5 This write-up is based on three articles published at lwn.net:
7 - <https://lwn.net/Articles/649115/> Pathname lookup in Linux
8 - <https://lwn.net/Articles/649729/> RCU-walk: faster pathname lookup in Linux
9 - <https://lwn.net/Articles/650786/> A walk among the symlinks
11 Written by Neil Brown with help from Al Viro and Jon Corbet.
12 It has subsequently been updated to reflect changes in the kernel
15 - per-directory parallel name lookup.
16 - ``openat2()`` resolution restriction flags.
18 Introduction to pathname lookup
19 ===============================
21 The most obvious aspect of pathname lookup, which very little
22 exploration is needed to discover, is that it is complex. There are
23 many rules, special cases, and implementation alternatives that all
24 combine to confuse the unwary reader. Computer science has long been
25 acquainted with such complexity and has tools to help manage it. One
26 tool that we will make extensive use of is "divide and conquer". For
27 the early parts of the analysis we will divide off symlinks - leaving
28 them until the final part. Well before we get to symlinks we have
29 another major division based on the VFS's approach to locking which
30 will allow us to review "REF-walk" and "RCU-walk" separately. But we
31 are getting ahead of ourselves. There are some important low level
32 distinctions we need to clarify first.
34 There are two sorts of ...
35 --------------------------
37 .. _openat: http://man7.org/linux/man-pages/man2/openat.2.html
39 Pathnames (sometimes "file names"), used to identify objects in the
40 filesystem, will be familiar to most readers. They contain two sorts
41 of elements: "slashes" that are sequences of one or more "``/``"
42 characters, and "components" that are sequences of one or more
43 non-"``/``" characters. These form two kinds of paths. Those that
44 start with slashes are "absolute" and start from the filesystem root.
45 The others are "relative" and start from the current directory, or
46 from some other location specified by a file descriptor given to
47 "``*at()``" system calls such as `openat() <openat_>`_.
49 .. _execveat: http://man7.org/linux/man-pages/man2/execveat.2.html
51 It is tempting to describe the second kind as starting with a
52 component, but that isn't always accurate: a pathname can lack both
53 slashes and components, it can be empty, in other words. This is
54 generally forbidden in POSIX, but some of those "``*at()``" system calls
55 in Linux permit it when the ``AT_EMPTY_PATH`` flag is given. For
56 example, if you have an open file descriptor on an executable file you
57 can execute it by calling `execveat() <execveat_>`_ passing
58 the file descriptor, an empty path, and the ``AT_EMPTY_PATH`` flag.
60 These paths can be divided into two sections: the final component and
61 everything else. The "everything else" is the easy bit. In all cases
62 it must identify a directory that already exists, otherwise an error
63 such as ``ENOENT`` or ``ENOTDIR`` will be reported.
65 The final component is not so simple. Not only do different system
66 calls interpret it quite differently (e.g. some create it, some do
67 not), but it might not even exist: neither the empty pathname nor the
68 pathname that is just slashes have a final component. If it does
69 exist, it could be "``.``" or "``..``" which are handled quite differently
70 from other components.
72 .. _POSIX: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
74 If a pathname ends with a slash, such as "``/tmp/foo/``" it might be
75 tempting to consider that to have an empty final component. In many
76 ways that would lead to correct results, but not always. In
77 particular, ``mkdir()`` and ``rmdir()`` each create or remove a directory named
78 by the final component, and they are required to work with pathnames
79 ending in "``/``". According to POSIX_:
81 A pathname that contains at least one non-<slash> character and
82 that ends with one or more trailing <slash> characters shall not
83 be resolved successfully unless the last pathname component before
84 the trailing <slash> characters names an existing directory or a
85 directory entry that is to be created for a directory immediately
86 after the pathname is resolved.
88 The Linux pathname walking code (mostly in ``fs/namei.c``) deals with
89 all of these issues: breaking the path into components, handling the
90 "everything else" quite separately from the final component, and
91 checking that the trailing slash is not used where it isn't
92 permitted. It also addresses the important issue of concurrent
95 While one process is looking up a pathname, another might be making
96 changes that affect that lookup. One fairly extreme case is that if
97 "a/b" were renamed to "a/c/b" while another process were looking up
98 "a/b/..", that process might successfully resolve on "a/c".
99 Most races are much more subtle, and a big part of the task of
100 pathname lookup is to prevent them from having damaging effects. Many
101 of the possible races are seen most clearly in the context of the
102 "dcache" and an understanding of that is central to understanding
105 More than just a cache
106 ----------------------
108 The "dcache" caches information about names in each filesystem to
109 make them quickly available for lookup. Each entry (known as a
110 "dentry") contains three significant fields: a component name, a
111 pointer to a parent dentry, and a pointer to the "inode" which
112 contains further information about the object in that parent with
113 the given name. The inode pointer can be ``NULL`` indicating that the
114 name doesn't exist in the parent. While there can be linkage in the
115 dentry of a directory to the dentries of the children, that linkage is
116 not used for pathname lookup, and so will not be considered here.
118 The dcache has a number of uses apart from accelerating lookup. One
119 that will be particularly relevant is that it is closely integrated
120 with the mount table that records which filesystem is mounted where.
121 What the mount table actually stores is which dentry is mounted on top
122 of which other dentry.
124 When considering the dcache, we have another of our "two types"
125 distinctions: there are two types of filesystems.
127 Some filesystems ensure that the information in the dcache is always
128 completely accurate (though not necessarily complete). This can allow
129 the VFS to determine if a particular file does or doesn't exist
130 without checking with the filesystem, and means that the VFS can
131 protect the filesystem against certain races and other problems.
132 These are typically "local" filesystems such as ext3, XFS, and Btrfs.
134 Other filesystems don't provide that guarantee because they cannot.
135 These are typically filesystems that are shared across a network,
136 whether remote filesystems like NFS and 9P, or cluster filesystems
137 like ocfs2 or cephfs. These filesystems allow the VFS to revalidate
138 cached information, and must provide their own protection against
139 awkward races. The VFS can detect these filesystems by the
140 ``DCACHE_OP_REVALIDATE`` flag being set in the dentry.
142 REF-walk: simple concurrency management with refcounts and spinlocks
143 --------------------------------------------------------------------
145 With all of those divisions carefully classified, we can now start
146 looking at the actual process of walking along a path. In particular
147 we will start with the handling of the "everything else" part of a
148 pathname, and focus on the "REF-walk" approach to concurrency
149 management. This code is found in the ``link_path_walk()`` function, if
150 you ignore all the places that only run when "``LOOKUP_RCU``"
151 (indicating the use of RCU-walk) is set.
153 .. _Meet the Lockers: https://lwn.net/Articles/453685/
155 REF-walk is fairly heavy-handed with locks and reference counts. Not
156 as heavy-handed as in the old "big kernel lock" days, but certainly not
157 afraid of taking a lock when one is needed. It uses a variety of
158 different concurrency controls. A background understanding of the
159 various primitives is assumed, or can be gleaned from elsewhere such
160 as in `Meet the Lockers`_.
162 The locking mechanisms used by REF-walk include:
167 This uses the lockref primitive to provide both a spinlock and a
168 reference count. The special-sauce of this primitive is that the
169 conceptual sequence "lock; inc_ref; unlock;" can often be performed
170 with a single atomic memory operation.
172 Holding a reference on a dentry ensures that the dentry won't suddenly
173 be freed and used for something else, so the values in various fields
174 will behave as expected. It also protects the ``->d_inode`` reference
175 to the inode to some extent.
177 The association between a dentry and its inode is fairly permanent.
178 For example, when a file is renamed, the dentry and inode move
179 together to the new location. When a file is created the dentry will
180 initially be negative (i.e. ``d_inode`` is ``NULL``), and will be assigned
181 to the new inode as part of the act of creation.
183 When a file is deleted, this can be reflected in the cache either by
184 setting ``d_inode`` to ``NULL``, or by removing it from the hash table
185 (described shortly) used to look up the name in the parent directory.
186 If the dentry is still in use the second option is used as it is
187 perfectly legal to keep using an open file after it has been deleted
188 and having the dentry around helps. If the dentry is not otherwise in
189 use (i.e. if the refcount in ``d_lockref`` is one), only then will
190 ``d_inode`` be set to ``NULL``. Doing it this way is more efficient for a
193 So as long as a counted reference is held to a dentry, a non-``NULL`` ``->d_inode``
194 value will never be changed.
199 ``d_lock`` is a synonym for the spinlock that is part of ``d_lockref`` above.
200 For our purposes, holding this lock protects against the dentry being
201 renamed or unlinked. In particular, its parent (``d_parent``), and its
202 name (``d_name``) cannot be changed, and it cannot be removed from the
205 When looking for a name in a directory, REF-walk takes ``d_lock`` on
206 each candidate dentry that it finds in the hash table and then checks
207 that the parent and name are correct. So it doesn't lock the parent
208 while searching in the cache; it only locks children.
210 When looking for the parent for a given name (to handle "``..``"),
211 REF-walk can take ``d_lock`` to get a stable reference to ``d_parent``,
212 but it first tries a more lightweight approach. As seen in
213 ``dget_parent()``, if a reference can be claimed on the parent, and if
214 subsequently ``d_parent`` can be seen to have not changed, then there is
215 no need to actually take the lock on the child.
220 Looking up a given name in a given directory involves computing a hash
221 from the two values (the name and the dentry of the directory),
222 accessing that slot in a hash table, and searching the linked list
225 When a dentry is renamed, the name and the parent dentry can both
226 change so the hash will almost certainly change too. This would move the
227 dentry to a different chain in the hash table. If a filename search
228 happened to be looking at a dentry that was moved in this way,
229 it might end up continuing the search down the wrong chain,
230 and so miss out on part of the correct chain.
232 The name-lookup process (``d_lookup()``) does *not* try to prevent this
233 from happening, but only to detect when it happens.
234 ``rename_lock`` is a seqlock that is updated whenever any dentry is
235 renamed. If ``d_lookup`` finds that a rename happened while it
236 unsuccessfully scanned a chain in the hash table, it simply tries
239 ``rename_lock`` is also used to detect and defend against potential attacks
240 against ``LOOKUP_BENEATH`` and ``LOOKUP_IN_ROOT`` when resolving ".." (where
241 the parent directory is moved outside the root, bypassing the ``path_equal()``
242 check). If ``rename_lock`` is updated during the lookup and the path encounters
243 a "..", a potential attack occurred and ``handle_dots()`` will bail out with
249 ``i_rwsem`` is a read/write semaphore that serializes all changes to a particular
250 directory. This ensures that, for example, an ``unlink()`` and a ``rename()``
251 cannot both happen at the same time. It also keeps the directory
252 stable while the filesystem is asked to look up a name that is not
253 currently in the dcache or, optionally, when the list of entries in a
254 directory is being retrieved with ``readdir()``.
256 This has a complementary role to that of ``d_lock``: ``i_rwsem`` on a
257 directory protects all of the names in that directory, while ``d_lock``
258 on a name protects just one name in a directory. Most changes to the
259 dcache hold ``i_rwsem`` on the relevant directory inode and briefly take
260 ``d_lock`` on one or more the dentries while the change happens. One
261 exception is when idle dentries are removed from the dcache due to
262 memory pressure. This uses ``d_lock``, but ``i_rwsem`` plays no role.
264 The semaphore affects pathname lookup in two distinct ways. Firstly it
265 prevents changes during lookup of a name in a directory. ``walk_component()`` uses
266 ``lookup_fast()`` first which, in turn, checks to see if the name is in the cache,
267 using only ``d_lock`` locking. If the name isn't found, then ``walk_component()``
268 falls back to ``lookup_slow()`` which takes a shared lock on ``i_rwsem``, checks again that
269 the name isn't in the cache, and then calls in to the filesystem to get a
270 definitive answer. A new dentry will be added to the cache regardless of
273 Secondly, when pathname lookup reaches the final component, it will
274 sometimes need to take an exclusive lock on ``i_rwsem`` before performing the last lookup so
275 that the required exclusion can be achieved. How path lookup chooses
276 to take, or not take, ``i_rwsem`` is one of the
277 issues addressed in a subsequent section.
279 If two threads attempt to look up the same name at the same time - a
280 name that is not yet in the dcache - the shared lock on ``i_rwsem`` will
281 not prevent them both adding new dentries with the same name. As this
282 would result in confusion an extra level of interlocking is used,
283 based around a secondary hash table (``in_lookup_hashtable``) and a
284 per-dentry flag bit (``DCACHE_PAR_LOOKUP``).
286 To add a new dentry to the cache while only holding a shared lock on
287 ``i_rwsem``, a thread must call ``d_alloc_parallel()``. This allocates a
288 dentry, stores the required name and parent in it, checks if there
289 is already a matching dentry in the primary or secondary hash
290 tables, and if not, stores the newly allocated dentry in the secondary
291 hash table, with ``DCACHE_PAR_LOOKUP`` set.
293 If a matching dentry was found in the primary hash table then that is
294 returned and the caller can know that it lost a race with some other
295 thread adding the entry. If no matching dentry is found in either
296 cache, the newly allocated dentry is returned and the caller can
297 detect this from the presence of ``DCACHE_PAR_LOOKUP``. In this case it
298 knows that it has won any race and now is responsible for asking the
299 filesystem to perform the lookup and find the matching inode. When
300 the lookup is complete, it must call ``d_lookup_done()`` which clears
301 the flag and does some other house keeping, including removing the
302 dentry from the secondary hash table - it will normally have been
303 added to the primary hash table already. Note that a ``struct
304 waitqueue_head`` is passed to ``d_alloc_parallel()``, and
305 ``d_lookup_done()`` must be called while this ``waitqueue_head`` is still
308 If a matching dentry is found in the secondary hash table,
309 ``d_alloc_parallel()`` has a little more work to do. It first waits for
310 ``DCACHE_PAR_LOOKUP`` to be cleared, using a wait_queue that was passed
311 to the instance of ``d_alloc_parallel()`` that won the race and that
312 will be woken by the call to ``d_lookup_done()``. It then checks to see
313 if the dentry has now been added to the primary hash table. If it
314 has, the dentry is returned and the caller just sees that it lost any
315 race. If it hasn't been added to the primary hash table, the most
316 likely explanation is that some other dentry was added instead using
317 ``d_splice_alias()``. In any case, ``d_alloc_parallel()`` repeats all the
318 look ups from the start and will normally return something from the
324 ``mnt_count`` is a per-CPU reference counter on "``mount``" structures.
325 Per-CPU here means that incrementing the count is cheap as it only
326 uses CPU-local memory, but checking if the count is zero is expensive as
327 it needs to check with every CPU. Taking a ``mnt_count`` reference
328 prevents the mount structure from disappearing as the result of regular
329 unmount operations, but does not prevent a "lazy" unmount. So holding
330 ``mnt_count`` doesn't ensure that the mount remains in the namespace and,
331 in particular, doesn't stabilize the link to the mounted-on dentry. It
332 does, however, ensure that the ``mount`` data structure remains coherent,
333 and it provides a reference to the root dentry of the mounted
334 filesystem. So a reference through ``->mnt_count`` provides a stable
335 reference to the mounted dentry, but not the mounted-on dentry.
340 ``mount_lock`` is a global seqlock, a bit like ``rename_lock``. It can be used to
341 check if any change has been made to any mount points.
343 While walking down the tree (away from the root) this lock is used when
344 crossing a mount point to check that the crossing was safe. That is,
345 the value in the seqlock is read, then the code finds the mount that
346 is mounted on the current directory, if there is one, and increments
347 the ``mnt_count``. Finally the value in ``mount_lock`` is checked against
348 the old value. If there is no change, then the crossing was safe. If there
349 was a change, the ``mnt_count`` is decremented and the whole process is
352 When walking up the tree (towards the root) by following a ".." link,
353 a little more care is needed. In this case the seqlock (which
354 contains both a counter and a spinlock) is fully locked to prevent
355 any changes to any mount points while stepping up. This locking is
356 needed to stabilize the link to the mounted-on dentry, which the
357 refcount on the mount itself doesn't ensure.
359 ``mount_lock`` is also used to detect and defend against potential attacks
360 against ``LOOKUP_BENEATH`` and ``LOOKUP_IN_ROOT`` when resolving ".." (where
361 the parent directory is moved outside the root, bypassing the ``path_equal()``
362 check). If ``mount_lock`` is updated during the lookup and the path encounters
363 a "..", a potential attack occurred and ``handle_dots()`` will bail out with
369 Finally the global (but extremely lightweight) RCU read lock is held
370 from time to time to ensure certain data structures don't get freed
373 In particular it is held while scanning chains in the dcache hash
374 table, and the mount point hash table.
376 Bringing it together with ``struct nameidata``
377 ----------------------------------------------
379 .. _First edition Unix: https://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
381 Throughout the process of walking a path, the current status is stored
382 in a ``struct nameidata``, "namei" being the traditional name - dating
383 all the way back to `First Edition Unix`_ - of the function that
384 converts a "name" to an "inode". ``struct nameidata`` contains (among
390 A ``path`` contains a ``struct vfsmount`` (which is
391 embedded in a ``struct mount``) and a ``struct dentry``. Together these
392 record the current status of the walk. They start out referring to the
393 starting point (the current working directory, the root directory, or some other
394 directory identified by a file descriptor), and are updated on each
395 step. A reference through ``d_lockref`` and ``mnt_count`` is always
401 This is a string together with a length (i.e. *not* ``nul`` terminated)
402 that is the "next" component in the pathname.
407 This is one of ``LAST_NORM``, ``LAST_ROOT``, ``LAST_DOT`` or ``LAST_DOTDOT``.
408 The ``last`` field is only valid if the type is ``LAST_NORM``.
413 This is used to hold a reference to the effective root of the
414 filesystem. Often that reference won't be needed, so this field is
415 only assigned the first time it is used, or when a non-standard root
416 is requested. Keeping a reference in the ``nameidata`` ensures that
417 only one root is in effect for the entire path walk, even if it races
418 with a ``chroot()`` system call.
420 It should be noted that in the case of ``LOOKUP_IN_ROOT`` or
421 ``LOOKUP_BENEATH``, the effective root becomes the directory file descriptor
422 passed to ``openat2()`` (which exposes these ``LOOKUP_`` flags).
424 The root is needed when either of two conditions holds: (1) either the
425 pathname or a symbolic link starts with a "'/'", or (2) a "``..``"
426 component is being handled, since "``..``" from the root must always stay
427 at the root. The value used is usually the current root directory of
428 the calling process. An alternate root can be provided as when
429 ``sysctl()`` calls ``file_open_root()``, and when NFSv4 or Btrfs call
430 ``mount_subtree()``. In each case a pathname is being looked up in a very
431 specific part of the filesystem, and the lookup must not be allowed to
432 escape that subtree. It works a bit like a local ``chroot()``.
434 Ignoring the handling of symbolic links, we can now describe the
435 "``link_path_walk()``" function, which handles the lookup of everything
436 except the final component as:
438 Given a path (``name``) and a nameidata structure (``nd``), check that the
439 current directory has execute permission and then advance ``name``
440 over one component while updating ``last_type`` and ``last``. If that
441 was the final component, then return, otherwise call
442 ``walk_component()`` and repeat from the top.
444 ``walk_component()`` is even easier. If the component is ``LAST_DOTS``,
445 it calls ``handle_dots()`` which does the necessary locking as already
446 described. If it finds a ``LAST_NORM`` component it first calls
447 "``lookup_fast()``" which only looks in the dcache, but will ask the
448 filesystem to revalidate the result if it is that sort of filesystem.
449 If that doesn't get a good result, it calls "``lookup_slow()``" which
450 takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
451 to find a definitive answer.
453 As the last step of walk_component(), step_into() will be called either
454 directly from walk_component() or from handle_dots(). It calls
455 handle_mounts(), to check and handle mount points, in which a new
456 ``struct path`` is created containing a counted reference to the new dentry and
457 a reference to the new ``vfsmount`` which is only counted if it is
458 different from the previous ``vfsmount``. Then if there is
459 a symbolic link, step_into() calls pick_link() to deal with it,
460 otherwise it installs the new ``struct path`` in the ``struct nameidata``, and
461 drops the unneeded references.
463 This "hand-over-hand" sequencing of getting a reference to the new
464 dentry before dropping the reference to the previous dentry may
465 seem obvious, but is worth pointing out so that we will recognize its
466 analogue in the "RCU-walk" version.
468 Handling the final component
469 ----------------------------
471 ``link_path_walk()`` only walks as far as setting ``nd->last`` and
472 ``nd->last_type`` to refer to the final component of the path. It does
473 not call ``walk_component()`` that last time. Handling that final
474 component remains for the caller to sort out. Those callers are
475 path_lookupat(), path_parentat() and
476 path_openat() each of which handles the differing requirements of
477 different system calls.
479 ``path_parentat()`` is clearly the simplest - it just wraps a little bit
480 of housekeeping around ``link_path_walk()`` and returns the parent
481 directory and final component to the caller. The caller will be either
482 aiming to create a name (via ``filename_create()``) or remove or rename
483 a name (in which case ``user_path_parent()`` is used). They will use
484 ``i_rwsem`` to exclude other changes while they validate and then
485 perform their operation.
487 ``path_lookupat()`` is nearly as simple - it is used when an existing
488 object is wanted such as by ``stat()`` or ``chmod()``. It essentially just
489 calls ``walk_component()`` on the final component through a call to
490 ``lookup_last()``. ``path_lookupat()`` returns just the final dentry.
491 It is worth noting that when flag ``LOOKUP_MOUNTPOINT`` is set,
492 path_lookupat() will unset LOOKUP_JUMPED in nameidata so that in the
493 subsequent path traversal d_weak_revalidate() won't be called.
494 This is important when unmounting a filesystem that is inaccessible, such as
495 one provided by a dead NFS server.
497 Finally ``path_openat()`` is used for the ``open()`` system call; it
498 contains, in support functions starting with "open_last_lookups()", all the
499 complexity needed to handle the different subtleties of O_CREAT (with
500 or without O_EXCL), final "``/``" characters, and trailing symbolic
501 links. We will revisit this in the final part of this series, which
502 focuses on those symbolic links. "open_last_lookups()" will sometimes, but
503 not always, take ``i_rwsem``, depending on what it finds.
505 Each of these, or the functions which call them, need to be alert to
506 the possibility that the final component is not ``LAST_NORM``. If the
507 goal of the lookup is to create something, then any value for
508 ``last_type`` other than ``LAST_NORM`` will result in an error. For
509 example if ``path_parentat()`` reports ``LAST_DOTDOT``, then the caller
510 won't try to create that name. They also check for trailing slashes
511 by testing ``last.name[last.len]``. If there is any character beyond
512 the final component, it must be a trailing slash.
514 Revalidation and automounts
515 ---------------------------
517 Apart from symbolic links, there are only two parts of the "REF-walk"
518 process not yet covered. One is the handling of stale cache entries
519 and the other is automounts.
521 On filesystems that require it, the lookup routines will call the
522 ``->d_revalidate()`` dentry method to ensure that the cached information
523 is current. This will often confirm validity or update a few details
524 from a server. In some cases it may find that there has been change
525 further up the path and that something that was thought to be valid
526 previously isn't really. When this happens the lookup of the whole
527 path is aborted and retried with the "``LOOKUP_REVAL``" flag set. This
528 forces revalidation to be more thorough. We will see more details of
529 this retry process in the next article.
531 Automount points are locations in the filesystem where an attempt to
532 lookup a name can trigger changes to how that lookup should be
533 handled, in particular by mounting a filesystem there. These are
534 covered in greater detail in autofs.txt in the Linux documentation
535 tree, but a few notes specifically related to path lookup are in order
538 The Linux VFS has a concept of "managed" dentries. There are three
539 potentially interesting things about these dentries corresponding
540 to three different flags that might be set in ``dentry->d_flags``:
542 ``DCACHE_MANAGE_TRANSIT``
543 ~~~~~~~~~~~~~~~~~~~~~~~~~
545 If this flag has been set, then the filesystem has requested that the
546 ``d_manage()`` dentry operation be called before handling any possible
547 mount point. This can perform two particular services:
549 It can block to avoid races. If an automount point is being
550 unmounted, the ``d_manage()`` function will usually wait for that
551 process to complete before letting the new lookup proceed and possibly
552 trigger a new automount.
554 It can selectively allow only some processes to transit through a
555 mount point. When a server process is managing automounts, it may
556 need to access a directory without triggering normal automount
557 processing. That server process can identify itself to the ``autofs``
558 filesystem, which will then give it a special pass through
559 ``d_manage()`` by returning ``-EISDIR``.
564 This flag is set on every dentry that is mounted on. As Linux
565 supports multiple filesystem namespaces, it is possible that the
566 dentry may not be mounted on in *this* namespace, just in some
567 other. So this flag is seen as a hint, not a promise.
569 If this flag is set, and ``d_manage()`` didn't return ``-EISDIR``,
570 ``lookup_mnt()`` is called to examine the mount hash table (honoring the
571 ``mount_lock`` described earlier) and possibly return a new ``vfsmount``
572 and a new ``dentry`` (both with counted references).
574 ``DCACHE_NEED_AUTOMOUNT``
575 ~~~~~~~~~~~~~~~~~~~~~~~~~
577 If ``d_manage()`` allowed us to get this far, and ``lookup_mnt()`` didn't
578 find a mount point, then this flag causes the ``d_automount()`` dentry
579 operation to be called.
581 The ``d_automount()`` operation can be arbitrarily complex and may
582 communicate with server processes etc. but it should ultimately either
583 report that there was an error, that there was nothing to mount, or
584 should provide an updated ``struct path`` with new ``dentry`` and ``vfsmount``.
586 In the latter case, ``finish_automount()`` will be called to safely
587 install the new mount point into the mount table.
589 There is no new locking of import here and it is important that no
590 locks (only counted references) are held over this processing due to
591 the very real possibility of extended delays.
592 This will become more important next time when we examine RCU-walk
593 which is particularly sensitive to delays.
595 RCU-walk - faster pathname lookup in Linux
596 ==========================================
598 RCU-walk is another algorithm for performing pathname lookup in Linux.
599 It is in many ways similar to REF-walk and the two share quite a bit
600 of code. The significant difference in RCU-walk is how it allows for
601 the possibility of concurrent access.
603 We noted that REF-walk is complex because there are numerous details
604 and special cases. RCU-walk reduces this complexity by simply
605 refusing to handle a number of cases -- it instead falls back to
606 REF-walk. The difficulty with RCU-walk comes from a different
607 direction: unfamiliarity. The locking rules when depending on RCU are
608 quite different from traditional locking, so we will spend a little extra
609 time when we come to those.
611 Clear demarcation of roles
612 --------------------------
614 The easiest way to manage concurrency is to forcibly stop any other
615 thread from changing the data structures that a given thread is
616 looking at. In cases where no other thread would even think of
617 changing the data and lots of different threads want to read at the
618 same time, this can be very costly. Even when using locks that permit
619 multiple concurrent readers, the simple act of updating the count of
620 the number of current readers can impose an unwanted cost. So the
621 goal when reading a shared data structure that no other process is
622 changing is to avoid writing anything to memory at all. Take no
623 locks, increment no counts, leave no footprints.
625 The REF-walk mechanism already described certainly doesn't follow this
626 principle, but then it is really designed to work when there may well
627 be other threads modifying the data. RCU-walk, in contrast, is
628 designed for the common situation where there are lots of frequent
629 readers and only occasional writers. This may not be common in all
630 parts of the filesystem tree, but in many parts it will be. For the
631 other parts it is important that RCU-walk can quickly fall back to
634 Pathname lookup always starts in RCU-walk mode but only remains there
635 as long as what it is looking for is in the cache and is stable. It
636 dances lightly down the cached filesystem image, leaving no footprints
637 and carefully watching where it is, to be sure it doesn't trip. If it
638 notices that something has changed or is changing, or if something
639 isn't in the cache, then it tries to stop gracefully and switch to
642 This stopping requires getting a counted reference on the current
643 ``vfsmount`` and ``dentry``, and ensuring that these are still valid -
644 that a path walk with REF-walk would have found the same entries.
645 This is an invariant that RCU-walk must guarantee. It can only make
646 decisions, such as selecting the next step, that are decisions which
647 REF-walk could also have made if it were walking down the tree at the
648 same time. If the graceful stop succeeds, the rest of the path is
649 processed with the reliable, if slightly sluggish, REF-walk. If
650 RCU-walk finds it cannot stop gracefully, it simply gives up and
651 restarts from the top with REF-walk.
653 This pattern of "try RCU-walk, if that fails try REF-walk" can be
654 clearly seen in functions like filename_lookup(),
656 do_filp_open(), and do_file_open_root(). These four
657 correspond roughly to the three ``path_*()`` functions we met earlier,
658 each of which calls ``link_path_walk()``. The ``path_*()`` functions are
659 called using different mode flags until a mode is found which works.
660 They are first called with ``LOOKUP_RCU`` set to request "RCU-walk". If
661 that fails with the error ``ECHILD`` they are called again with no
662 special flag to request "REF-walk". If either of those report the
663 error ``ESTALE`` a final attempt is made with ``LOOKUP_REVAL`` set (and no
664 ``LOOKUP_RCU``) to ensure that entries found in the cache are forcibly
665 revalidated - normally entries are only revalidated if the filesystem
666 determines that they are too old to trust.
668 The ``LOOKUP_RCU`` attempt may drop that flag internally and switch to
669 REF-walk, but will never then try to switch back to RCU-walk. Places
670 that trip up RCU-walk are much more likely to be near the leaves and
671 so it is very unlikely that there will be much, if any, benefit from
674 RCU and seqlocks: fast and light
675 --------------------------------
677 RCU is, unsurprisingly, critical to RCU-walk mode. The
678 ``rcu_read_lock()`` is held for the entire time that RCU-walk is walking
679 down a path. The particular guarantee it provides is that the key
680 data structures - dentries, inodes, super_blocks, and mounts - will
681 not be freed while the lock is held. They might be unlinked or
682 invalidated in one way or another, but the memory will not be
683 repurposed so values in various fields will still be meaningful. This
684 is the only guarantee that RCU provides; everything else is done using
687 As we saw above, REF-walk holds a counted reference to the current
688 dentry and the current vfsmount, and does not release those references
689 before taking references to the "next" dentry or vfsmount. It also
690 sometimes takes the ``d_lock`` spinlock. These references and locks are
691 taken to prevent certain changes from happening. RCU-walk must not
692 take those references or locks and so cannot prevent such changes.
693 Instead, it checks to see if a change has been made, and aborts or
696 To preserve the invariant mentioned above (that RCU-walk may only make
697 decisions that REF-walk could have made), it must make the checks at
698 or near the same places that REF-walk holds the references. So, when
699 REF-walk increments a reference count or takes a spinlock, RCU-walk
700 samples the status of a seqlock using ``read_seqcount_begin()`` or a
701 similar function. When REF-walk decrements the count or drops the
702 lock, RCU-walk checks if the sampled status is still valid using
703 ``read_seqcount_retry()`` or similar.
705 However, there is a little bit more to seqlocks than that. If
706 RCU-walk accesses two different fields in a seqlock-protected
707 structure, or accesses the same field twice, there is no a priori
708 guarantee of any consistency between those accesses. When consistency
709 is needed - which it usually is - RCU-walk must take a copy and then
710 use ``read_seqcount_retry()`` to validate that copy.
712 ``read_seqcount_retry()`` not only checks the sequence number, but also
713 imposes a memory barrier so that no memory-read instruction from
714 *before* the call can be delayed until *after* the call, either by the
715 CPU or by the compiler. A simple example of this can be seen in
716 ``slow_dentry_cmp()`` which, for filesystems which do not use simple
717 byte-wise name equality, calls into the filesystem to compare a name
718 against a dentry. The length and name pointer are copied into local
719 variables, then ``read_seqcount_retry()`` is called to confirm the two
720 are consistent, and only then is ``->d_compare()`` called. When
721 standard filename comparison is used, ``dentry_cmp()`` is called
722 instead. Notably it does *not* use ``read_seqcount_retry()``, but
723 instead has a large comment explaining why the consistency guarantee
724 isn't necessary. A subsequent ``read_seqcount_retry()`` will be
725 sufficient to catch any problem that could occur at this point.
727 With that little refresher on seqlocks out of the way we can look at
728 the bigger picture of how RCU-walk uses seqlocks.
730 ``mount_lock`` and ``nd->m_seq``
731 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
733 We already met the ``mount_lock`` seqlock when REF-walk used it to
734 ensure that crossing a mount point is performed safely. RCU-walk uses
735 it for that too, but for quite a bit more.
737 Instead of taking a counted reference to each ``vfsmount`` as it
738 descends the tree, RCU-walk samples the state of ``mount_lock`` at the
739 start of the walk and stores this initial sequence number in the
740 ``struct nameidata`` in the ``m_seq`` field. This one lock and one
741 sequence number are used to validate all accesses to all ``vfsmounts``,
742 and all mount point crossings. As changes to the mount table are
743 relatively rare, it is reasonable to fall back on REF-walk any time
744 that any "mount" or "unmount" happens.
746 ``m_seq`` is checked (using ``read_seqretry()``) at the end of an RCU-walk
747 sequence, whether switching to REF-walk for the rest of the path or
748 when the end of the path is reached. It is also checked when stepping
749 down over a mount point (in ``__follow_mount_rcu()``) or up (in
750 ``follow_dotdot_rcu()``). If it is ever found to have changed, the
751 whole RCU-walk sequence is aborted and the path is processed again by
754 If RCU-walk finds that ``mount_lock`` hasn't changed then it can be sure
755 that, had REF-walk taken counted references on each vfsmount, the
756 results would have been the same. This ensures the invariant holds,
757 at least for vfsmount structures.
759 ``dentry->d_seq`` and ``nd->seq``
760 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
762 In place of taking a count or lock on ``d_reflock``, RCU-walk samples
763 the per-dentry ``d_seq`` seqlock, and stores the sequence number in the
764 ``seq`` field of the nameidata structure, so ``nd->seq`` should always be
765 the current sequence number of ``nd->dentry``. This number needs to be
766 revalidated after copying, and before using, the name, parent, or
769 The handling of the name we have already looked at, and the parent is
770 only accessed in ``follow_dotdot_rcu()`` which fairly trivially follows
771 the required pattern, though it does so for three different cases.
773 When not at a mount point, ``d_parent`` is followed and its ``d_seq`` is
774 collected. When we are at a mount point, we instead follow the
775 ``mnt->mnt_mountpoint`` link to get a new dentry and collect its
776 ``d_seq``. Then, after finally finding a ``d_parent`` to follow, we must
777 check if we have landed on a mount point and, if so, must find that
778 mount point and follow the ``mnt->mnt_root`` link. This would imply a
779 somewhat unusual, but certainly possible, circumstance where the
780 starting point of the path lookup was in part of the filesystem that
781 was mounted on, and so not visible from the root.
783 The inode pointer, stored in ``->d_inode``, is a little more
784 interesting. The inode will always need to be accessed at least
785 twice, once to determine if it is NULL and once to verify access
786 permissions. Symlink handling requires a validated inode pointer too.
787 Rather than revalidating on each access, a copy is made on the first
788 access and it is stored in the ``inode`` field of ``nameidata`` from where
789 it can be safely accessed without further validation.
791 ``lookup_fast()`` is the only lookup routine that is used in RCU-mode,
792 ``lookup_slow()`` being too slow and requiring locks. It is in
793 ``lookup_fast()`` that we find the important "hand over hand" tracking
794 of the current dentry.
796 The current ``dentry`` and current ``seq`` number are passed to
797 ``__d_lookup_rcu()`` which, on success, returns a new ``dentry`` and a
798 new ``seq`` number. ``lookup_fast()`` then copies the inode pointer and
799 revalidates the new ``seq`` number. It then validates the old ``dentry``
800 with the old ``seq`` number one last time and only then continues. This
801 process of getting the ``seq`` number of the new dentry and then
802 checking the ``seq`` number of the old exactly mirrors the process of
803 getting a counted reference to the new dentry before dropping that for
804 the old dentry which we saw in REF-walk.
806 No ``inode->i_rwsem`` or even ``rename_lock``
807 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
809 A semaphore is a fairly heavyweight lock that can only be taken when it is
810 permissible to sleep. As ``rcu_read_lock()`` forbids sleeping,
811 ``inode->i_rwsem`` plays no role in RCU-walk. If some other thread does
812 take ``i_rwsem`` and modifies the directory in a way that RCU-walk needs
813 to notice, the result will be either that RCU-walk fails to find the
814 dentry that it is looking for, or it will find a dentry which
815 ``read_seqretry()`` won't validate. In either case it will drop down to
816 REF-walk mode which can take whatever locks are needed.
818 Though ``rename_lock`` could be used by RCU-walk as it doesn't require
819 any sleeping, RCU-walk doesn't bother. REF-walk uses ``rename_lock`` to
820 protect against the possibility of hash chains in the dcache changing
821 while they are being searched. This can result in failing to find
822 something that actually is there. When RCU-walk fails to find
823 something in the dentry cache, whether it is really there or not, it
824 already drops down to REF-walk and tries again with appropriate
825 locking. This neatly handles all cases, so adding extra checks on
826 rename_lock would bring no significant value.
828 ``unlazy walk()`` and ``complete_walk()``
829 -----------------------------------------
831 That "dropping down to REF-walk" typically involves a call to
832 ``unlazy_walk()``, so named because "RCU-walk" is also sometimes
833 referred to as "lazy walk". ``unlazy_walk()`` is called when
834 following the path down to the current vfsmount/dentry pair seems to
835 have proceeded successfully, but the next step is problematic. This
836 can happen if the next name cannot be found in the dcache, if
837 permission checking or name revalidation couldn't be achieved while
838 the ``rcu_read_lock()`` is held (which forbids sleeping), if an
839 automount point is found, or in a couple of cases involving symlinks.
840 It is also called from ``complete_walk()`` when the lookup has reached
841 the final component, or the very end of the path, depending on which
842 particular flavor of lookup is used.
844 Other reasons for dropping out of RCU-walk that do not trigger a call
845 to ``unlazy_walk()`` are when some inconsistency is found that cannot be
846 handled immediately, such as ``mount_lock`` or one of the ``d_seq``
847 seqlocks reporting a change. In these cases the relevant function
848 will return ``-ECHILD`` which will percolate up until it triggers a new
849 attempt from the top using REF-walk.
851 For those cases where ``unlazy_walk()`` is an option, it essentially
852 takes a reference on each of the pointers that it holds (vfsmount,
853 dentry, and possibly some symbolic links) and then verifies that the
854 relevant seqlocks have not been changed. If there have been changes,
855 it, too, aborts with ``-ECHILD``, otherwise the transition to REF-walk
856 has been a success and the lookup process continues.
858 Taking a reference on those pointers is not quite as simple as just
859 incrementing a counter. That works to take a second reference if you
860 already have one (often indirectly through another object), but it
861 isn't sufficient if you don't actually have a counted reference at
862 all. For ``dentry->d_lockref``, it is safe to increment the reference
863 counter to get a reference unless it has been explicitly marked as
864 "dead" which involves setting the counter to ``-128``.
865 ``lockref_get_not_dead()`` achieves this.
867 For ``mnt->mnt_count`` it is safe to take a reference as long as
868 ``mount_lock`` is then used to validate the reference. If that
869 validation fails, it may *not* be safe to just drop that reference in
870 the standard way of calling ``mnt_put()`` - an unmount may have
871 progressed too far. So the code in ``legitimize_mnt()``, when it
872 finds that the reference it got might not be safe, checks the
873 ``MNT_SYNC_UMOUNT`` flag to determine if a simple ``mnt_put()`` is
874 correct, or if it should just decrement the count and pretend none of
877 Taking care in filesystems
878 --------------------------
880 RCU-walk depends almost entirely on cached information and often will
881 not call into the filesystem at all. However there are two places,
882 besides the already-mentioned component-name comparison, where the
883 file system might be included in RCU-walk, and it must know to be
886 If the filesystem has non-standard permission-checking requirements -
887 such as a networked filesystem which may need to check with the server
888 - the ``i_op->permission`` interface might be called during RCU-walk.
889 In this case an extra "``MAY_NOT_BLOCK``" flag is passed so that it
890 knows not to sleep, but to return ``-ECHILD`` if it cannot complete
891 promptly. ``i_op->permission`` is given the inode pointer, not the
892 dentry, so it doesn't need to worry about further consistency checks.
893 However if it accesses any other filesystem data structures, it must
894 ensure they are safe to be accessed with only the ``rcu_read_lock()``
895 held. This typically means they must be freed using ``kfree_rcu()`` or
898 .. _READ_ONCE: https://lwn.net/Articles/624126/
900 If the filesystem may need to revalidate dcache entries, then
901 ``d_op->d_revalidate`` may be called in RCU-walk too. This interface
902 *is* passed the dentry but does not have access to the ``inode`` or the
903 ``seq`` number from the ``nameidata``, so it needs to be extra careful
904 when accessing fields in the dentry. This "extra care" typically
905 involves using `READ_ONCE() <READ_ONCE_>`_ to access fields, and verifying the
906 result is not NULL before using it. This pattern can be seen in
907 ``nfs_lookup_revalidate()``.
912 In various places in the details of REF-walk and RCU-walk, and also in
913 the big picture, there are a couple of related patterns that are worth
916 The first is "try quickly and check, if that fails try slowly". We
917 can see that in the high-level approach of first trying RCU-walk and
918 then trying REF-walk, and in places where ``unlazy_walk()`` is used to
919 switch to REF-walk for the rest of the path. We also saw it earlier
920 in ``dget_parent()`` when following a "``..``" link. It tries a quick way
921 to get a reference, then falls back to taking locks if needed.
923 The second pattern is "try quickly and check, if that fails try
924 again - repeatedly". This is seen with the use of ``rename_lock`` and
925 ``mount_lock`` in REF-walk. RCU-walk doesn't make use of this pattern -
926 if anything goes wrong it is much safer to just abort and try a more
929 The emphasis here is "try quickly and check". It should probably be
930 "try quickly *and carefully*, then check". The fact that checking is
931 needed is a reminder that the system is dynamic and only a limited
932 number of things are safe at all. The most likely cause of errors in
933 this whole process is assuming something is safe when in reality it
934 isn't. Careful consideration of what exactly guarantees the safety of
935 each access is sometimes necessary.
937 A walk among the symlinks
938 =========================
940 There are several basic issues that we will examine to understand the
941 handling of symbolic links: the symlink stack, together with cache
942 lifetimes, will help us understand the overall recursive handling of
943 symlinks and lead to the special care needed for the final component.
944 Then a consideration of access-time updates and summary of the various
945 flags controlling lookup will finish the story.
950 There are only two sorts of filesystem objects that can usefully
951 appear in a path prior to the final component: directories and symlinks.
952 Handling directories is quite straightforward: the new directory
953 simply becomes the starting point at which to interpret the next
954 component on the path. Handling symbolic links requires a bit more
957 Conceptually, symbolic links could be handled by editing the path. If
958 a component name refers to a symbolic link, then that component is
959 replaced by the body of the link and, if that body starts with a '/',
960 then all preceding parts of the path are discarded. This is what the
961 "``readlink -f``" command does, though it also edits out "``.``" and
964 Directly editing the path string is not really necessary when looking
965 up a path, and discarding early components is pointless as they aren't
966 looked at anyway. Keeping track of all remaining components is
967 important, but they can of course be kept separately; there is no need
968 to concatenate them. As one symlink may easily refer to another,
969 which in turn can refer to a third, we may need to keep the remaining
970 components of several paths, each to be processed when the preceding
971 ones are completed. These path remnants are kept on a stack of
974 There are two reasons for placing limits on how many symlinks can
975 occur in a single path lookup. The most obvious is to avoid loops.
976 If a symlink referred to itself either directly or through
977 intermediaries, then following the symlink can never complete
978 successfully - the error ``ELOOP`` must be returned. Loops can be
979 detected without imposing limits, but limits are the simplest solution
980 and, given the second reason for restriction, quite sufficient.
982 .. _outlined recently: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
984 The second reason was `outlined recently`_ by Linus:
986 Because it's a latency and DoS issue too. We need to react well to
987 true loops, but also to "very deep" non-loops. It's not about memory
988 use, it's about users triggering unreasonable CPU resources.
990 Linux imposes a limit on the length of any pathname: ``PATH_MAX``, which
991 is 4096. There are a number of reasons for this limit; not letting the
992 kernel spend too much time on just one path is one of them. With
993 symbolic links you can effectively generate much longer paths so some
994 sort of limit is needed for the same reason. Linux imposes a limit of
995 at most 40 (MAXSYMLINKS) symlinks in any one path lookup. It previously imposed
996 a further limit of eight on the maximum depth of recursion, but that was
997 raised to 40 when a separate stack was implemented, so there is now
1000 The ``nameidata`` structure that we met in an earlier article contains a
1001 small stack that can be used to store the remaining part of up to two
1002 symlinks. In many cases this will be sufficient. If it isn't, a
1003 separate stack is allocated with room for 40 symlinks. Pathname
1004 lookup will never exceed that stack as, once the 40th symlink is
1005 detected, an error is returned.
1007 It might seem that the name remnants are all that needs to be stored on
1008 this stack, but we need a bit more. To see that, we need to move on to
1011 Storage and lifetime of cached symlinks
1012 ---------------------------------------
1014 Like other filesystem resources, such as inodes and directory
1015 entries, symlinks are cached by Linux to avoid repeated costly access
1016 to external storage. It is particularly important for RCU-walk to be
1017 able to find and temporarily hold onto these cached entries, so that
1018 it doesn't need to drop down into REF-walk.
1020 .. _object-oriented design pattern: https://lwn.net/Articles/446317/
1022 While each filesystem is free to make its own choice, symlinks are
1023 typically stored in one of two places. Short symlinks are often
1024 stored directly in the inode. When a filesystem allocates a ``struct
1025 inode`` it typically allocates extra space to store private data (a
1026 common `object-oriented design pattern`_ in the kernel). This will
1027 sometimes include space for a symlink. The other common location is
1028 in the page cache, which normally stores the content of files. The
1029 pathname in a symlink can be seen as the content of that symlink and
1030 can easily be stored in the page cache just like file content.
1032 When neither of these is suitable, the next most likely scenario is
1033 that the filesystem will allocate some temporary memory and copy or
1034 construct the symlink content into that memory whenever it is needed.
1036 When the symlink is stored in the inode, it has the same lifetime as
1037 the inode which, itself, is protected by RCU or by a counted reference
1038 on the dentry. This means that the mechanisms that pathname lookup
1039 uses to access the dcache and icache (inode cache) safely are quite
1040 sufficient for accessing some cached symlinks safely. In these cases,
1041 the ``i_link`` pointer in the inode is set to point to wherever the
1042 symlink is stored and it can be accessed directly whenever needed.
1044 When the symlink is stored in the page cache or elsewhere, the
1045 situation is not so straightforward. A reference on a dentry or even
1046 on an inode does not imply any reference on cached pages of that
1047 inode, and even an ``rcu_read_lock()`` is not sufficient to ensure that
1048 a page will not disappear. So for these symlinks the pathname lookup
1049 code needs to ask the filesystem to provide a stable reference and,
1050 significantly, needs to release that reference when it is finished
1053 Taking a reference to a cache page is often possible even in RCU-walk
1054 mode. It does require making changes to memory, which is best avoided,
1055 but that isn't necessarily a big cost and it is better than dropping
1056 out of RCU-walk mode completely. Even filesystems that allocate
1057 space to copy the symlink into can use ``GFP_ATOMIC`` to often successfully
1058 allocate memory without the need to drop out of RCU-walk. If a
1059 filesystem cannot successfully get a reference in RCU-walk mode, it
1060 must return ``-ECHILD`` and ``unlazy_walk()`` will be called to return to
1061 REF-walk mode in which the filesystem is allowed to sleep.
1063 The place for all this to happen is the ``i_op->get_link()`` inode
1064 method. This is called both in RCU-walk and REF-walk. In RCU-walk the
1065 ``dentry*`` argument is NULL, ``->get_link()`` can return -ECHILD to drop out of
1066 RCU-walk. Much like the ``i_op->permission()`` method we
1067 looked at previously, ``->get_link()`` would need to be careful that
1068 all the data structures it references are safe to be accessed while
1069 holding no counted reference, only the RCU lock. A callback
1070 ``struct delayed_called`` will be passed to ``->get_link()``:
1071 file systems can set their own put_link function and argument through
1072 set_delayed_call(). Later on, when VFS wants to put link, it will call
1073 do_delayed_call() to invoke that callback function with the argument.
1075 In order for the reference to each symlink to be dropped when the walk completes,
1076 whether in RCU-walk or REF-walk, the symlink stack needs to contain,
1077 along with the path remnants:
1079 - the ``struct path`` to provide a reference to the previous path
1080 - the ``const char *`` to provide a reference to the to previous name
1081 - the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
1082 - the ``struct delayed_call`` for later invocation.
1084 This means that each entry in the symlink stack needs to hold five
1085 pointers and an integer instead of just one pointer (the path
1086 remnant). On a 64-bit system, this is about 40 bytes per entry;
1087 with 40 entries it adds up to 1600 bytes total, which is less than
1088 half a page. So it might seem like a lot, but is by no means
1091 Note that, in a given stack frame, the path remnant (``name``) is not
1092 part of the symlink that the other fields refer to. It is the remnant
1093 to be followed once that symlink has been fully parsed.
1095 Following the symlink
1096 ---------------------
1098 The main loop in ``link_path_walk()`` iterates seamlessly over all
1099 components in the path and all of the non-final symlinks. As symlinks
1100 are processed, the ``name`` pointer is adjusted to point to a new
1101 symlink, or is restored from the stack, so that much of the loop
1102 doesn't need to notice. Getting this ``name`` variable on and off the
1103 stack is very straightforward; pushing and popping the references is
1104 a little more complex.
1106 When a symlink is found, walk_component() calls pick_link() via step_into()
1107 which returns the link from the filesystem.
1108 Providing that operation is successful, the old path ``name`` is placed on the
1109 stack, and the new value is used as the ``name`` for a while. When the end of
1110 the path is found (i.e. ``*name`` is ``'\0'``) the old ``name`` is restored
1111 off the stack and path walking continues.
1113 Pushing and popping the reference pointers (inode, cookie, etc.) is more
1114 complex in part because of the desire to handle tail recursion. When
1115 the last component of a symlink itself points to a symlink, we
1116 want to pop the symlink-just-completed off the stack before pushing
1117 the symlink-just-found to avoid leaving empty path remnants that would
1118 just get in the way.
1120 It is most convenient to push the new symlink references onto the
1121 stack in ``walk_component()`` immediately when the symlink is found;
1122 ``walk_component()`` is also the last piece of code that needs to look at the
1123 old symlink as it walks that last component. So it is quite
1124 convenient for ``walk_component()`` to release the old symlink and pop
1125 the references just before pushing the reference information for the
1126 new symlink. It is guided in this by three flags: ``WALK_NOFOLLOW`` which
1127 forbids it from following a symlink if it finds one, ``WALK_MORE``
1128 which indicates that it is yet too early to release the
1129 current symlink, and ``WALK_TRAILING`` which indicates that it is on the final
1130 component of the lookup, so we will check userspace flag ``LOOKUP_FOLLOW`` to
1131 decide whether follow it when it is a symlink and call ``may_follow_link()`` to
1132 check if we have privilege to follow it.
1134 Symlinks with no final component
1135 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1137 A pair of special-case symlinks deserve a little further explanation.
1138 Both result in a new ``struct path`` (with mount and dentry) being set
1139 up in the ``nameidata``, and result in pick_link() returning ``NULL``.
1141 The more obvious case is a symlink to "``/``". All symlinks starting
1142 with "``/``" are detected in pick_link() which resets the ``nameidata``
1143 to point to the effective filesystem root. If the symlink only
1144 contains "``/``" then there is nothing more to do, no components at all,
1145 so ``NULL`` is returned to indicate that the symlink can be released and
1146 the stack frame discarded.
1148 The other case involves things in ``/proc`` that look like symlinks but
1149 aren't really (and are therefore commonly referred to as "magic-links")::
1151 $ ls -l /proc/self/fd/1
1152 lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
1154 Every open file descriptor in any process is represented in ``/proc`` by
1155 something that looks like a symlink. It is really a reference to the
1156 target file, not just the name of it. When you ``readlink`` these
1157 objects you get a name that might refer to the same file - unless it
1158 has been unlinked or mounted over. When ``walk_component()`` follows
1159 one of these, the ``->get_link()`` method in "procfs" doesn't return
1160 a string name, but instead calls nd_jump_link() which updates the
1161 ``nameidata`` in place to point to that target. ``->get_link()`` then
1162 returns ``NULL``. Again there is no final component and pick_link()
1165 Following the symlink in the final component
1166 --------------------------------------------
1168 All this leads to ``link_path_walk()`` walking down every component, and
1169 following all symbolic links it finds, until it reaches the final
1170 component. This is just returned in the ``last`` field of ``nameidata``.
1171 For some callers, this is all they need; they want to create that
1172 ``last`` name if it doesn't exist or give an error if it does. Other
1173 callers will want to follow a symlink if one is found, and possibly
1174 apply special handling to the last component of that symlink, rather
1175 than just the last component of the original file name. These callers
1176 potentially need to call ``link_path_walk()`` again and again on
1177 successive symlinks until one is found that doesn't point to another
1180 This case is handled by relevant callers of link_path_walk(), such as
1181 path_lookupat(), path_openat() using a loop that calls link_path_walk(),
1182 and then handles the final component by calling open_last_lookups() or
1183 lookup_last(). If it is a symlink that needs to be followed,
1184 open_last_lookups() or lookup_last() will set things up properly and
1185 return the path so that the loop repeats, calling
1186 link_path_walk() again. This could loop as many as 40 times if the last
1187 component of each symlink is another symlink.
1189 Of the various functions that examine the final component,
1190 open_last_lookups() is the most interesting as it works in tandem
1191 with do_open() for opening a file. Part of open_last_lookups() runs
1192 with ``i_rwsem`` held and this part is in a separate function: lookup_open().
1194 Explaining open_last_lookups() and do_open() completely is beyond the scope
1195 of this article, but a few highlights should help those interested in exploring
1198 1. Rather than just finding the target file, do_open() is used after
1199 open_last_lookup() to open
1200 it. If the file was found in the dcache, then ``vfs_open()`` is used for
1201 this. If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
1202 the filesystem provides it) to combine the final lookup with the open, or
1203 will perform the separate ``i_op->lookup()`` and ``i_op->create()`` steps
1204 directly. In the later case the actual "open" of this newly found or
1205 created file will be performed by vfs_open(), just as if the name
1206 were found in the dcache.
1208 2. vfs_open() can fail with ``-EOPENSTALE`` if the cached information
1209 wasn't quite current enough. If it's in RCU-walk ``-ECHILD`` will be returned
1210 otherwise ``-ESTALE`` is returned. When ``-ESTALE`` is returned, the caller may
1211 retry with ``LOOKUP_REVAL`` flag set.
1213 3. An open with O_CREAT **does** follow a symlink in the final component,
1214 unlike other creation system calls (like ``mkdir``). So the sequence::
1217 echo hello > /tmp/foo
1219 will create a file called ``/tmp/bar``. This is not permitted if
1220 ``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
1221 like for a non-creating open: lookup_last() or open_last_lookup()
1222 returns a non ``NULL`` value, and link_path_walk() gets called and the
1223 open process continues on the symlink that was found.
1225 Updating the access time
1226 ------------------------
1228 We previously said of RCU-walk that it would "take no locks, increment
1229 no counts, leave no footprints." We have since seen that some
1230 "footprints" can be needed when handling symlinks as a counted
1231 reference (or even a memory allocation) may be needed. But these
1232 footprints are best kept to a minimum.
1234 One other place where walking down a symlink can involve leaving
1235 footprints in a way that doesn't affect directories is in updating access times.
1236 In Unix (and Linux) every filesystem object has a "last accessed
1237 time", or "``atime``". Passing through a directory to access a file
1238 within is not considered to be an access for the purposes of
1239 ``atime``; only listing the contents of a directory can update its ``atime``.
1240 Symlinks are different it seems. Both reading a symlink (with ``readlink()``)
1241 and looking up a symlink on the way to some other destination can
1242 update the atime on that symlink.
1244 .. _clearest statement: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
1246 It is not clear why this is the case; POSIX has little to say on the
1247 subject. The `clearest statement`_ is that, if a particular implementation
1248 updates a timestamp in a place not specified by POSIX, this must be
1249 documented "except that any changes caused by pathname resolution need
1250 not be documented". This seems to imply that POSIX doesn't really
1251 care about access-time updates during pathname lookup.
1253 .. _Linux 1.3.87: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
1255 An examination of history shows that prior to `Linux 1.3.87`_, the ext2
1256 filesystem, at least, didn't update atime when following a link.
1257 Unfortunately we have no record of why that behavior was changed.
1259 In any case, access time must now be updated and that operation can be
1260 quite complex. Trying to stay in RCU-walk while doing it is best
1261 avoided. Fortunately it is often permitted to skip the ``atime``
1262 update. Because ``atime`` updates cause performance problems in various
1263 areas, Linux supports the ``relatime`` mount option, which generally
1264 limits the updates of ``atime`` to once per day on files that aren't
1265 being changed (and symlinks never change once created). Even without
1266 ``relatime``, many filesystems record ``atime`` with a one-second
1267 granularity, so only one update per second is required.
1269 It is easy to test if an ``atime`` update is needed while in RCU-walk
1270 mode and, if it isn't, the update can be skipped and RCU-walk mode
1271 continues. Only when an ``atime`` update is actually required does the
1272 path walk drop down to REF-walk. All of this is handled in the
1273 ``get_link()`` function.
1278 A suitable way to wrap up this tour of pathname walking is to list
1279 the various flags that can be stored in the ``nameidata`` to guide the
1280 lookup process. Many of these are only meaningful on the final
1281 component, others reflect the current state of the pathname lookup, and some
1282 apply restrictions to all path components encountered in the path lookup.
1284 And then there is ``LOOKUP_EMPTY``, which doesn't fit conceptually with
1285 the others. If this is not set, an empty pathname causes an error
1286 very early on. If it is set, empty pathnames are not considered to be
1292 We have already met two global state flags: ``LOOKUP_RCU`` and
1293 ``LOOKUP_REVAL``. These select between one of three overall approaches
1294 to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation.
1296 ``LOOKUP_PARENT`` indicates that the final component hasn't been reached
1297 yet. This is primarily used to tell the audit subsystem the full
1298 context of a particular access being audited.
1300 ``ND_ROOT_PRESET`` indicates that the ``root`` field in the ``nameidata`` was
1301 provided by the caller, so it shouldn't be released when it is no
1304 ``ND_JUMPED`` means that the current dentry was chosen not because
1305 it had the right name but for some other reason. This happens when
1306 following "``..``", following a symlink to ``/``, crossing a mount point
1307 or accessing a "``/proc/$PID/fd/$FD``" symlink (also known as a "magic
1308 link"). In this case the filesystem has not been asked to revalidate the
1309 name (with ``d_revalidate()``). In such cases the inode may still need
1310 to be revalidated, so ``d_op->d_weak_revalidate()`` is called if
1311 ``ND_JUMPED`` is set when the look completes - which may be at the
1312 final component or, when creating, unlinking, or renaming, at the penultimate component.
1314 Resolution-restriction flags
1315 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1317 In order to allow userspace to protect itself against certain race conditions
1318 and attack scenarios involving changing path components, a series of flags are
1319 available which apply restrictions to all path components encountered during
1320 path lookup. These flags are exposed through ``openat2()``'s ``resolve`` field.
1322 ``LOOKUP_NO_SYMLINKS`` blocks all symlink traversals (including magic-links).
1323 This is distinctly different from ``LOOKUP_FOLLOW``, because the latter only
1324 relates to restricting the following of trailing symlinks.
1326 ``LOOKUP_NO_MAGICLINKS`` blocks all magic-link traversals. Filesystems must
1327 ensure that they return errors from ``nd_jump_link()``, because that is how
1328 ``LOOKUP_NO_MAGICLINKS`` and other magic-link restrictions are implemented.
1330 ``LOOKUP_NO_XDEV`` blocks all ``vfsmount`` traversals (this includes both
1331 bind-mounts and ordinary mounts). Note that the ``vfsmount`` which contains the
1332 lookup is determined by the first mountpoint the path lookup reaches --
1333 absolute paths start with the ``vfsmount`` of ``/``, and relative paths start
1334 with the ``dfd``'s ``vfsmount``. Magic-links are only permitted if the
1335 ``vfsmount`` of the path is unchanged.
1337 ``LOOKUP_BENEATH`` blocks any path components which resolve outside the
1338 starting point of the resolution. This is done by blocking ``nd_jump_root()``
1339 as well as blocking ".." if it would jump outside the starting point.
1340 ``rename_lock`` and ``mount_lock`` are used to detect attacks against the
1341 resolution of "..". Magic-links are also blocked.
1343 ``LOOKUP_IN_ROOT`` resolves all path components as though the starting point
1344 were the filesystem root. ``nd_jump_root()`` brings the resolution back to
1345 the starting point, and ".." at the starting point will act as a no-op. As with
1346 ``LOOKUP_BENEATH``, ``rename_lock`` and ``mount_lock`` are used to detect
1347 attacks against ".." resolution. Magic-links are also blocked.
1349 Final-component flags
1350 ~~~~~~~~~~~~~~~~~~~~~
1352 Some of these flags are only set when the final component is being
1353 considered. Others are only checked for when considering that final
1356 ``LOOKUP_AUTOMOUNT`` ensures that, if the final component is an automount
1357 point, then the mount is triggered. Some operations would trigger it
1358 anyway, but operations like ``stat()`` deliberately don't. ``statfs()``
1359 needs to trigger the mount but otherwise behaves a lot like ``stat()``, so
1360 it sets ``LOOKUP_AUTOMOUNT``, as does "``quotactl()``" and the handling of
1363 ``LOOKUP_FOLLOW`` has a similar function to ``LOOKUP_AUTOMOUNT`` but for
1364 symlinks. Some system calls set or clear it implicitly, while
1365 others have API flags such as ``AT_SYMLINK_FOLLOW`` and
1366 ``UMOUNT_NOFOLLOW`` to control it. Its effect is similar to
1367 ``WALK_GET`` that we already met, but it is used in a different way.
1369 ``LOOKUP_DIRECTORY`` insists that the final component is a directory.
1370 Various callers set this and it is also set when the final component
1371 is found to be followed by a slash.
1373 Finally ``LOOKUP_OPEN``, ``LOOKUP_CREATE``, ``LOOKUP_EXCL``, and
1374 ``LOOKUP_RENAME_TARGET`` are not used directly by the VFS but are made
1375 available to the filesystem and particularly the ``->d_revalidate()``
1376 method. A filesystem can choose not to bother revalidating too hard
1377 if it knows that it will be asked to open or create the file soon.
1378 These flags were previously useful for ``->lookup()`` too but with the
1379 introduction of ``->atomic_open()`` they are less relevant there.
1384 Despite its complexity, all this pathname lookup code appears to be
1385 in good shape - various parts are certainly easier to understand now
1386 than even a couple of releases ago. But that doesn't mean it is
1387 "finished". As already mentioned, RCU-walk currently only follows
1388 symlinks that are stored in the inode so, while it handles many ext4
1389 symlinks, it doesn't help with NFS, XFS, or Btrfs. That support
1390 is not likely to be long delayed.
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