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1 <head> 2 <style> p { max-width:50em} ol, ul {max-width: 40em}</style> 3 </head> 4 5 Pathname lookup in Linux. 6 ========================= 7 8 This write-up is based on three articles published at lwn.net: 9 10 - <https://lwn.net/Articles/649115/> Pathname lookup in Linux 11 - <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux 12 - <https://lwn.net/Articles/650786/> A walk among the symlinks 13 14 Written by Neil Brown with help from Al Viro and Jon Corbet. 15 16 Introduction 17 ------------ 18 19 The most obvious aspect of pathname lookup, which very little 20 exploration is needed to discover, is that it is complex. There are 21 many rules, special cases, and implementation alternatives that all 22 combine to confuse the unwary reader. Computer science has long been 23 acquainted with such complexity and has tools to help manage it. One 24 tool that we will make extensive use of is "divide and conquer". For 25 the early parts of the analysis we will divide off symlinks - leaving 26 them until the final part. Well before we get to symlinks we have 27 another major division based on the VFS's approach to locking which 28 will allow us to review "REF-walk" and "RCU-walk" separately. But we 29 are getting ahead of ourselves. There are some important low level 30 distinctions we need to clarify first. 31 32 There are two sorts of ... 33 -------------------------- 34 35 [`openat()`]: http://man7.org/linux/man-pages/man2/openat.2.html 36 37 Pathnames (sometimes "file names"), used to identify objects in the 38 filesystem, will be familiar to most readers. They contain two sorts 39 of elements: "slashes" that are sequences of one or more "`/`" 40 characters, and "components" that are sequences of one or more 41 non-"`/`" characters. These form two kinds of paths. Those that 42 start with slashes are "absolute" and start from the filesystem root. 43 The others are "relative" and start from the current directory, or 44 from some other location specified by a file descriptor given to a 45 "xxx`at`" system call such as "[`openat()`]". 46 47 [`execveat()`]: http://man7.org/linux/man-pages/man2/execveat.2.html 48 49 It is tempting to describe the second kind as starting with a 50 component, but that isn't always accurate: a pathname can lack both 51 slashes and components, it can be empty, in other words. This is 52 generally forbidden in POSIX, but some of those "xxx`at`" system calls 53 in Linux permit it when the `AT_EMPTY_PATH` flag is given. For 54 example, if you have an open file descriptor on an executable file you 55 can execute it by calling [`execveat()`] passing the file descriptor, 56 an empty path, and the `AT_EMPTY_PATH` flag. 57 58 These paths can be divided into two sections: the final component and 59 everything else. The "everything else" is the easy bit. In all cases 60 it must identify a directory that already exists, otherwise an error 61 such as `ENOENT` or `ENOTDIR` will be reported. 62 63 The final component is not so simple. Not only do different system 64 calls interpret it quite differently (e.g. some create it, some do 65 not), but it might not even exist: neither the empty pathname nor the 66 pathname that is just slashes have a final component. If it does 67 exist, it could be "`.`" or "`..`" which are handled quite differently 68 from other components. 69 70 [POSIX]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12 71 72 If a pathname ends with a slash, such as "`/tmp/foo/`" it might be 73 tempting to consider that to have an empty final component. In many 74 ways that would lead to correct results, but not always. In 75 particular, `mkdir()` and `rmdir()` each create or remove a directory named 76 by the final component, and they are required to work with pathnames 77 ending in "`/`". According to [POSIX] 78 79 > A pathname that contains at least one non- <slash> character and 80 > that ends with one or more trailing <slash> characters shall not 81 > be resolved successfully unless the last pathname component before 82 > the trailing <slash> characters names an existing directory or a 83 > directory entry that is to be created for a directory immediately 84 > after the pathname is resolved. 85 86 The Linux pathname walking code (mostly in `fs/namei.c`) deals with 87 all of these issues: breaking the path into components, handling the 88 "everything else" quite separately from the final component, and 89 checking that the trailing slash is not used where it isn't 90 permitted. It also addresses the important issue of concurrent 91 access. 92 93 While one process is looking up a pathname, another might be making 94 changes that affect that lookup. One fairly extreme case is that if 95 "a/b" were renamed to "a/c/b" while another process were looking up 96 "a/b/..", that process might successfully resolve on "a/c". 97 Most races are much more subtle, and a big part of the task of 98 pathname lookup is to prevent them from having damaging effects. Many 99 of the possible races are seen most clearly in the context of the 100 "dcache" and an understanding of that is central to understanding 101 pathname lookup. 102 103 More than just a cache. 104 ----------------------- 105 106 The "dcache" caches information about names in each filesystem to 107 make them quickly available for lookup. Each entry (known as a 108 "dentry") contains three significant fields: a component name, a 109 pointer to a parent dentry, and a pointer to the "inode" which 110 contains further information about the object in that parent with 111 the given name. The inode pointer can be `NULL` indicating that the 112 name doesn't exist in the parent. While there can be linkage in the 113 dentry of a directory to the dentries of the children, that linkage is 114 not used for pathname lookup, and so will not be considered here. 115 116 The dcache has a number of uses apart from accelerating lookup. One 117 that will be particularly relevant is that it is closely integrated 118 with the mount table that records which filesystem is mounted where. 119 What the mount table actually stores is which dentry is mounted on top 120 of which other dentry. 121 122 When considering the dcache, we have another of our "two types" 123 distinctions: there are two types of filesystems. 124 125 Some filesystems ensure that the information in the dcache is always 126 completely accurate (though not necessarily complete). This can allow 127 the VFS to determine if a particular file does or doesn't exist 128 without checking with the filesystem, and means that the VFS can 129 protect the filesystem against certain races and other problems. 130 These are typically "local" filesystems such as ext3, XFS, and Btrfs. 131 132 Other filesystems don't provide that guarantee because they cannot. 133 These are typically filesystems that are shared across a network, 134 whether remote filesystems like NFS and 9P, or cluster filesystems 135 like ocfs2 or cephfs. These filesystems allow the VFS to revalidate 136 cached information, and must provide their own protection against 137 awkward races. The VFS can detect these filesystems by the 138 `DCACHE_OP_REVALIDATE` flag being set in the dentry. 139 140 REF-walk: simple concurrency management with refcounts and spinlocks 141 -------------------------------------------------------------------- 142 143 With all of those divisions carefully classified, we can now start 144 looking at the actual process of walking along a path. In particular 145 we will start with the handling of the "everything else" part of a 146 pathname, and focus on the "REF-walk" approach to concurrency 147 management. This code is found in the `link_path_walk()` function, if 148 you ignore all the places that only run when "`LOOKUP_RCU`" 149 (indicating the use of RCU-walk) is set. 150 151 [Meet the Lockers]: https://lwn.net/Articles/453685/ 152 153 REF-walk is fairly heavy-handed with locks and reference counts. Not 154 as heavy-handed as in the old "big kernel lock" days, but certainly not 155 afraid of taking a lock when one is needed. It uses a variety of 156 different concurrency controls. A background understanding of the 157 various primitives is assumed, or can be gleaned from elsewhere such 158 as in [Meet the Lockers]. 159 160 The locking mechanisms used by REF-walk include: 161 162 ### dentry->d_lockref ### 163 164 This uses the lockref primitive to provide both a spinlock and a 165 reference count. The special-sauce of this primitive is that the 166 conceptual sequence "lock; inc_ref; unlock;" can often be performed 167 with a single atomic memory operation. 168 169 Holding a reference on a dentry ensures that the dentry won't suddenly 170 be freed and used for something else, so the values in various fields 171 will behave as expected. It also protects the `->d_inode` reference 172 to the inode to some extent. 173 174 The association between a dentry and its inode is fairly permanent. 175 For example, when a file is renamed, the dentry and inode move 176 together to the new location. When a file is created the dentry will 177 initially be negative (i.e. `d_inode` is `NULL`), and will be assigned 178 to the new inode as part of the act of creation. 179 180 When a file is deleted, this can be reflected in the cache either by 181 setting `d_inode` to `NULL`, or by removing it from the hash table 182 (described shortly) used to look up the name in the parent directory. 183 If the dentry is still in use the second option is used as it is 184 perfectly legal to keep using an open file after it has been deleted 185 and having the dentry around helps. If the dentry is not otherwise in 186 use (i.e. if the refcount in `d_lockref` is one), only then will 187 `d_inode` be set to `NULL`. Doing it this way is more efficient for a 188 very common case. 189 190 So as long as a counted reference is held to a dentry, a non-`NULL` `->d_inode` 191 value will never be changed. 192 193 ### dentry->d_lock ### 194 195 `d_lock` is a synonym for the spinlock that is part of `d_lockref` above. 196 For our purposes, holding this lock protects against the dentry being 197 renamed or unlinked. In particular, its parent (`d_parent`), and its 198 name (`d_name`) cannot be changed, and it cannot be removed from the 199 dentry hash table. 200 201 When looking for a name in a directory, REF-walk takes `d_lock` on 202 each candidate dentry that it finds in the hash table and then checks 203 that the parent and name are correct. So it doesn't lock the parent 204 while searching in the cache; it only locks children. 205 206 When looking for the parent for a given name (to handle "`..`"), 207 REF-walk can take `d_lock` to get a stable reference to `d_parent`, 208 but it first tries a more lightweight approach. As seen in 209 `dget_parent()`, if a reference can be claimed on the parent, and if 210 subsequently `d_parent` can be seen to have not changed, then there is 211 no need to actually take the lock on the child. 212 213 ### rename_lock ### 214 215 Looking up a given name in a given directory involves computing a hash 216 from the two values (the name and the dentry of the directory), 217 accessing that slot in a hash table, and searching the linked list 218 that is found there. 219 220 When a dentry is renamed, the name and the parent dentry can both 221 change so the hash will almost certainly change too. This would move the 222 dentry to a different chain in the hash table. If a filename search 223 happened to be looking at a dentry that was moved in this way, 224 it might end up continuing the search down the wrong chain, 225 and so miss out on part of the correct chain. 226 227 The name-lookup process (`d_lookup()`) does _not_ try to prevent this 228 from happening, but only to detect when it happens. 229 `rename_lock` is a seqlock that is updated whenever any dentry is 230 renamed. If `d_lookup` finds that a rename happened while it 231 unsuccessfully scanned a chain in the hash table, it simply tries 232 again. 233 234 ### inode->i_mutex ### 235 236 `i_mutex` is a mutex that serializes all changes to a particular 237 directory. This ensures that, for example, an `unlink()` and a `rename()` 238 cannot both happen at the same time. It also keeps the directory 239 stable while the filesystem is asked to look up a name that is not 240 currently in the dcache. 241 242 This has a complementary role to that of `d_lock`: `i_mutex` on a 243 directory protects all of the names in that directory, while `d_lock` 244 on a name protects just one name in a directory. Most changes to the 245 dcache hold `i_mutex` on the relevant directory inode and briefly take 246 `d_lock` on one or more the dentries while the change happens. One 247 exception is when idle dentries are removed from the dcache due to 248 memory pressure. This uses `d_lock`, but `i_mutex` plays no role. 249 250 The mutex affects pathname lookup in two distinct ways. Firstly it 251 serializes lookup of a name in a directory. `walk_component()` uses 252 `lookup_fast()` first which, in turn, checks to see if the name is in the cache, 253 using only `d_lock` locking. If the name isn't found, then `walk_component()` 254 falls back to `lookup_slow()` which takes `i_mutex`, checks again that 255 the name isn't in the cache, and then calls in to the filesystem to get a 256 definitive answer. A new dentry will be added to the cache regardless of 257 the result. 258 259 Secondly, when pathname lookup reaches the final component, it will 260 sometimes need to take `i_mutex` before performing the last lookup so 261 that the required exclusion can be achieved. How path lookup chooses 262 to take, or not take, `i_mutex` is one of the 263 issues addressed in a subsequent section. 264 265 ### mnt->mnt_count ### 266 267 `mnt_count` is a per-CPU reference counter on "`mount`" structures. 268 Per-CPU here means that incrementing the count is cheap as it only 269 uses CPU-local memory, but checking if the count is zero is expensive as 270 it needs to check with every CPU. Taking a `mnt_count` reference 271 prevents the mount structure from disappearing as the result of regular 272 unmount operations, but does not prevent a "lazy" unmount. So holding 273 `mnt_count` doesn't ensure that the mount remains in the namespace and, 274 in particular, doesn't stabilize the link to the mounted-on dentry. It 275 does, however, ensure that the `mount` data structure remains coherent, 276 and it provides a reference to the root dentry of the mounted 277 filesystem. So a reference through `->mnt_count` provides a stable 278 reference to the mounted dentry, but not the mounted-on dentry. 279 280 ### mount_lock ### 281 282 `mount_lock` is a global seqlock, a bit like `rename_lock`. It can be used to 283 check if any change has been made to any mount points. 284 285 While walking down the tree (away from the root) this lock is used when 286 crossing a mount point to check that the crossing was safe. That is, 287 the value in the seqlock is read, then the code finds the mount that 288 is mounted on the current directory, if there is one, and increments 289 the `mnt_count`. Finally the value in `mount_lock` is checked against 290 the old value. If there is no change, then the crossing was safe. If there 291 was a change, the `mnt_count` is decremented and the whole process is 292 retried. 293 294 When walking up the tree (towards the root) by following a ".." link, 295 a little more care is needed. In this case the seqlock (which 296 contains both a counter and a spinlock) is fully locked to prevent 297 any changes to any mount points while stepping up. This locking is 298 needed to stabilize the link to the mounted-on dentry, which the 299 refcount on the mount itself doesn't ensure. 300 301 ### RCU ### 302 303 Finally the global (but extremely lightweight) RCU read lock is held 304 from time to time to ensure certain data structures don't get freed 305 unexpectedly. 306 307 In particular it is held while scanning chains in the dcache hash 308 table, and the mount point hash table. 309 310 Bringing it together with `struct nameidata` 311 -------------------------------------------- 312 313 [First edition Unix]: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s 314 315 Throughout the process of walking a path, the current status is stored 316 in a `struct nameidata`, "namei" being the traditional name - dating 317 all the way back to [First Edition Unix] - of the function that 318 converts a "name" to an "inode". `struct nameidata` contains (among 319 other fields): 320 321 ### `struct path path` ### 322 323 A `path` contains a `struct vfsmount` (which is 324 embedded in a `struct mount`) and a `struct dentry`. Together these 325 record the current status of the walk. They start out referring to the 326 starting point (the current working directory, the root directory, or some other 327 directory identified by a file descriptor), and are updated on each 328 step. A reference through `d_lockref` and `mnt_count` is always 329 held. 330 331 ### `struct qstr last` ### 332 333 This is a string together with a length (i.e. _not_ `nul` terminated) 334 that is the "next" component in the pathname. 335 336 ### `int last_type` ### 337 338 This is one of `LAST_NORM`, `LAST_ROOT`, `LAST_DOT`, `LAST_DOTDOT`, or 339 `LAST_BIND`. The `last` field is only valid if the type is 340 `LAST_NORM`. `LAST_BIND` is used when following a symlink and no 341 components of the symlink have been processed yet. Others should be 342 fairly self-explanatory. 343 344 ### `struct path root` ### 345 346 This is used to hold a reference to the effective root of the 347 filesystem. Often that reference won't be needed, so this field is 348 only assigned the first time it is used, or when a non-standard root 349 is requested. Keeping a reference in the `nameidata` ensures that 350 only one root is in effect for the entire path walk, even if it races 351 with a `chroot()` system call. 352 353 The root is needed when either of two conditions holds: (1) either the 354 pathname or a symbolic link starts with a "'/'", or (2) a "`..`" 355 component is being handled, since "`..`" from the root must always stay 356 at the root. The value used is usually the current root directory of 357 the calling process. An alternate root can be provided as when 358 `sysctl()` calls `file_open_root()`, and when NFSv4 or Btrfs call 359 `mount_subtree()`. In each case a pathname is being looked up in a very 360 specific part of the filesystem, and the lookup must not be allowed to 361 escape that subtree. It works a bit like a local `chroot()`. 362 363 Ignoring the handling of symbolic links, we can now describe the 364 "`link_path_walk()`" function, which handles the lookup of everything 365 except the final component as: 366 367 > Given a path (`name`) and a nameidata structure (`nd`), check that the 368 > current directory has execute permission and then advance `name` 369 > over one component while updating `last_type` and `last`. If that 370 > was the final component, then return, otherwise call 371 > `walk_component()` and repeat from the top. 372 373 `walk_component()` is even easier. If the component is `LAST_DOTS`, 374 it calls `handle_dots()` which does the necessary locking as already 375 described. If it finds a `LAST_NORM` component it first calls 376 "`lookup_fast()`" which only looks in the dcache, but will ask the 377 filesystem to revalidate the result if it is that sort of filesystem. 378 If that doesn't get a good result, it calls "`lookup_slow()`" which 379 takes the `i_mutex`, rechecks the cache, and then asks the filesystem 380 to find a definitive answer. Each of these will call 381 `follow_managed()` (as described below) to handle any mount points. 382 383 In the absence of symbolic links, `walk_component()` creates a new 384 `struct path` containing a counted reference to the new dentry and a 385 reference to the new `vfsmount` which is only counted if it is 386 different from the previous `vfsmount`. It then calls 387 `path_to_nameidata()` to install the new `struct path` in the 388 `struct nameidata` and drop the unneeded references. 389 390 This "hand-over-hand" sequencing of getting a reference to the new 391 dentry before dropping the reference to the previous dentry may 392 seem obvious, but is worth pointing out so that we will recognize its 393 analogue in the "RCU-walk" version. 394 395 Handling the final component. 396 ----------------------------- 397 398 `link_path_walk()` only walks as far as setting `nd->last` and 399 `nd->last_type` to refer to the final component of the path. It does 400 not call `walk_component()` that last time. Handling that final 401 component remains for the caller to sort out. Those callers are 402 `path_lookupat()`, `path_parentat()`, `path_mountpoint()` and 403 `path_openat()` each of which handles the differing requirements of 404 different system calls. 405 406 `path_parentat()` is clearly the simplest - it just wraps a little bit 407 of housekeeping around `link_path_walk()` and returns the parent 408 directory and final component to the caller. The caller will be either 409 aiming to create a name (via `filename_create()`) or remove or rename 410 a name (in which case `user_path_parent()` is used). They will use 411 `i_mutex` to exclude other changes while they validate and then 412 perform their operation. 413 414 `path_lookupat()` is nearly as simple - it is used when an existing 415 object is wanted such as by `stat()` or `chmod()`. It essentially just 416 calls `walk_component()` on the final component through a call to 417 `lookup_last()`. `path_lookupat()` returns just the final dentry. 418 419 `path_mountpoint()` handles the special case of unmounting which must 420 not try to revalidate the mounted filesystem. It effectively 421 contains, through a call to `mountpoint_last()`, an alternate 422 implementation of `lookup_slow()` which skips that step. This is 423 important when unmounting a filesystem that is inaccessible, such as 424 one provided by a dead NFS server. 425 426 Finally `path_openat()` is used for the `open()` system call; it 427 contains, in support functions starting with "`do_last()`", all the 428 complexity needed to handle the different subtleties of O_CREAT (with 429 or without O_EXCL), final "`/`" characters, and trailing symbolic 430 links. We will revisit this in the final part of this series, which 431 focuses on those symbolic links. "`do_last()`" will sometimes, but 432 not always, take `i_mutex`, depending on what it finds. 433 434 Each of these, or the functions which call them, need to be alert to 435 the possibility that the final component is not `LAST_NORM`. If the 436 goal of the lookup is to create something, then any value for 437 `last_type` other than `LAST_NORM` will result in an error. For 438 example if `path_parentat()` reports `LAST_DOTDOT`, then the caller 439 won't try to create that name. They also check for trailing slashes 440 by testing `last.name[last.len]`. If there is any character beyond 441 the final component, it must be a trailing slash. 442 443 Revalidation and automounts 444 --------------------------- 445 446 Apart from symbolic links, there are only two parts of the "REF-walk" 447 process not yet covered. One is the handling of stale cache entries 448 and the other is automounts. 449 450 On filesystems that require it, the lookup routines will call the 451 `->d_revalidate()` dentry method to ensure that the cached information 452 is current. This will often confirm validity or update a few details 453 from a server. In some cases it may find that there has been change 454 further up the path and that something that was thought to be valid 455 previously isn't really. When this happens the lookup of the whole 456 path is aborted and retried with the "`LOOKUP_REVAL`" flag set. This 457 forces revalidation to be more thorough. We will see more details of 458 this retry process in the next article. 459 460 Automount points are locations in the filesystem where an attempt to 461 lookup a name can trigger changes to how that lookup should be 462 handled, in particular by mounting a filesystem there. These are 463 covered in greater detail in autofs4.txt in the Linux documentation 464 tree, but a few notes specifically related to path lookup are in order 465 here. 466 467 The Linux VFS has a concept of "managed" dentries which is reflected 468 in function names such as "`follow_managed()`". There are three 469 potentially interesting things about these dentries corresponding 470 to three different flags that might be set in `dentry->d_flags`: 471 472 ### `DCACHE_MANAGE_TRANSIT` ### 473 474 If this flag has been set, then the filesystem has requested that the 475 `d_manage()` dentry operation be called before handling any possible 476 mount point. This can perform two particular services: 477 478 It can block to avoid races. If an automount point is being 479 unmounted, the `d_manage()` function will usually wait for that 480 process to complete before letting the new lookup proceed and possibly 481 trigger a new automount. 482 483 It can selectively allow only some processes to transit through a 484 mount point. When a server process is managing automounts, it may 485 need to access a directory without triggering normal automount 486 processing. That server process can identify itself to the `autofs` 487 filesystem, which will then give it a special pass through 488 `d_manage()` by returning `-EISDIR`. 489 490 ### `DCACHE_MOUNTED` ### 491 492 This flag is set on every dentry that is mounted on. As Linux 493 supports multiple filesystem namespaces, it is possible that the 494 dentry may not be mounted on in *this* namespace, just in some 495 other. So this flag is seen as a hint, not a promise. 496 497 If this flag is set, and `d_manage()` didn't return `-EISDIR`, 498 `lookup_mnt()` is called to examine the mount hash table (honoring the 499 `mount_lock` described earlier) and possibly return a new `vfsmount` 500 and a new `dentry` (both with counted references). 501 502 ### `DCACHE_NEED_AUTOMOUNT` ### 503 504 If `d_manage()` allowed us to get this far, and `lookup_mnt()` didn't 505 find a mount point, then this flag causes the `d_automount()` dentry 506 operation to be called. 507 508 The `d_automount()` operation can be arbitrarily complex and may 509 communicate with server processes etc. but it should ultimately either 510 report that there was an error, that there was nothing to mount, or 511 should provide an updated `struct path` with new `dentry` and `vfsmount`. 512 513 In the latter case, `finish_automount()` will be called to safely 514 install the new mount point into the mount table. 515 516 There is no new locking of import here and it is important that no 517 locks (only counted references) are held over this processing due to 518 the very real possibility of extended delays. 519 This will become more important next time when we examine RCU-walk 520 which is particularly sensitive to delays. 521 522 RCU-walk - faster pathname lookup in Linux 523 ========================================== 524 525 RCU-walk is another algorithm for performing pathname lookup in Linux. 526 It is in many ways similar to REF-walk and the two share quite a bit 527 of code. The significant difference in RCU-walk is how it allows for 528 the possibility of concurrent access. 529 530 We noted that REF-walk is complex because there are numerous details 531 and special cases. RCU-walk reduces this complexity by simply 532 refusing to handle a number of cases -- it instead falls back to 533 REF-walk. The difficulty with RCU-walk comes from a different 534 direction: unfamiliarity. The locking rules when depending on RCU are 535 quite different from traditional locking, so we will spend a little extra 536 time when we come to those. 537 538 Clear demarcation of roles 539 -------------------------- 540 541 The easiest way to manage concurrency is to forcibly stop any other 542 thread from changing the data structures that a given thread is 543 looking at. In cases where no other thread would even think of 544 changing the data and lots of different threads want to read at the 545 same time, this can be very costly. Even when using locks that permit 546 multiple concurrent readers, the simple act of updating the count of 547 the number of current readers can impose an unwanted cost. So the 548 goal when reading a shared data structure that no other process is 549 changing is to avoid writing anything to memory at all. Take no 550 locks, increment no counts, leave no footprints. 551 552 The REF-walk mechanism already described certainly doesn't follow this 553 principle, but then it is really designed to work when there may well 554 be other threads modifying the data. RCU-walk, in contrast, is 555 designed for the common situation where there are lots of frequent 556 readers and only occasional writers. This may not be common in all 557 parts of the filesystem tree, but in many parts it will be. For the 558 other parts it is important that RCU-walk can quickly fall back to 559 using REF-walk. 560 561 Pathname lookup always starts in RCU-walk mode but only remains there 562 as long as what it is looking for is in the cache and is stable. It 563 dances lightly down the cached filesystem image, leaving no footprints 564 and carefully watching where it is, to be sure it doesn't trip. If it 565 notices that something has changed or is changing, or if something 566 isn't in the cache, then it tries to stop gracefully and switch to 567 REF-walk. 568 569 This stopping requires getting a counted reference on the current 570 `vfsmount` and `dentry`, and ensuring that these are still valid - 571 that a path walk with REF-walk would have found the same entries. 572 This is an invariant that RCU-walk must guarantee. It can only make 573 decisions, such as selecting the next step, that are decisions which 574 REF-walk could also have made if it were walking down the tree at the 575 same time. If the graceful stop succeeds, the rest of the path is 576 processed with the reliable, if slightly sluggish, REF-walk. If 577 RCU-walk finds it cannot stop gracefully, it simply gives up and 578 restarts from the top with REF-walk. 579 580 This pattern of "try RCU-walk, if that fails try REF-walk" can be 581 clearly seen in functions like `filename_lookup()`, 582 `filename_parentat()`, `filename_mountpoint()`, 583 `do_filp_open()`, and `do_file_open_root()`. These five 584 correspond roughly to the four `path_`* functions we met earlier, 585 each of which calls `link_path_walk()`. The `path_*` functions are 586 called using different mode flags until a mode is found which works. 587 They are first called with `LOOKUP_RCU` set to request "RCU-walk". If 588 that fails with the error `ECHILD` they are called again with no 589 special flag to request "REF-walk". If either of those report the 590 error `ESTALE` a final attempt is made with `LOOKUP_REVAL` set (and no 591 `LOOKUP_RCU`) to ensure that entries found in the cache are forcibly 592 revalidated - normally entries are only revalidated if the filesystem 593 determines that they are too old to trust. 594 595 The `LOOKUP_RCU` attempt may drop that flag internally and switch to 596 REF-walk, but will never then try to switch back to RCU-walk. Places 597 that trip up RCU-walk are much more likely to be near the leaves and 598 so it is very unlikely that there will be much, if any, benefit from 599 switching back. 600 601 RCU and seqlocks: fast and light 602 -------------------------------- 603 604 RCU is, unsurprisingly, critical to RCU-walk mode. The 605 `rcu_read_lock()` is held for the entire time that RCU-walk is walking 606 down a path. The particular guarantee it provides is that the key 607 data structures - dentries, inodes, super_blocks, and mounts - will 608 not be freed while the lock is held. They might be unlinked or 609 invalidated in one way or another, but the memory will not be 610 repurposed so values in various fields will still be meaningful. This 611 is the only guarantee that RCU provides; everything else is done using 612 seqlocks. 613 614 As we saw above, REF-walk holds a counted reference to the current 615 dentry and the current vfsmount, and does not release those references 616 before taking references to the "next" dentry or vfsmount. It also 617 sometimes takes the `d_lock` spinlock. These references and locks are 618 taken to prevent certain changes from happening. RCU-walk must not 619 take those references or locks and so cannot prevent such changes. 620 Instead, it checks to see if a change has been made, and aborts or 621 retries if it has. 622 623 To preserve the invariant mentioned above (that RCU-walk may only make 624 decisions that REF-walk could have made), it must make the checks at 625 or near the same places that REF-walk holds the references. So, when 626 REF-walk increments a reference count or takes a spinlock, RCU-walk 627 samples the status of a seqlock using `read_seqcount_begin()` or a 628 similar function. When REF-walk decrements the count or drops the 629 lock, RCU-walk checks if the sampled status is still valid using 630 `read_seqcount_retry()` or similar. 631 632 However, there is a little bit more to seqlocks than that. If 633 RCU-walk accesses two different fields in a seqlock-protected 634 structure, or accesses the same field twice, there is no a priori 635 guarantee of any consistency between those accesses. When consistency 636 is needed - which it usually is - RCU-walk must take a copy and then 637 use `read_seqcount_retry()` to validate that copy. 638 639 `read_seqcount_retry()` not only checks the sequence number, but also 640 imposes a memory barrier so that no memory-read instruction from 641 *before* the call can be delayed until *after* the call, either by the 642 CPU or by the compiler. A simple example of this can be seen in 643 `slow_dentry_cmp()` which, for filesystems which do not use simple 644 byte-wise name equality, calls into the filesystem to compare a name 645 against a dentry. The length and name pointer are copied into local 646 variables, then `read_seqcount_retry()` is called to confirm the two 647 are consistent, and only then is `->d_compare()` called. When 648 standard filename comparison is used, `dentry_cmp()` is called 649 instead. Notably it does _not_ use `read_seqcount_retry()`, but 650 instead has a large comment explaining why the consistency guarantee 651 isn't necessary. A subsequent `read_seqcount_retry()` will be 652 sufficient to catch any problem that could occur at this point. 653 654 With that little refresher on seqlocks out of the way we can look at 655 the bigger picture of how RCU-walk uses seqlocks. 656 657 ### `mount_lock` and `nd->m_seq` ### 658 659 We already met the `mount_lock` seqlock when REF-walk used it to 660 ensure that crossing a mount point is performed safely. RCU-walk uses 661 it for that too, but for quite a bit more. 662 663 Instead of taking a counted reference to each `vfsmount` as it 664 descends the tree, RCU-walk samples the state of `mount_lock` at the 665 start of the walk and stores this initial sequence number in the 666 `struct nameidata` in the `m_seq` field. This one lock and one 667 sequence number are used to validate all accesses to all `vfsmounts`, 668 and all mount point crossings. As changes to the mount table are 669 relatively rare, it is reasonable to fall back on REF-walk any time 670 that any "mount" or "unmount" happens. 671 672 `m_seq` is checked (using `read_seqretry()`) at the end of an RCU-walk 673 sequence, whether switching to REF-walk for the rest of the path or 674 when the end of the path is reached. It is also checked when stepping 675 down over a mount point (in `__follow_mount_rcu()`) or up (in 676 `follow_dotdot_rcu()`). If it is ever found to have changed, the 677 whole RCU-walk sequence is aborted and the path is processed again by 678 REF-walk. 679 680 If RCU-walk finds that `mount_lock` hasn't changed then it can be sure 681 that, had REF-walk taken counted references on each vfsmount, the 682 results would have been the same. This ensures the invariant holds, 683 at least for vfsmount structures. 684 685 ### `dentry->d_seq` and `nd->seq`. ### 686 687 In place of taking a count or lock on `d_reflock`, RCU-walk samples 688 the per-dentry `d_seq` seqlock, and stores the sequence number in the 689 `seq` field of the nameidata structure, so `nd->seq` should always be 690 the current sequence number of `nd->dentry`. This number needs to be 691 revalidated after copying, and before using, the name, parent, or 692 inode of the dentry. 693 694 The handling of the name we have already looked at, and the parent is 695 only accessed in `follow_dotdot_rcu()` which fairly trivially follows 696 the required pattern, though it does so for three different cases. 697 698 When not at a mount point, `d_parent` is followed and its `d_seq` is 699 collected. When we are at a mount point, we instead follow the 700 `mnt->mnt_mountpoint` link to get a new dentry and collect its 701 `d_seq`. Then, after finally finding a `d_parent` to follow, we must 702 check if we have landed on a mount point and, if so, must find that 703 mount point and follow the `mnt->mnt_root` link. This would imply a 704 somewhat unusual, but certainly possible, circumstance where the 705 starting point of the path lookup was in part of the filesystem that 706 was mounted on, and so not visible from the root. 707 708 The inode pointer, stored in `->d_inode`, is a little more 709 interesting. The inode will always need to be accessed at least 710 twice, once to determine if it is NULL and once to verify access 711 permissions. Symlink handling requires a validated inode pointer too. 712 Rather than revalidating on each access, a copy is made on the first 713 access and it is stored in the `inode` field of `nameidata` from where 714 it can be safely accessed without further validation. 715 716 `lookup_fast()` is the only lookup routine that is used in RCU-mode, 717 `lookup_slow()` being too slow and requiring locks. It is in 718 `lookup_fast()` that we find the important "hand over hand" tracking 719 of the current dentry. 720 721 The current `dentry` and current `seq` number are passed to 722 `__d_lookup_rcu()` which, on success, returns a new `dentry` and a 723 new `seq` number. `lookup_fast()` then copies the inode pointer and 724 revalidates the new `seq` number. It then validates the old `dentry` 725 with the old `seq` number one last time and only then continues. This 726 process of getting the `seq` number of the new dentry and then 727 checking the `seq` number of the old exactly mirrors the process of 728 getting a counted reference to the new dentry before dropping that for 729 the old dentry which we saw in REF-walk. 730 731 ### No `inode->i_mutex` or even `rename_lock` ### 732 733 A mutex is a fairly heavyweight lock that can only be taken when it is 734 permissible to sleep. As `rcu_read_lock()` forbids sleeping, 735 `inode->i_mutex` plays no role in RCU-walk. If some other thread does 736 take `i_mutex` and modifies the directory in a way that RCU-walk needs 737 to notice, the result will be either that RCU-walk fails to find the 738 dentry that it is looking for, or it will find a dentry which 739 `read_seqretry()` won't validate. In either case it will drop down to 740 REF-walk mode which can take whatever locks are needed. 741 742 Though `rename_lock` could be used by RCU-walk as it doesn't require 743 any sleeping, RCU-walk doesn't bother. REF-walk uses `rename_lock` to 744 protect against the possibility of hash chains in the dcache changing 745 while they are being searched. This can result in failing to find 746 something that actually is there. When RCU-walk fails to find 747 something in the dentry cache, whether it is really there or not, it 748 already drops down to REF-walk and tries again with appropriate 749 locking. This neatly handles all cases, so adding extra checks on 750 rename_lock would bring no significant value. 751 752 `unlazy walk()` and `complete_walk()` 753 ------------------------------------- 754 755 That "dropping down to REF-walk" typically involves a call to 756 `unlazy_walk()`, so named because "RCU-walk" is also sometimes 757 referred to as "lazy walk". `unlazy_walk()` is called when 758 following the path down to the current vfsmount/dentry pair seems to 759 have proceeded successfully, but the next step is problematic. This 760 can happen if the next name cannot be found in the dcache, if 761 permission checking or name revalidation couldn't be achieved while 762 the `rcu_read_lock()` is held (which forbids sleeping), if an 763 automount point is found, or in a couple of cases involving symlinks. 764 It is also called from `complete_walk()` when the lookup has reached 765 the final component, or the very end of the path, depending on which 766 particular flavor of lookup is used. 767 768 Other reasons for dropping out of RCU-walk that do not trigger a call 769 to `unlazy_walk()` are when some inconsistency is found that cannot be 770 handled immediately, such as `mount_lock` or one of the `d_seq` 771 seqlocks reporting a change. In these cases the relevant function 772 will return `-ECHILD` which will percolate up until it triggers a new 773 attempt from the top using REF-walk. 774 775 For those cases where `unlazy_walk()` is an option, it essentially 776 takes a reference on each of the pointers that it holds (vfsmount, 777 dentry, and possibly some symbolic links) and then verifies that the 778 relevant seqlocks have not been changed. If there have been changes, 779 it, too, aborts with `-ECHILD`, otherwise the transition to REF-walk 780 has been a success and the lookup process continues. 781 782 Taking a reference on those pointers is not quite as simple as just 783 incrementing a counter. That works to take a second reference if you 784 already have one (often indirectly through another object), but it 785 isn't sufficient if you don't actually have a counted reference at 786 all. For `dentry->d_lockref`, it is safe to increment the reference 787 counter to get a reference unless it has been explicitly marked as 788 "dead" which involves setting the counter to `-128`. 789 `lockref_get_not_dead()` achieves this. 790 791 For `mnt->mnt_count` it is safe to take a reference as long as 792 `mount_lock` is then used to validate the reference. If that 793 validation fails, it may *not* be safe to just drop that reference in 794 the standard way of calling `mnt_put()` - an unmount may have 795 progressed too far. So the code in `legitimize_mnt()`, when it 796 finds that the reference it got might not be safe, checks the 797 `MNT_SYNC_UMOUNT` flag to determine if a simple `mnt_put()` is 798 correct, or if it should just decrement the count and pretend none of 799 this ever happened. 800 801 Taking care in filesystems 802 --------------------------- 803 804 RCU-walk depends almost entirely on cached information and often will 805 not call into the filesystem at all. However there are two places, 806 besides the already-mentioned component-name comparison, where the 807 file system might be included in RCU-walk, and it must know to be 808 careful. 809 810 If the filesystem has non-standard permission-checking requirements - 811 such as a networked filesystem which may need to check with the server 812 - the `i_op->permission` interface might be called during RCU-walk. 813 In this case an extra "`MAY_NOT_BLOCK`" flag is passed so that it 814 knows not to sleep, but to return `-ECHILD` if it cannot complete 815 promptly. `i_op->permission` is given the inode pointer, not the 816 dentry, so it doesn't need to worry about further consistency checks. 817 However if it accesses any other filesystem data structures, it must 818 ensure they are safe to be accessed with only the `rcu_read_lock()` 819 held. This typically means they must be freed using `kfree_rcu()` or 820 similar. 821 822 [`READ_ONCE()`]: https://lwn.net/Articles/624126/ 823 824 If the filesystem may need to revalidate dcache entries, then 825 `d_op->d_revalidate` may be called in RCU-walk too. This interface 826 *is* passed the dentry but does not have access to the `inode` or the 827 `seq` number from the `nameidata`, so it needs to be extra careful 828 when accessing fields in the dentry. This "extra care" typically 829 involves using [`READ_ONCE()`] to access fields, and verifying the 830 result is not NULL before using it. This pattern can be seen in 831 `nfs_lookup_revalidate()`. 832 833 A pair of patterns 834 ------------------ 835 836 In various places in the details of REF-walk and RCU-walk, and also in 837 the big picture, there are a couple of related patterns that are worth 838 being aware of. 839 840 The first is "try quickly and check, if that fails try slowly". We 841 can see that in the high-level approach of first trying RCU-walk and 842 then trying REF-walk, and in places where `unlazy_walk()` is used to 843 switch to REF-walk for the rest of the path. We also saw it earlier 844 in `dget_parent()` when following a "`..`" link. It tries a quick way 845 to get a reference, then falls back to taking locks if needed. 846 847 The second pattern is "try quickly and check, if that fails try 848 again - repeatedly". This is seen with the use of `rename_lock` and 849 `mount_lock` in REF-walk. RCU-walk doesn't make use of this pattern - 850 if anything goes wrong it is much safer to just abort and try a more 851 sedate approach. 852 853 The emphasis here is "try quickly and check". It should probably be 854 "try quickly _and carefully,_ then check". The fact that checking is 855 needed is a reminder that the system is dynamic and only a limited 856 number of things are safe at all. The most likely cause of errors in 857 this whole process is assuming something is safe when in reality it 858 isn't. Careful consideration of what exactly guarantees the safety of 859 each access is sometimes necessary. 860 861 A walk among the symlinks 862 ========================= 863 864 There are several basic issues that we will examine to understand the 865 handling of symbolic links: the symlink stack, together with cache 866 lifetimes, will help us understand the overall recursive handling of 867 symlinks and lead to the special care needed for the final component. 868 Then a consideration of access-time updates and summary of the various 869 flags controlling lookup will finish the story. 870 871 The symlink stack 872 ----------------- 873 874 There are only two sorts of filesystem objects that can usefully 875 appear in a path prior to the final component: directories and symlinks. 876 Handling directories is quite straightforward: the new directory 877 simply becomes the starting point at which to interpret the next 878 component on the path. Handling symbolic links requires a bit more 879 work. 880 881 Conceptually, symbolic links could be handled by editing the path. If 882 a component name refers to a symbolic link, then that component is 883 replaced by the body of the link and, if that body starts with a '/', 884 then all preceding parts of the path are discarded. This is what the 885 "`readlink -f`" command does, though it also edits out "`.`" and 886 "`..`" components. 887 888 Directly editing the path string is not really necessary when looking 889 up a path, and discarding early components is pointless as they aren't 890 looked at anyway. Keeping track of all remaining components is 891 important, but they can of course be kept separately; there is no need 892 to concatenate them. As one symlink may easily refer to another, 893 which in turn can refer to a third, we may need to keep the remaining 894 components of several paths, each to be processed when the preceding 895 ones are completed. These path remnants are kept on a stack of 896 limited size. 897 898 There are two reasons for placing limits on how many symlinks can 899 occur in a single path lookup. The most obvious is to avoid loops. 900 If a symlink referred to itself either directly or through 901 intermediaries, then following the symlink can never complete 902 successfully - the error `ELOOP` must be returned. Loops can be 903 detected without imposing limits, but limits are the simplest solution 904 and, given the second reason for restriction, quite sufficient. 905 906 [outlined recently]: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550 907 908 The second reason was [outlined recently] by Linus: 909 910 > Because it's a latency and DoS issue too. We need to react well to 911 > true loops, but also to "very deep" non-loops. It's not about memory 912 > use, it's about users triggering unreasonable CPU resources. 913 914 Linux imposes a limit on the length of any pathname: `PATH_MAX`, which 915 is 4096. There are a number of reasons for this limit; not letting the 916 kernel spend too much time on just one path is one of them. With 917 symbolic links you can effectively generate much longer paths so some 918 sort of limit is needed for the same reason. Linux imposes a limit of 919 at most 40 symlinks in any one path lookup. It previously imposed a 920 further limit of eight on the maximum depth of recursion, but that was 921 raised to 40 when a separate stack was implemented, so there is now 922 just the one limit. 923 924 The `nameidata` structure that we met in an earlier article contains a 925 small stack that can be used to store the remaining part of up to two 926 symlinks. In many cases this will be sufficient. If it isn't, a 927 separate stack is allocated with room for 40 symlinks. Pathname 928 lookup will never exceed that stack as, once the 40th symlink is 929 detected, an error is returned. 930 931 It might seem that the name remnants are all that needs to be stored on 932 this stack, but we need a bit more. To see that, we need to move on to 933 cache lifetimes. 934 935 Storage and lifetime of cached symlinks 936 --------------------------------------- 937 938 Like other filesystem resources, such as inodes and directory 939 entries, symlinks are cached by Linux to avoid repeated costly access 940 to external storage. It is particularly important for RCU-walk to be 941 able to find and temporarily hold onto these cached entries, so that 942 it doesn't need to drop down into REF-walk. 943 944 [object-oriented design pattern]: https://lwn.net/Articles/446317/ 945 946 While each filesystem is free to make its own choice, symlinks are 947 typically stored in one of two places. Short symlinks are often 948 stored directly in the inode. When a filesystem allocates a `struct 949 inode` it typically allocates extra space to store private data (a 950 common [object-oriented design pattern] in the kernel). This will 951 sometimes include space for a symlink. The other common location is 952 in the page cache, which normally stores the content of files. The 953 pathname in a symlink can be seen as the content of that symlink and 954 can easily be stored in the page cache just like file content. 955 956 When neither of these is suitable, the next most likely scenario is 957 that the filesystem will allocate some temporary memory and copy or 958 construct the symlink content into that memory whenever it is needed. 959 960 When the symlink is stored in the inode, it has the same lifetime as 961 the inode which, itself, is protected by RCU or by a counted reference 962 on the dentry. This means that the mechanisms that pathname lookup 963 uses to access the dcache and icache (inode cache) safely are quite 964 sufficient for accessing some cached symlinks safely. In these cases, 965 the `i_link` pointer in the inode is set to point to wherever the 966 symlink is stored and it can be accessed directly whenever needed. 967 968 When the symlink is stored in the page cache or elsewhere, the 969 situation is not so straightforward. A reference on a dentry or even 970 on an inode does not imply any reference on cached pages of that 971 inode, and even an `rcu_read_lock()` is not sufficient to ensure that 972 a page will not disappear. So for these symlinks the pathname lookup 973 code needs to ask the filesystem to provide a stable reference and, 974 significantly, needs to release that reference when it is finished 975 with it. 976 977 Taking a reference to a cache page is often possible even in RCU-walk 978 mode. It does require making changes to memory, which is best avoided, 979 but that isn't necessarily a big cost and it is better than dropping 980 out of RCU-walk mode completely. Even filesystems that allocate 981 space to copy the symlink into can use `GFP_ATOMIC` to often successfully 982 allocate memory without the need to drop out of RCU-walk. If a 983 filesystem cannot successfully get a reference in RCU-walk mode, it 984 must return `-ECHILD` and `unlazy_walk()` will be called to return to 985 REF-walk mode in which the filesystem is allowed to sleep. 986 987 The place for all this to happen is the `i_op->follow_link()` inode 988 method. In the present mainline code this is never actually called in 989 RCU-walk mode as the rewrite is not quite complete. It is likely that 990 in a future release this method will be passed an `inode` pointer when 991 called in RCU-walk mode so it both (1) knows to be careful, and (2) has the 992 validated pointer. Much like the `i_op->permission()` method we 993 looked at previously, `->follow_link()` would need to be careful that 994 all the data structures it references are safe to be accessed while 995 holding no counted reference, only the RCU lock. Though getting a 996 reference with `->follow_link()` is not yet done in RCU-walk mode, the 997 code is ready to release the reference when that does happen. 998 999 This need to drop the reference to a symlink adds significant 1000 complexity. It requires a reference to the inode so that the 1001 `i_op->put_link()` inode operation can be called. In REF-walk, that 1002 reference is kept implicitly through a reference to the dentry, so 1003 keeping the `struct path` of the symlink is easiest. For RCU-walk, 1004 the pointer to the inode is kept separately. To allow switching from 1005 RCU-walk back to REF-walk in the middle of processing nested symlinks 1006 we also need the seq number for the dentry so we can confirm that 1007 switching back was safe. 1008 1009 Finally, when providing a reference to a symlink, the filesystem also 1010 provides an opaque "cookie" that must be passed to `->put_link()` so that it 1011 knows what to free. This might be the allocated memory area, or a 1012 pointer to the `struct page` in the page cache, or something else 1013 completely. Only the filesystem knows what it is. 1014 1015 In order for the reference to each symlink to be dropped when the walk completes, 1016 whether in RCU-walk or REF-walk, the symlink stack needs to contain, 1017 along with the path remnants: 1018 1019 - the `struct path` to provide a reference to the inode in REF-walk 1020 - the `struct inode *` to provide a reference to the inode in RCU-walk 1021 - the `seq` to allow the path to be safely switched from RCU-walk to REF-walk 1022 - the `cookie` that tells `->put_path()` what to put. 1023 1024 This means that each entry in the symlink stack needs to hold five 1025 pointers and an integer instead of just one pointer (the path 1026 remnant). On a 64-bit system, this is about 40 bytes per entry; 1027 with 40 entries it adds up to 1600 bytes total, which is less than 1028 half a page. So it might seem like a lot, but is by no means 1029 excessive. 1030 1031 Note that, in a given stack frame, the path remnant (`name`) is not 1032 part of the symlink that the other fields refer to. It is the remnant 1033 to be followed once that symlink has been fully parsed. 1034 1035 Following the symlink 1036 --------------------- 1037 1038 The main loop in `link_path_walk()` iterates seamlessly over all 1039 components in the path and all of the non-final symlinks. As symlinks 1040 are processed, the `name` pointer is adjusted to point to a new 1041 symlink, or is restored from the stack, so that much of the loop 1042 doesn't need to notice. Getting this `name` variable on and off the 1043 stack is very straightforward; pushing and popping the references is 1044 a little more complex. 1045 1046 When a symlink is found, `walk_component()` returns the value `1` 1047 (`0` is returned for any other sort of success, and a negative number 1048 is, as usual, an error indicator). This causes `get_link()` to be 1049 called; it then gets the link from the filesystem. Providing that 1050 operation is successful, the old path `name` is placed on the stack, 1051 and the new value is used as the `name` for a while. When the end of 1052 the path is found (i.e. `*name` is `'\0'`) the old `name` is restored 1053 off the stack and path walking continues. 1054 1055 Pushing and popping the reference pointers (inode, cookie, etc.) is more 1056 complex in part because of the desire to handle tail recursion. When 1057 the last component of a symlink itself points to a symlink, we 1058 want to pop the symlink-just-completed off the stack before pushing 1059 the symlink-just-found to avoid leaving empty path remnants that would 1060 just get in the way. 1061 1062 It is most convenient to push the new symlink references onto the 1063 stack in `walk_component()` immediately when the symlink is found; 1064 `walk_component()` is also the last piece of code that needs to look at the 1065 old symlink as it walks that last component. So it is quite 1066 convenient for `walk_component()` to release the old symlink and pop 1067 the references just before pushing the reference information for the 1068 new symlink. It is guided in this by two flags; `WALK_GET`, which 1069 gives it permission to follow a symlink if it finds one, and 1070 `WALK_PUT`, which tells it to release the current symlink after it has been 1071 followed. `WALK_PUT` is tested first, leading to a call to 1072 `put_link()`. `WALK_GET` is tested subsequently (by 1073 `should_follow_link()`) leading to a call to `pick_link()` which sets 1074 up the stack frame. 1075 1076 ### Symlinks with no final component ### 1077 1078 A pair of special-case symlinks deserve a little further explanation. 1079 Both result in a new `struct path` (with mount and dentry) being set 1080 up in the `nameidata`, and result in `get_link()` returning `NULL`. 1081 1082 The more obvious case is a symlink to "`/`". All symlinks starting 1083 with "`/`" are detected in `get_link()` which resets the `nameidata` 1084 to point to the effective filesystem root. If the symlink only 1085 contains "`/`" then there is nothing more to do, no components at all, 1086 so `NULL` is returned to indicate that the symlink can be released and 1087 the stack frame discarded. 1088 1089 The other case involves things in `/proc` that look like symlinks but 1090 aren't really. 1091 1092 > $ ls -l /proc/self/fd/1 1093 > lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4 1094 1095 Every open file descriptor in any process is represented in `/proc` by 1096 something that looks like a symlink. It is really a reference to the 1097 target file, not just the name of it. When you `readlink` these 1098 objects you get a name that might refer to the same file - unless it 1099 has been unlinked or mounted over. When `walk_component()` follows 1100 one of these, the `->follow_link()` method in "procfs" doesn't return 1101 a string name, but instead calls `nd_jump_link()` which updates the 1102 `nameidata` in place to point to that target. `->follow_link()` then 1103 returns `NULL`. Again there is no final component and `get_link()` 1104 reports this by leaving the `last_type` field of `nameidata` as 1105 `LAST_BIND`. 1106 1107 Following the symlink in the final component 1108 -------------------------------------------- 1109 1110 All this leads to `link_path_walk()` walking down every component, and 1111 following all symbolic links it finds, until it reaches the final 1112 component. This is just returned in the `last` field of `nameidata`. 1113 For some callers, this is all they need; they want to create that 1114 `last` name if it doesn't exist or give an error if it does. Other 1115 callers will want to follow a symlink if one is found, and possibly 1116 apply special handling to the last component of that symlink, rather 1117 than just the last component of the original file name. These callers 1118 potentially need to call `link_path_walk()` again and again on 1119 successive symlinks until one is found that doesn't point to another 1120 symlink. 1121 1122 This case is handled by the relevant caller of `link_path_walk()`, such as 1123 `path_lookupat()` using a loop that calls `link_path_walk()`, and then 1124 handles the final component. If the final component is a symlink 1125 that needs to be followed, then `trailing_symlink()` is called to set 1126 things up properly and the loop repeats, calling `link_path_walk()` 1127 again. This could loop as many as 40 times if the last component of 1128 each symlink is another symlink. 1129 1130 The various functions that examine the final component and possibly 1131 report that it is a symlink are `lookup_last()`, `mountpoint_last()` 1132 and `do_last()`, each of which use the same convention as 1133 `walk_component()` of returning `1` if a symlink was found that needs 1134 to be followed. 1135 1136 Of these, `do_last()` is the most interesting as it is used for 1137 opening a file. Part of `do_last()` runs with `i_mutex` held and this 1138 part is in a separate function: `lookup_open()`. 1139 1140 Explaining `do_last()` completely is beyond the scope of this article, 1141 but a few highlights should help those interested in exploring the 1142 code. 1143 1144 1. Rather than just finding the target file, `do_last()` needs to open 1145 it. If the file was found in the dcache, then `vfs_open()` is used for 1146 this. If not, then `lookup_open()` will either call `atomic_open()` (if 1147 the filesystem provides it) to combine the final lookup with the open, or 1148 will perform the separate `lookup_real()` and `vfs_create()` steps 1149 directly. In the later case the actual "open" of this newly found or 1150 created file will be performed by `vfs_open()`, just as if the name 1151 were found in the dcache. 1152 1153 2. `vfs_open()` can fail with `-EOPENSTALE` if the cached information 1154 wasn't quite current enough. Rather than restarting the lookup from 1155 the top with `LOOKUP_REVAL` set, `lookup_open()` is called instead, 1156 giving the filesystem a chance to resolve small inconsistencies. 1157 If that doesn't work, only then is the lookup restarted from the top. 1158 1159 3. An open with O_CREAT **does** follow a symlink in the final component, 1160 unlike other creation system calls (like `mkdir`). So the sequence: 1161 1162 > ln -s bar /tmp/foo 1163 > echo hello > /tmp/foo 1164 1165 will create a file called `/tmp/bar`. This is not permitted if 1166 `O_EXCL` is set but otherwise is handled for an O_CREAT open much 1167 like for a non-creating open: `should_follow_link()` returns `1`, and 1168 so does `do_last()` so that `trailing_symlink()` gets called and the 1169 open process continues on the symlink that was found. 1170 1171 Updating the access time 1172 ------------------------ 1173 1174 We previously said of RCU-walk that it would "take no locks, increment 1175 no counts, leave no footprints." We have since seen that some 1176 "footprints" can be needed when handling symlinks as a counted 1177 reference (or even a memory allocation) may be needed. But these 1178 footprints are best kept to a minimum. 1179 1180 One other place where walking down a symlink can involve leaving 1181 footprints in a way that doesn't affect directories is in updating access times. 1182 In Unix (and Linux) every filesystem object has a "last accessed 1183 time", or "`atime`". Passing through a directory to access a file 1184 within is not considered to be an access for the purposes of 1185 `atime`; only listing the contents of a directory can update its `atime`. 1186 Symlinks are different it seems. Both reading a symlink (with `readlink()`) 1187 and looking up a symlink on the way to some other destination can 1188 update the atime on that symlink. 1189 1190 [clearest statement]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08 1191 1192 It is not clear why this is the case; POSIX has little to say on the 1193 subject. The [clearest statement] is that, if a particular implementation 1194 updates a timestamp in a place not specified by POSIX, this must be 1195 documented "except that any changes caused by pathname resolution need 1196 not be documented". This seems to imply that POSIX doesn't really 1197 care about access-time updates during pathname lookup. 1198 1199 [Linux 1.3.87]: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8 1200 1201 An examination of history shows that prior to [Linux 1.3.87], the ext2 1202 filesystem, at least, didn't update atime when following a link. 1203 Unfortunately we have no record of why that behavior was changed. 1204 1205 In any case, access time must now be updated and that operation can be 1206 quite complex. Trying to stay in RCU-walk while doing it is best 1207 avoided. Fortunately it is often permitted to skip the `atime` 1208 update. Because `atime` updates cause performance problems in various 1209 areas, Linux supports the `relatime` mount option, which generally 1210 limits the updates of `atime` to once per day on files that aren't 1211 being changed (and symlinks never change once created). Even without 1212 `relatime`, many filesystems record `atime` with a one-second 1213 granularity, so only one update per second is required. 1214 1215 It is easy to test if an `atime` update is needed while in RCU-walk 1216 mode and, if it isn't, the update can be skipped and RCU-walk mode 1217 continues. Only when an `atime` update is actually required does the 1218 path walk drop down to REF-walk. All of this is handled in the 1219 `get_link()` function. 1220 1221 A few flags 1222 ----------- 1223 1224 A suitable way to wrap up this tour of pathname walking is to list 1225 the various flags that can be stored in the `nameidata` to guide the 1226 lookup process. Many of these are only meaningful on the final 1227 component, others reflect the current state of the pathname lookup. 1228 And then there is `LOOKUP_EMPTY`, which doesn't fit conceptually with 1229 the others. If this is not set, an empty pathname causes an error 1230 very early on. If it is set, empty pathnames are not considered to be 1231 an error. 1232 1233 ### Global state flags ### 1234 1235 We have already met two global state flags: `LOOKUP_RCU` and 1236 `LOOKUP_REVAL`. These select between one of three overall approaches 1237 to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation. 1238 1239 `LOOKUP_PARENT` indicates that the final component hasn't been reached 1240 yet. This is primarily used to tell the audit subsystem the full 1241 context of a particular access being audited. 1242 1243 `LOOKUP_ROOT` indicates that the `root` field in the `nameidata` was 1244 provided by the caller, so it shouldn't be released when it is no 1245 longer needed. 1246 1247 `LOOKUP_JUMPED` means that the current dentry was chosen not because 1248 it had the right name but for some other reason. This happens when 1249 following "`..`", following a symlink to `/`, crossing a mount point 1250 or accessing a "`/proc/$PID/fd/$FD`" symlink. In this case the 1251 filesystem has not been asked to revalidate the name (with 1252 `d_revalidate()`). In such cases the inode may still need to be 1253 revalidated, so `d_op->d_weak_revalidate()` is called if 1254 `LOOKUP_JUMPED` is set when the look completes - which may be at the 1255 final component or, when creating, unlinking, or renaming, at the penultimate component. 1256 1257 ### Final-component flags ### 1258 1259 Some of these flags are only set when the final component is being 1260 considered. Others are only checked for when considering that final 1261 component. 1262 1263 `LOOKUP_AUTOMOUNT` ensures that, if the final component is an automount 1264 point, then the mount is triggered. Some operations would trigger it 1265 anyway, but operations like `stat()` deliberately don't. `statfs()` 1266 needs to trigger the mount but otherwise behaves a lot like `stat()`, so 1267 it sets `LOOKUP_AUTOMOUNT`, as does "`quotactl()`" and the handling of 1268 "`mount --bind`". 1269 1270 `LOOKUP_FOLLOW` has a similar function to `LOOKUP_AUTOMOUNT` but for 1271 symlinks. Some system calls set or clear it implicitly, while 1272 others have API flags such as `AT_SYMLINK_FOLLOW` and 1273 `UMOUNT_NOFOLLOW` to control it. Its effect is similar to 1274 `WALK_GET` that we already met, but it is used in a different way. 1275 1276 `LOOKUP_DIRECTORY` insists that the final component is a directory. 1277 Various callers set this and it is also set when the final component 1278 is found to be followed by a slash. 1279 1280 Finally `LOOKUP_OPEN`, `LOOKUP_CREATE`, `LOOKUP_EXCL`, and 1281 `LOOKUP_RENAME_TARGET` are not used directly by the VFS but are made 1282 available to the filesystem and particularly the `->d_revalidate()` 1283 method. A filesystem can choose not to bother revalidating too hard 1284 if it knows that it will be asked to open or create the file soon. 1285 These flags were previously useful for `->lookup()` too but with the 1286 introduction of `->atomic_open()` they are less relevant there. 1287 1288 End of the road 1289 --------------- 1290 1291 Despite its complexity, all this pathname lookup code appears to be 1292 in good shape - various parts are certainly easier to understand now 1293 than even a couple of releases ago. But that doesn't mean it is 1294 "finished". As already mentioned, RCU-walk currently only follows 1295 symlinks that are stored in the inode so, while it handles many ext4 1296 symlinks, it doesn't help with NFS, XFS, or Btrfs. That support 1297 is not likely to be long delayed.