplicate a filesystem context, copying any options noted and duplicating 448 or additionally referencing any resources held therein. This is available 449 for use where a filesystem has to get a mount within a mount, such as NFS4 450 does by internally mounting the root of the target server and then doing a 451 private pathwalk to the target directory. 452 453 The purpose in the new context is inherited from the old one. 454 455 * :: 456 457 void put_fs_context(struct fs_context *fc); 458 459 Destroy a filesystem context, releasing any resources it holds. This 460 calls the ->free() operation. This is intended to be called by anyone who 461 created a filesystem context. 462 463 .. Warning:: 464 465 filesystem contexts are not refcounted, so this causes unconditional 466 destruction. 467 468In all the above operations, apart from the put op, the return is a mount 469context pointer or a negative error code. 470 471For the remaining operations, if an error occurs, a negative error code will be 472returned. 473 474 * :: 475 476 int vfs_parse_fs_param(struct fs_context *fc, 477 struct fs_parameter *param); 478 479 Supply a single mount parameter to the filesystem context. This includes 480 the specification of the source/device which is specified as the "source" 481 parameter (which may be specified multiple times if the filesystem 482 supports that). 483 484 param specifies the parameter key name and the value. The parameter is 485 first checked to see if it corresponds to a standard mount flag (in which 486 case it is used to set an SB_xxx flag and consumed) or a security option 487 (in which case the LSM consumes it) before it is passed on to the 488 filesystem. 489 490 The parameter value is typed and can be one of: 491 492 ==================== ============================= 493 fs_value_is_flag Parameter not given a value 494 fs_value_is_string Value is a string 495 fs_value_is_blob Value is a binary blob 496 fs_value_is_filename Value is a filename* + dirfd 497 fs_value_is_file Value is an open file (file*) 498 ==================== ============================= 499 500 If there is a value, that value is stored in a union in the struct in one 501 of param->{string,blob,name,file}. Note that the function may steal and 502 clear the pointer, but then becomes responsible for disposing of the 503 object. 504 505 * :: 506 507 int vfs_parse_fs_qstr(struct fs_context *fc, const char *key, 508 const struct qstr *value); 509 510 A wrapper around vfs_parse_fs_param() that copies the value string it is 511 passed. 512 513 * :: 514 515 int vfs_parse_fs_string(struct fs_context *fc, const char *key, 516 const char *value); 517 518 A wrapper around vfs_parse_fs_param() that copies the value string it is 519 passed. 520 521 * :: 522 523 int generic_parse_monolithic(struct fs_context *fc, void *data); 524 525 Parse a sys_mount() data page, assuming the form to be a text list 526 consisting of key[=val] options separated by commas. Each item in the 527 list is passed to vfs_mount_option(). This is the default when the 528 ->parse_monolithic() method is NULL. 529 530 * :: 531 532 int vfs_get_tree(struct fs_context *fc); 533 534 Get or create the mountable root and superblock, using the parameters in 535 the filesystem context to select/configure the superblock. This invokes 536 the ->get_tree() method. 537 538 * :: 539 540 struct vfsmount *vfs_create_mount(struct fs_context *fc); 541 542 Create a mount given the parameters in the specified filesystem context. 543 Note that this does not attach the mount to anything. 544 545 546Superblock Creation Helpers 547=========================== 548 549A number of VFS helpers are available for use by filesystems for the creation 550or looking up of superblocks. 551 552 * :: 553 554 struct super_block * 555 sget_fc(struct fs_context *fc, 556 int (*test)(struct super_block *sb, struct fs_context *fc), 557 int (*set)(struct super_block *sb, struct fs_context *fc)); 558 559 This is the core routine. If test is non-NULL, it searches for an 560 existing superblock matching the criteria held in the fs_context, using 561 the test function to match them. If no match is found, a new superblock 562 is created and the set function is called to set it up. 563 564 Prior to the set function being called, fc->s_fs_info will be transferred 565 to sb->s_fs_info - and fc->s_fs_info will be cleared if set returns 566 success (ie. 0). 567 568The following helpers all wrap sget_fc(): 569 570 (1) vfs_get_single_super 571 572 Only one such superblock may exist in the system. Any further 573 attempt to get a new superblock gets this one (and any parameter 574 differences are ignored). 575 576 (2) vfs_get_keyed_super 577 578 Multiple superblocks of this type may exist and they're keyed on 579 their s_fs_info pointer (for example this may refer to a 580 namespace). 581 582 (3) vfs_get_independent_super 583 584 Multiple independent superblocks of this type may exist. This 585 function never matches an existing one and always creates a new 586 one. 587 588 589Parameter Description 590===================== 591 592Parameters are described using structures defined in linux/fs_parser.h. 593There's a core description struct that links everything together:: 594 595 struct fs_parameter_description { 596 const struct fs_parameter_spec *specs; 597 const struct fs_parameter_enum *enums; 598 }; 599 600For example:: 601 602 enum { 603 Opt_autocell, 604 Opt_bar, 605 Opt_dyn, 606 Opt_foo, 607 Opt_source, 608 }; 609 610 static const struct fs_parameter_description afs_fs_parameters = { 611 .specs = afs_param_specs, 612 .enums = afs_param_enums, 613 }; 614 615The members are as follows: 616 617 (1) :: 618 619 const struct fs_parameter_specification *specs; 620 621 Table of parameter specifications, terminated with a null entry, where the 622 entries are of type:: 623 624 struct fs_parameter_spec { 625 const char *name; 626 u8 opt; 627 enum fs_parameter_type type:8; 628 unsigned short flags; 629 }; 630 631 The 'name' field is a string to match exactly to the parameter key (no 632 wildcards, patterns and no case-independence) and 'opt' is the value that 633 will be returned by the fs_parser() function in the case of a successful 634 match. 635 636 The 'type' field indicates the desired value type and must be one of: 637 638 ======================= ======================= ===================== 639 TYPE NAME EXPECTED VALUE RESULT IN 640 ======================= ======================= ===================== 641 fs_param_is_flag No value n/a 642 fs_param_is_bool Boolean value result->boolean 643 fs_param_is_u32 32-bit unsigned int result->uint_32 644 fs_param_is_u32_octal 32-bit octal int result->uint_32 645 fs_param_is_u32_hex 32-bit hex int result->uint_32 646 fs_param_is_s32 32-bit signed int result->int_32 647 fs_param_is_u64 64-bit unsigned int result->uint_64 648 fs_param_is_enum Enum value name result->uint_32 649 fs_param_is_string Arbitrary string param->string 650 fs_param_is_blockdev Blockdev path * Needs lookup 651 fs_param_is_fd File descriptor result->int_32 652 fs_param_is_uid User ID (u32) result->uid 653 fs_param_is_gid Group ID (u32) result->gid 654 ======================= ======================= ===================== 655 656 Note that if the value is of fs_param_is_bool type, fs_parse() will try 657 to match any string value against "0", "1", "no", "yes", "false", "true". 658 659 Each parameter can also be qualified with 'flags': 660 661 ======================= ================================================ 662 fs_param_v_optional The value is optional 663 fs_param_neg_with_no result->negated set if key is prefixed with "no" 664 fs_param_neg_with_empty result->negated set if value is "" 665 fs_param_deprecated The parameter is deprecated. 666 ======================= ================================================ 667 668 These are wrapped with a number of convenience wrappers: 669 670 ======================= =============================================== 671 MACRO SPECIFIES 672 ======================= =============================================== 673 fsparam_flag() fs_param_is_flag 674 fsparam_flag_no() fs_param_is_flag, fs_param_neg_with_no 675 fsparam_bool() fs_param_is_bool 676 fsparam_u32() fs_param_is_u32 677 fsparam_u32oct() fs_param_is_u32_octal 678 fsparam_s32() fs_param_is_s32 679 fsparam_u64() fs_param_is_u64 680 fsparam_enum() fs_param_is_enum 681 fsparam_string() fs_param_is_string 682 fsparam_bdev() fs_param_is_blockdev 683 fsparam_fd() fs_param_is_fd 684 fsparam_uid() fs_param_is_uid 685 fsparam_gid() fs_param_is_gid 686 ======================= =============================================== 687 688 all of which take two arguments, name string and option number - for 689 example:: 690 691 static const struct fs_parameter_spec afs_param_specs[] = { 692 fsparam_flag ("autocell", Opt_autocell), 693 fsparam_flag ("dyn", Opt_dyn), 694 fsparam_string ("source", Opt_source), 695 fsparam_flag_no ("foo", Opt_foo), 696 {} 697 }; 698 699 An addition macro, __fsparam() is provided that takes an additional pair 700 of arguments to specify the type and the flags for anything that doesn't 701 match one of the above macros. 702 703 (2) :: 704 705 const struct fs_parameter_enum *enums; 706 707 Table of enum value names to integer mappings, terminated with a null 708 entry. This is of type:: 709 710 struct fs_parameter_enum { 711 u8 opt; 712 char name[14]; 713 u8 value; 714 }; 715 716 Where the array is an unsorted list of { parameter ID, name }-keyed 717 elements that indicate the value to map to, e.g.:: 718 719 static const struct fs_parameter_enum afs_param_enums[] = { 720 { Opt_bar, "x", 1}, 721 { Opt_bar, "y", 23}, 722 { Opt_bar, "z", 42}, 723 }; 724 725 If a parameter of type fs_param_is_enum is encountered, fs_parse() will 726 try to look the value up in the enum table and the result will be stored 727 in the parse result. 728 729The parser should be pointed to by the parser pointer in the file_system_type 730struct as this will provide validation on registration (if 731CONFIG_VALIDATE_FS_PARSER=y) and will allow the description to be queried from 732userspace using the fsinfo() syscall. 733 734 735Parameter Helper Functions 736========================== 737 738A number of helper functions are provided to help a filesystem or an LSM 739process the parameters it is given. 740 741 * :: 742 743 int lookup_constant(const struct constant_table tbl[], 744 const char *name, int not_found); 745 746 Look up a constant by name in a table of name -> integer mappings. The 747 table is an array of elements of the following type:: 748 749 struct constant_table { 750 const char *name; 751 int value; 752 }; 753 754 If a match is found, the corresponding value is returned. If a match 755 isn't found, the not_found value is returned instead. 756 757 * :: 758 759 bool fs_validate_description(const char *name, 760 const struct fs_parameter_description *desc); 761 762 This performs some validation checks on a parameter description. It 763 returns true if the description is good and false if it is not. It will 764 log errors to the kernel log buffer if validation fails. 765 766 * :: 767 768 int fs_parse(struct fs_context *fc, 769 const struct fs_parameter_description *desc, 770 struct fs_parameter *param, 771 struct fs_parse_result *result); 772 773 This is the main interpreter of parameters. It uses the parameter 774 description to look up a parameter by key name and to convert that to an 775 option number (which it returns). 776 777 If successful, and if the parameter type indicates the result is a 778 boolean, integer, enum, uid, or gid type, the value is converted by this 779 function and the result stored in 780 result->{boolean,int_32,uint_32,uint_64,uid,gid}. 781 782 If a match isn't initially made, the key is prefixed with "no" and no 783 value is present then an attempt will be made to look up the key with the 784 prefix removed. If this matches a parameter for which the type has flag 785 fs_param_neg_with_no set, then a match will be made and result->negated 786 will be set to true. 787 788 If the parameter isn't matched, -ENOPARAM will be returned; if the 789 parameter is matched, but the value is erroneous, -EINVAL will be 790 returned; otherwise the parameter's option number will be returned. 791 792 * :: 793 794 int fs_lookup_param(struct fs_context *fc, 795 struct fs_parameter *value, 796 bool want_bdev, 797 unsigned int flags, 798 struct path *_path); 799 800 This takes a parameter that carries a string or filename type and attempts 801 to do a path lookup on it. If the parameter expects a blockdev, a check 802 is made that the inode actually represents one. 803 804 Returns 0 if successful and ``*_path`` will be set; returns a negative 805 error code if not. 806