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5 Pathname lookup in Linux.
6 =========================
8 This write-up is based on three articles published at lwn.net:
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
14 Written by Neil Brown with help from Al Viro and Jon Corbet.
16 Introduction
17 ------------
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.
32 There are two sorts of ...
33 --------------------------
35 [`openat()`]: http://man7.org/linux/man-pages/man2/openat.2.html
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()`]".
47 [`execveat()`]: http://man7.org/linux/man-pages/man2/execveat.2.html
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.
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.
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.
70 [POSIX]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
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]
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.
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.
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.
103 More than just a cache.
104 -----------------------
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.
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.
122 When considering the dcache, we have another of our "two types"
123 distinctions: there are two types of filesystems.
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.
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.
140 REF-walk: simple concurrency management with refcounts and spinlocks
141 --------------------------------------------------------------------
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.
151 [Meet the Lockers]: https://lwn.net/Articles/453685/
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].
160 The locking mechanisms used by REF-walk include:
162 ### dentry->d_lockref ###
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.
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.
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.
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.
190 So as long as a counted reference is held to a dentry, a non-`NULL` `->d_inode`
191 value will never be changed.
193 ### dentry->d_lock ###
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.
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.
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.
213 ### rename_lock ###
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.
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.
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.
234 ### inode->i_mutex ###
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.
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.
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.
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.
265 ### mnt->mnt_count ###
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.
280 ### mount_lock ###
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.
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.
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.
301 ### RCU ###
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.
307 In particular it is held while scanning chains in the dcache hash
308 table, and the mount point hash table.
310 Bringing it together with `struct nameidata`
311 --------------------------------------------
313 [First edition Unix]: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
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):
321 ### `struct path path` ###
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.
331 ### `struct qstr last` ###
333 This is a string together with a length (i.e. _not_ `nul` terminated)
334 that is the "next" component in the pathname.
336 ### `int last_type` ###
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.
344 ### `struct path root` ###
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.
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()`.
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:
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.
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.
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.
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.
395 Handling the final component.
396 -----------------------------
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.
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.
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.
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.
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.
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.
443 Revalidation and automounts
444 ---------------------------
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.
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.
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.
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`:
472 ### `DCACHE_MANAGE_TRANSIT` ###
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:
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.
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`.
490 ### `DCACHE_MOUNTED` ###
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.
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).
502 ### `DCACHE_NEED_AUTOMOUNT` ###
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.
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`.
513 In the latter case, `finish_automount()` will be called to safely
514 install the new mount point into the mount table.
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.
522 RCU-walk - faster pathname lookup in Linux
523 ==========================================
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.
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.
538 Clear demarcation of roles
539 --------------------------
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.
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.
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.
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.
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.
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.
601 RCU and seqlocks: fast and light
602 --------------------------------
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.
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.
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.
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.
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.
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.
657 ### `mount_lock` and `nd->m_seq` ###
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.
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.
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.
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.
685 ### `dentry->d_seq` and `nd->seq`. ###
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.
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.
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.
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.
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.
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.
731 ### No `inode->i_mutex` or even `rename_lock` ###
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.
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.
752 `unlazy walk()` and `complete_walk()`
753 -------------------------------------
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.
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.
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.
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.
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.
801 Taking care in filesystems
802 ---------------------------
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.
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.
822 [`READ_ONCE()`]: https://lwn.net/Articles/624126/
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 `ACCESS_ONCE()` or the newer [`READ_ONCE()`] to access
830 fields, and verifying the result is not NULL before using it. This
831 pattern can be see in `nfs_lookup_revalidate()`.
833 A pair of patterns
834 ------------------
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.
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.
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.
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.
861 A walk among the symlinks
862 =========================
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.
871 The symlink stack
872 -----------------
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.
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.
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.
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.
906 [outlined recently]: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
908 The second reason was [outlined recently] by Linus:
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.
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.
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.
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.
935 Storage and lifetime of cached symlinks
936 ---------------------------------------
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.
944 [object-oriented design pattern]: https://lwn.net/Articles/446317/
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.
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.
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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
1035 Following the symlink
1036 ---------------------
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.
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.
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.
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.
1076 ### Symlinks with no final component ###
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`.
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.
1089 The other case involves things in `/proc` that look like symlinks but
1090 aren't really.
1092 > $ ls -l /proc/self/fd/1
1093 > lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
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`.
1107 Following the symlink in the final component
1108 --------------------------------------------
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.
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.
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.
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()`.
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.
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.
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.
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:
1162 > ln -s bar /tmp/foo
1163 > echo hello > /tmp/foo
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.
1171 Updating the access time
1172 ------------------------
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.
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.
1190 [clearest statement]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
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.
1199 [Linux 1.3.87]: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
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.
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.
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.
1221 A few flags
1222 -----------
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.
1233 ### Global state flags ###
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.
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.
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.
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.
1257 ### Final-component flags ###
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.
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`".
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.
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.
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.
1288 End of the road
1289 ---------------
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.