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1===============
2Pathname lookup
3===============
4
5This write-up is based on three articles published at lwn.net:
6
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
10
11Written by Neil Brown with help from Al Viro and Jon Corbet.
12It has subsequently been updated to reflect changes in the kernel
13including:
14
15- per-directory parallel name lookup.
16
17Introduction to pathname lookup
18===============================
19
20The most obvious aspect of pathname lookup, which very little
21exploration is needed to discover, is that it is complex. There are
22many rules, special cases, and implementation alternatives that all
23combine to confuse the unwary reader. Computer science has long been
24acquainted with such complexity and has tools to help manage it. One
25tool that we will make extensive use of is "divide and conquer". For
26the early parts of the analysis we will divide off symlinks - leaving
27them until the final part. Well before we get to symlinks we have
28another major division based on the VFS's approach to locking which
29will allow us to review "REF-walk" and "RCU-walk" separately. But we
30are getting ahead of ourselves. There are some important low level
31distinctions we need to clarify first.
32
33There are two sorts of ...
34--------------------------
35
36.. _openat: http://man7.org/linux/man-pages/man2/openat.2.html
37
38Pathnames (sometimes "file names"), used to identify objects in the
39filesystem, will be familiar to most readers. They contain two sorts
40of elements: "slashes" that are sequences of one or more "``/``"
41characters, and "components" that are sequences of one or more
42non-"``/``" characters. These form two kinds of paths. Those that
43start with slashes are "absolute" and start from the filesystem root.
44The others are "relative" and start from the current directory, or
45from some other location specified by a file descriptor given to a
46"``XXXat``" system call such as `openat() <openat_>`_.
47
48.. _execveat: http://man7.org/linux/man-pages/man2/execveat.2.html
49
50It is tempting to describe the second kind as starting with a
51component, but that isn't always accurate: a pathname can lack both
52slashes and components, it can be empty, in other words. This is
53generally forbidden in POSIX, but some of those "xxx``at``" system calls
54in Linux permit it when the ``AT_EMPTY_PATH`` flag is given. For
55example, if you have an open file descriptor on an executable file you
56can execute it by calling `execveat() <execveat_>`_ passing
57the file descriptor, an empty path, and the ``AT_EMPTY_PATH`` flag.
58
59These paths can be divided into two sections: the final component and
60everything else. The "everything else" is the easy bit. In all cases
61it must identify a directory that already exists, otherwise an error
62such as ``ENOENT`` or ``ENOTDIR`` will be reported.
63
64The final component is not so simple. Not only do different system
65calls interpret it quite differently (e.g. some create it, some do
66not), but it might not even exist: neither the empty pathname nor the
67pathname that is just slashes have a final component. If it does
68exist, it could be "``.``" or "``..``" which are handled quite differently
69from other components.
70
71.. _POSIX: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12
72
73If a pathname ends with a slash, such as "``/tmp/foo/``" it might be
74tempting to consider that to have an empty final component. In many
75ways that would lead to correct results, but not always. In
76particular, ``mkdir()`` and ``rmdir()`` each create or remove a directory named
77by the final component, and they are required to work with pathnames
78ending in "``/``". According to POSIX_
79
80 A pathname that contains at least one non- <slash> character and
81 that ends with one or more trailing <slash> characters shall not
82 be resolved successfully unless the last pathname component before
83 the trailing <slash> characters names an existing directory or a
84 directory entry that is to be created for a directory immediately
85 after the pathname is resolved.
86
87The Linux pathname walking code (mostly in ``fs/namei.c``) deals with
88all of these issues: breaking the path into components, handling the
89"everything else" quite separately from the final component, and
90checking that the trailing slash is not used where it isn't
91permitted. It also addresses the important issue of concurrent
92access.
93
94While one process is looking up a pathname, another might be making
95changes that affect that lookup. One fairly extreme case is that if
96"a/b" were renamed to "a/c/b" while another process were looking up
97"a/b/..", that process might successfully resolve on "a/c".
98Most races are much more subtle, and a big part of the task of
99pathname lookup is to prevent them from having damaging effects. Many
100of the possible races are seen most clearly in the context of the
101"dcache" and an understanding of that is central to understanding
102pathname lookup.
103
104More than just a cache
105----------------------
106
107The "dcache" caches information about names in each filesystem to
108make them quickly available for lookup. Each entry (known as a
109"dentry") contains three significant fields: a component name, a
110pointer to a parent dentry, and a pointer to the "inode" which
111contains further information about the object in that parent with
112the given name. The inode pointer can be ``NULL`` indicating that the
113name doesn't exist in the parent. While there can be linkage in the
114dentry of a directory to the dentries of the children, that linkage is
115not used for pathname lookup, and so will not be considered here.
116
117The dcache has a number of uses apart from accelerating lookup. One
118that will be particularly relevant is that it is closely integrated
119with the mount table that records which filesystem is mounted where.
120What the mount table actually stores is which dentry is mounted on top
121of which other dentry.
122
123When considering the dcache, we have another of our "two types"
124distinctions: there are two types of filesystems.
125
126Some filesystems ensure that the information in the dcache is always
127completely accurate (though not necessarily complete). This can allow
128the VFS to determine if a particular file does or doesn't exist
129without checking with the filesystem, and means that the VFS can
130protect the filesystem against certain races and other problems.
131These are typically "local" filesystems such as ext3, XFS, and Btrfs.
132
133Other filesystems don't provide that guarantee because they cannot.
134These are typically filesystems that are shared across a network,
135whether remote filesystems like NFS and 9P, or cluster filesystems
136like ocfs2 or cephfs. These filesystems allow the VFS to revalidate
137cached information, and must provide their own protection against
138awkward races. The VFS can detect these filesystems by the
139``DCACHE_OP_REVALIDATE`` flag being set in the dentry.
140
141REF-walk: simple concurrency management with refcounts and spinlocks
142--------------------------------------------------------------------
143
144With all of those divisions carefully classified, we can now start
145looking at the actual process of walking along a path. In particular
146we will start with the handling of the "everything else" part of a
147pathname, and focus on the "REF-walk" approach to concurrency
148management. This code is found in the ``link_path_walk()`` function, if
149you ignore all the places that only run when "``LOOKUP_RCU``"
150(indicating the use of RCU-walk) is set.
151
152.. _Meet the Lockers: https://lwn.net/Articles/453685/
153
154REF-walk is fairly heavy-handed with locks and reference counts. Not
155as heavy-handed as in the old "big kernel lock" days, but certainly not
156afraid of taking a lock when one is needed. It uses a variety of
157different concurrency controls. A background understanding of the
158various primitives is assumed, or can be gleaned from elsewhere such
159as in `Meet the Lockers`_.
160
161The locking mechanisms used by REF-walk include:
162
163dentry->d_lockref
164~~~~~~~~~~~~~~~~~
165
166This uses the lockref primitive to provide both a spinlock and a
167reference count. The special-sauce of this primitive is that the
168conceptual sequence "lock; inc_ref; unlock;" can often be performed
169with a single atomic memory operation.
170
171Holding a reference on a dentry ensures that the dentry won't suddenly
172be freed and used for something else, so the values in various fields
173will behave as expected. It also protects the ``->d_inode`` reference
174to the inode to some extent.
175
176The association between a dentry and its inode is fairly permanent.
177For example, when a file is renamed, the dentry and inode move
178together to the new location. When a file is created the dentry will
179initially be negative (i.e. ``d_inode`` is ``NULL``), and will be assigned
180to the new inode as part of the act of creation.
181
182When a file is deleted, this can be reflected in the cache either by
183setting ``d_inode`` to ``NULL``, or by removing it from the hash table
184(described shortly) used to look up the name in the parent directory.
185If the dentry is still in use the second option is used as it is
186perfectly legal to keep using an open file after it has been deleted
187and having the dentry around helps. If the dentry is not otherwise in
188use (i.e. if the refcount in ``d_lockref`` is one), only then will
189``d_inode`` be set to ``NULL``. Doing it this way is more efficient for a
190very common case.
191
192So as long as a counted reference is held to a dentry, a non-``NULL`` ``->d_inode``
193value will never be changed.
194
195dentry->d_lock
196~~~~~~~~~~~~~~
197
198``d_lock`` is a synonym for the spinlock that is part of ``d_lockref`` above.
199For our purposes, holding this lock protects against the dentry being
200renamed or unlinked. In particular, its parent (``d_parent``), and its
201name (``d_name``) cannot be changed, and it cannot be removed from the
202dentry hash table.
203
204When looking for a name in a directory, REF-walk takes ``d_lock`` on
205each candidate dentry that it finds in the hash table and then checks
206that the parent and name are correct. So it doesn't lock the parent
207while searching in the cache; it only locks children.
208
209When looking for the parent for a given name (to handle "``..``"),
210REF-walk can take ``d_lock`` to get a stable reference to ``d_parent``,
211but it first tries a more lightweight approach. As seen in
212``dget_parent()``, if a reference can be claimed on the parent, and if
213subsequently ``d_parent`` can be seen to have not changed, then there is
214no need to actually take the lock on the child.
215
216rename_lock
217~~~~~~~~~~~
218
219Looking up a given name in a given directory involves computing a hash
220from the two values (the name and the dentry of the directory),
221accessing that slot in a hash table, and searching the linked list
222that is found there.
223
224When a dentry is renamed, the name and the parent dentry can both
225change so the hash will almost certainly change too. This would move the
226dentry to a different chain in the hash table. If a filename search
227happened to be looking at a dentry that was moved in this way,
228it might end up continuing the search down the wrong chain,
229and so miss out on part of the correct chain.
230
231The name-lookup process (``d_lookup()``) does _not_ try to prevent this
232from happening, but only to detect when it happens.
233``rename_lock`` is a seqlock that is updated whenever any dentry is
234renamed. If ``d_lookup`` finds that a rename happened while it
235unsuccessfully scanned a chain in the hash table, it simply tries
236again.
237
238inode->i_rwsem
239~~~~~~~~~~~~~~
240
241``i_rwsem`` is a read/write semaphore that serializes all changes to a particular
242directory. This ensures that, for example, an ``unlink()`` and a ``rename()``
243cannot both happen at the same time. It also keeps the directory
244stable while the filesystem is asked to look up a name that is not
245currently in the dcache or, optionally, when the list of entries in a
246directory is being retrieved with ``readdir()``.
247
248This has a complementary role to that of ``d_lock``: ``i_rwsem`` on a
249directory protects all of the names in that directory, while ``d_lock``
250on a name protects just one name in a directory. Most changes to the
251dcache hold ``i_rwsem`` on the relevant directory inode and briefly take
252``d_lock`` on one or more the dentries while the change happens. One
253exception is when idle dentries are removed from the dcache due to
254memory pressure. This uses ``d_lock``, but ``i_rwsem`` plays no role.
255
256The semaphore affects pathname lookup in two distinct ways. Firstly it
257prevents changes during lookup of a name in a directory. ``walk_component()`` uses
258``lookup_fast()`` first which, in turn, checks to see if the name is in the cache,
259using only ``d_lock`` locking. If the name isn't found, then ``walk_component()``
260falls back to ``lookup_slow()`` which takes a shared lock on ``i_rwsem``, checks again that
261the name isn't in the cache, and then calls in to the filesystem to get a
262definitive answer. A new dentry will be added to the cache regardless of
263the result.
264
265Secondly, when pathname lookup reaches the final component, it will
266sometimes need to take an exclusive lock on ``i_rwsem`` before performing the last lookup so
267that the required exclusion can be achieved. How path lookup chooses
268to take, or not take, ``i_rwsem`` is one of the
269issues addressed in a subsequent section.
270
271If two threads attempt to look up the same name at the same time - a
272name that is not yet in the dcache - the shared lock on ``i_rwsem`` will
273not prevent them both adding new dentries with the same name. As this
274would result in confusion an extra level of interlocking is used,
275based around a secondary hash table (``in_lookup_hashtable``) and a
276per-dentry flag bit (``DCACHE_PAR_LOOKUP``).
277
278To add a new dentry to the cache while only holding a shared lock on
279``i_rwsem``, a thread must call ``d_alloc_parallel()``. This allocates a
280dentry, stores the required name and parent in it, checks if there
281is already a matching dentry in the primary or secondary hash
282tables, and if not, stores the newly allocated dentry in the secondary
283hash table, with ``DCACHE_PAR_LOOKUP`` set.
284
285If a matching dentry was found in the primary hash table then that is
286returned and the caller can know that it lost a race with some other
287thread adding the entry. If no matching dentry is found in either
288cache, the newly allocated dentry is returned and the caller can
289detect this from the presence of ``DCACHE_PAR_LOOKUP``. In this case it
290knows that it has won any race and now is responsible for asking the
291filesystem to perform the lookup and find the matching inode. When
292the lookup is complete, it must call ``d_lookup_done()`` which clears
293the flag and does some other house keeping, including removing the
294dentry from the secondary hash table - it will normally have been
295added to the primary hash table already. Note that a ``struct
296waitqueue_head`` is passed to ``d_alloc_parallel()``, and
297``d_lookup_done()`` must be called while this ``waitqueue_head`` is still
298in scope.
299
300If a matching dentry is found in the secondary hash table,
301``d_alloc_parallel()`` has a little more work to do. It first waits for
302``DCACHE_PAR_LOOKUP`` to be cleared, using a wait_queue that was passed
303to the instance of ``d_alloc_parallel()`` that won the race and that
304will be woken by the call to ``d_lookup_done()``. It then checks to see
305if the dentry has now been added to the primary hash table. If it
306has, the dentry is returned and the caller just sees that it lost any
307race. If it hasn't been added to the primary hash table, the most
308likely explanation is that some other dentry was added instead using
309``d_splice_alias()``. In any case, ``d_alloc_parallel()`` repeats all the
310look ups from the start and will normally return something from the
311primary hash table.
312
313mnt->mnt_count
314~~~~~~~~~~~~~~
315
316``mnt_count`` is a per-CPU reference counter on "``mount``" structures.
317Per-CPU here means that incrementing the count is cheap as it only
318uses CPU-local memory, but checking if the count is zero is expensive as
319it needs to check with every CPU. Taking a ``mnt_count`` reference
320prevents the mount structure from disappearing as the result of regular
321unmount operations, but does not prevent a "lazy" unmount. So holding
322``mnt_count`` doesn't ensure that the mount remains in the namespace and,
323in particular, doesn't stabilize the link to the mounted-on dentry. It
324does, however, ensure that the ``mount`` data structure remains coherent,
325and it provides a reference to the root dentry of the mounted
326filesystem. So a reference through ``->mnt_count`` provides a stable
327reference to the mounted dentry, but not the mounted-on dentry.
328
329mount_lock
330~~~~~~~~~~
331
332``mount_lock`` is a global seqlock, a bit like ``rename_lock``. It can be used to
333check if any change has been made to any mount points.
334
335While walking down the tree (away from the root) this lock is used when
336crossing a mount point to check that the crossing was safe. That is,
337the value in the seqlock is read, then the code finds the mount that
338is mounted on the current directory, if there is one, and increments
339the ``mnt_count``. Finally the value in ``mount_lock`` is checked against
340the old value. If there is no change, then the crossing was safe. If there
341was a change, the ``mnt_count`` is decremented and the whole process is
342retried.
343
344When walking up the tree (towards the root) by following a ".." link,
345a little more care is needed. In this case the seqlock (which
346contains both a counter and a spinlock) is fully locked to prevent
347any changes to any mount points while stepping up. This locking is
348needed to stabilize the link to the mounted-on dentry, which the
349refcount on the mount itself doesn't ensure.
350
351RCU
352~~~
353
354Finally the global (but extremely lightweight) RCU read lock is held
355from time to time to ensure certain data structures don't get freed
356unexpectedly.
357
358In particular it is held while scanning chains in the dcache hash
359table, and the mount point hash table.
360
361Bringing it together with ``struct nameidata``
362----------------------------------------------
363
364.. _First edition Unix: http://minnie.tuhs.org/cgi-bin/utree.pl?file=V1/u2.s
365
366Throughout the process of walking a path, the current status is stored
367in a ``struct nameidata``, "namei" being the traditional name - dating
368all the way back to `First Edition Unix`_ - of the function that
369converts a "name" to an "inode". ``struct nameidata`` contains (among
370other fields):
371
372``struct path path``
373~~~~~~~~~~~~~~~~~~~~
374
375A ``path`` contains a ``struct vfsmount`` (which is
376embedded in a ``struct mount``) and a ``struct dentry``. Together these
377record the current status of the walk. They start out referring to the
378starting point (the current working directory, the root directory, or some other
379directory identified by a file descriptor), and are updated on each
380step. A reference through ``d_lockref`` and ``mnt_count`` is always
381held.
382
383``struct qstr last``
384~~~~~~~~~~~~~~~~~~~~
385
386This is a string together with a length (i.e. _not_ ``nul`` terminated)
387that is the "next" component in the pathname.
388
389``int last_type``
390~~~~~~~~~~~~~~~~~
391
392This is one of ``LAST_NORM``, ``LAST_ROOT``, ``LAST_DOT``, ``LAST_DOTDOT``, or
393``LAST_BIND``. The ``last`` field is only valid if the type is
394``LAST_NORM``. ``LAST_BIND`` is used when following a symlink and no
395components of the symlink have been processed yet. Others should be
396fairly self-explanatory.
397
398``struct path root``
399~~~~~~~~~~~~~~~~~~~~
400
401This is used to hold a reference to the effective root of the
402filesystem. Often that reference won't be needed, so this field is
403only assigned the first time it is used, or when a non-standard root
404is requested. Keeping a reference in the ``nameidata`` ensures that
405only one root is in effect for the entire path walk, even if it races
406with a ``chroot()`` system call.
407
408The root is needed when either of two conditions holds: (1) either the
409pathname or a symbolic link starts with a "'/'", or (2) a "``..``"
410component is being handled, since "``..``" from the root must always stay
411at the root. The value used is usually the current root directory of
412the calling process. An alternate root can be provided as when
413``sysctl()`` calls ``file_open_root()``, and when NFSv4 or Btrfs call
414``mount_subtree()``. In each case a pathname is being looked up in a very
415specific part of the filesystem, and the lookup must not be allowed to
416escape that subtree. It works a bit like a local ``chroot()``.
417
418Ignoring the handling of symbolic links, we can now describe the
419"``link_path_walk()``" function, which handles the lookup of everything
420except the final component as:
421
422 Given a path (``name``) and a nameidata structure (``nd``), check that the
423 current directory has execute permission and then advance ``name``
424 over one component while updating ``last_type`` and ``last``. If that
425 was the final component, then return, otherwise call
426 ``walk_component()`` and repeat from the top.
427
428``walk_component()`` is even easier. If the component is ``LAST_DOTS``,
429it calls ``handle_dots()`` which does the necessary locking as already
430described. If it finds a ``LAST_NORM`` component it first calls
431"``lookup_fast()``" which only looks in the dcache, but will ask the
432filesystem to revalidate the result if it is that sort of filesystem.
433If that doesn't get a good result, it calls "``lookup_slow()``" which
434takes ``i_rwsem``, rechecks the cache, and then asks the filesystem
435to find a definitive answer. Each of these will call
436``follow_managed()`` (as described below) to handle any mount points.
437
438In the absence of symbolic links, ``walk_component()`` creates a new
439``struct path`` containing a counted reference to the new dentry and a
440reference to the new ``vfsmount`` which is only counted if it is
441different from the previous ``vfsmount``. It then calls
442``path_to_nameidata()`` to install the new ``struct path`` in the
443``struct nameidata`` and drop the unneeded references.
444
445This "hand-over-hand" sequencing of getting a reference to the new
446dentry before dropping the reference to the previous dentry may
447seem obvious, but is worth pointing out so that we will recognize its
448analogue in the "RCU-walk" version.
449
450Handling the final component
451----------------------------
452
453``link_path_walk()`` only walks as far as setting ``nd->last`` and
454``nd->last_type`` to refer to the final component of the path. It does
455not call ``walk_component()`` that last time. Handling that final
456component remains for the caller to sort out. Those callers are
457``path_lookupat()``, ``path_parentat()``, ``path_mountpoint()`` and
458``path_openat()`` each of which handles the differing requirements of
459different system calls.
460
461``path_parentat()`` is clearly the simplest - it just wraps a little bit
462of housekeeping around ``link_path_walk()`` and returns the parent
463directory and final component to the caller. The caller will be either
464aiming to create a name (via ``filename_create()``) or remove or rename
465a name (in which case ``user_path_parent()`` is used). They will use
466``i_rwsem`` to exclude other changes while they validate and then
467perform their operation.
468
469``path_lookupat()`` is nearly as simple - it is used when an existing
470object is wanted such as by ``stat()`` or ``chmod()``. It essentially just
471calls ``walk_component()`` on the final component through a call to
472``lookup_last()``. ``path_lookupat()`` returns just the final dentry.
473
474``path_mountpoint()`` handles the special case of unmounting which must
475not try to revalidate the mounted filesystem. It effectively
476contains, through a call to ``mountpoint_last()``, an alternate
477implementation of ``lookup_slow()`` which skips that step. This is
478important when unmounting a filesystem that is inaccessible, such as
479one provided by a dead NFS server.
480
481Finally ``path_openat()`` is used for the ``open()`` system call; it
482contains, in support functions starting with "``do_last()``", all the
483complexity needed to handle the different subtleties of O_CREAT (with
484or without O_EXCL), final "``/``" characters, and trailing symbolic
485links. We will revisit this in the final part of this series, which
486focuses on those symbolic links. "``do_last()``" will sometimes, but
487not always, take ``i_rwsem``, depending on what it finds.
488
489Each of these, or the functions which call them, need to be alert to
490the possibility that the final component is not ``LAST_NORM``. If the
491goal of the lookup is to create something, then any value for
492``last_type`` other than ``LAST_NORM`` will result in an error. For
493example if ``path_parentat()`` reports ``LAST_DOTDOT``, then the caller
494won't try to create that name. They also check for trailing slashes
495by testing ``last.name[last.len]``. If there is any character beyond
496the final component, it must be a trailing slash.
497
498Revalidation and automounts
499---------------------------
500
501Apart from symbolic links, there are only two parts of the "REF-walk"
502process not yet covered. One is the handling of stale cache entries
503and the other is automounts.
504
505On filesystems that require it, the lookup routines will call the
506``->d_revalidate()`` dentry method to ensure that the cached information
507is current. This will often confirm validity or update a few details
508from a server. In some cases it may find that there has been change
509further up the path and that something that was thought to be valid
510previously isn't really. When this happens the lookup of the whole
511path is aborted and retried with the "``LOOKUP_REVAL``" flag set. This
512forces revalidation to be more thorough. We will see more details of
513this retry process in the next article.
514
515Automount points are locations in the filesystem where an attempt to
516lookup a name can trigger changes to how that lookup should be
517handled, in particular by mounting a filesystem there. These are
518covered in greater detail in autofs.txt in the Linux documentation
519tree, but a few notes specifically related to path lookup are in order
520here.
521
522The Linux VFS has a concept of "managed" dentries which is reflected
523in function names such as "``follow_managed()``". There are three
524potentially interesting things about these dentries corresponding
525to three different flags that might be set in ``dentry->d_flags``:
526
527``DCACHE_MANAGE_TRANSIT``
528~~~~~~~~~~~~~~~~~~~~~~~~~
529
530If this flag has been set, then the filesystem has requested that the
531``d_manage()`` dentry operation be called before handling any possible
532mount point. This can perform two particular services:
533
534It can block to avoid races. If an automount point is being
535unmounted, the ``d_manage()`` function will usually wait for that
536process to complete before letting the new lookup proceed and possibly
537trigger a new automount.
538
539It can selectively allow only some processes to transit through a
540mount point. When a server process is managing automounts, it may
541need to access a directory without triggering normal automount
542processing. That server process can identify itself to the ``autofs``
543filesystem, which will then give it a special pass through
544``d_manage()`` by returning ``-EISDIR``.
545
546``DCACHE_MOUNTED``
547~~~~~~~~~~~~~~~~~~
548
549This flag is set on every dentry that is mounted on. As Linux
550supports multiple filesystem namespaces, it is possible that the
551dentry may not be mounted on in *this* namespace, just in some
552other. So this flag is seen as a hint, not a promise.
553
554If this flag is set, and ``d_manage()`` didn't return ``-EISDIR``,
555``lookup_mnt()`` is called to examine the mount hash table (honoring the
556``mount_lock`` described earlier) and possibly return a new ``vfsmount``
557and a new ``dentry`` (both with counted references).
558
559``DCACHE_NEED_AUTOMOUNT``
560~~~~~~~~~~~~~~~~~~~~~~~~~
561
562If ``d_manage()`` allowed us to get this far, and ``lookup_mnt()`` didn't
563find a mount point, then this flag causes the ``d_automount()`` dentry
564operation to be called.
565
566The ``d_automount()`` operation can be arbitrarily complex and may
567communicate with server processes etc. but it should ultimately either
568report that there was an error, that there was nothing to mount, or
569should provide an updated ``struct path`` with new ``dentry`` and ``vfsmount``.
570
571In the latter case, ``finish_automount()`` will be called to safely
572install the new mount point into the mount table.
573
574There is no new locking of import here and it is important that no
575locks (only counted references) are held over this processing due to
576the very real possibility of extended delays.
577This will become more important next time when we examine RCU-walk
578which is particularly sensitive to delays.
579
580RCU-walk - faster pathname lookup in Linux
581==========================================
582
583RCU-walk is another algorithm for performing pathname lookup in Linux.
584It is in many ways similar to REF-walk and the two share quite a bit
585of code. The significant difference in RCU-walk is how it allows for
586the possibility of concurrent access.
587
588We noted that REF-walk is complex because there are numerous details
589and special cases. RCU-walk reduces this complexity by simply
590refusing to handle a number of cases -- it instead falls back to
591REF-walk. The difficulty with RCU-walk comes from a different
592direction: unfamiliarity. The locking rules when depending on RCU are
593quite different from traditional locking, so we will spend a little extra
594time when we come to those.
595
596Clear demarcation of roles
597--------------------------
598
599The easiest way to manage concurrency is to forcibly stop any other
600thread from changing the data structures that a given thread is
601looking at. In cases where no other thread would even think of
602changing the data and lots of different threads want to read at the
603same time, this can be very costly. Even when using locks that permit
604multiple concurrent readers, the simple act of updating the count of
605the number of current readers can impose an unwanted cost. So the
606goal when reading a shared data structure that no other process is
607changing is to avoid writing anything to memory at all. Take no
608locks, increment no counts, leave no footprints.
609
610The REF-walk mechanism already described certainly doesn't follow this
611principle, but then it is really designed to work when there may well
612be other threads modifying the data. RCU-walk, in contrast, is
613designed for the common situation where there are lots of frequent
614readers and only occasional writers. This may not be common in all
615parts of the filesystem tree, but in many parts it will be. For the
616other parts it is important that RCU-walk can quickly fall back to
617using REF-walk.
618
619Pathname lookup always starts in RCU-walk mode but only remains there
620as long as what it is looking for is in the cache and is stable. It
621dances lightly down the cached filesystem image, leaving no footprints
622and carefully watching where it is, to be sure it doesn't trip. If it
623notices that something has changed or is changing, or if something
624isn't in the cache, then it tries to stop gracefully and switch to
625REF-walk.
626
627This stopping requires getting a counted reference on the current
628``vfsmount`` and ``dentry``, and ensuring that these are still valid -
629that a path walk with REF-walk would have found the same entries.
630This is an invariant that RCU-walk must guarantee. It can only make
631decisions, such as selecting the next step, that are decisions which
632REF-walk could also have made if it were walking down the tree at the
633same time. If the graceful stop succeeds, the rest of the path is
634processed with the reliable, if slightly sluggish, REF-walk. If
635RCU-walk finds it cannot stop gracefully, it simply gives up and
636restarts from the top with REF-walk.
637
638This pattern of "try RCU-walk, if that fails try REF-walk" can be
639clearly seen in functions like ``filename_lookup()``,
640``filename_parentat()``, ``filename_mountpoint()``,
641``do_filp_open()``, and ``do_file_open_root()``. These five
642correspond roughly to the four ``path_``* functions we met earlier,
643each of which calls ``link_path_walk()``. The ``path_*`` functions are
644called using different mode flags until a mode is found which works.
645They are first called with ``LOOKUP_RCU`` set to request "RCU-walk". If
646that fails with the error ``ECHILD`` they are called again with no
647special flag to request "REF-walk". If either of those report the
648error ``ESTALE`` a final attempt is made with ``LOOKUP_REVAL`` set (and no
649``LOOKUP_RCU``) to ensure that entries found in the cache are forcibly
650revalidated - normally entries are only revalidated if the filesystem
651determines that they are too old to trust.
652
653The ``LOOKUP_RCU`` attempt may drop that flag internally and switch to
654REF-walk, but will never then try to switch back to RCU-walk. Places
655that trip up RCU-walk are much more likely to be near the leaves and
656so it is very unlikely that there will be much, if any, benefit from
657switching back.
658
659RCU and seqlocks: fast and light
660--------------------------------
661
662RCU is, unsurprisingly, critical to RCU-walk mode. The
663``rcu_read_lock()`` is held for the entire time that RCU-walk is walking
664down a path. The particular guarantee it provides is that the key
665data structures - dentries, inodes, super_blocks, and mounts - will
666not be freed while the lock is held. They might be unlinked or
667invalidated in one way or another, but the memory will not be
668repurposed so values in various fields will still be meaningful. This
669is the only guarantee that RCU provides; everything else is done using
670seqlocks.
671
672As we saw above, REF-walk holds a counted reference to the current
673dentry and the current vfsmount, and does not release those references
674before taking references to the "next" dentry or vfsmount. It also
675sometimes takes the ``d_lock`` spinlock. These references and locks are
676taken to prevent certain changes from happening. RCU-walk must not
677take those references or locks and so cannot prevent such changes.
678Instead, it checks to see if a change has been made, and aborts or
679retries if it has.
680
681To preserve the invariant mentioned above (that RCU-walk may only make
682decisions that REF-walk could have made), it must make the checks at
683or near the same places that REF-walk holds the references. So, when
684REF-walk increments a reference count or takes a spinlock, RCU-walk
685samples the status of a seqlock using ``read_seqcount_begin()`` or a
686similar function. When REF-walk decrements the count or drops the
687lock, RCU-walk checks if the sampled status is still valid using
688``read_seqcount_retry()`` or similar.
689
690However, there is a little bit more to seqlocks than that. If
691RCU-walk accesses two different fields in a seqlock-protected
692structure, or accesses the same field twice, there is no a priori
693guarantee of any consistency between those accesses. When consistency
694is needed - which it usually is - RCU-walk must take a copy and then
695use ``read_seqcount_retry()`` to validate that copy.
696
697``read_seqcount_retry()`` not only checks the sequence number, but also
698imposes a memory barrier so that no memory-read instruction from
699*before* the call can be delayed until *after* the call, either by the
700CPU or by the compiler. A simple example of this can be seen in
701``slow_dentry_cmp()`` which, for filesystems which do not use simple
702byte-wise name equality, calls into the filesystem to compare a name
703against a dentry. The length and name pointer are copied into local
704variables, then ``read_seqcount_retry()`` is called to confirm the two
705are consistent, and only then is ``->d_compare()`` called. When
706standard filename comparison is used, ``dentry_cmp()`` is called
707instead. Notably it does _not_ use ``read_seqcount_retry()``, but
708instead has a large comment explaining why the consistency guarantee
709isn't necessary. A subsequent ``read_seqcount_retry()`` will be
710sufficient to catch any problem that could occur at this point.
711
712With that little refresher on seqlocks out of the way we can look at
713the bigger picture of how RCU-walk uses seqlocks.
714
715``mount_lock`` and ``nd->m_seq``
716~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
717
718We already met the ``mount_lock`` seqlock when REF-walk used it to
719ensure that crossing a mount point is performed safely. RCU-walk uses
720it for that too, but for quite a bit more.
721
722Instead of taking a counted reference to each ``vfsmount`` as it
723descends the tree, RCU-walk samples the state of ``mount_lock`` at the
724start of the walk and stores this initial sequence number in the
725``struct nameidata`` in the ``m_seq`` field. This one lock and one
726sequence number are used to validate all accesses to all ``vfsmounts``,
727and all mount point crossings. As changes to the mount table are
728relatively rare, it is reasonable to fall back on REF-walk any time
729that any "mount" or "unmount" happens.
730
731``m_seq`` is checked (using ``read_seqretry()``) at the end of an RCU-walk
732sequence, whether switching to REF-walk for the rest of the path or
733when the end of the path is reached. It is also checked when stepping
734down over a mount point (in ``__follow_mount_rcu()``) or up (in
735``follow_dotdot_rcu()``). If it is ever found to have changed, the
736whole RCU-walk sequence is aborted and the path is processed again by
737REF-walk.
738
739If RCU-walk finds that ``mount_lock`` hasn't changed then it can be sure
740that, had REF-walk taken counted references on each vfsmount, the
741results would have been the same. This ensures the invariant holds,
742at least for vfsmount structures.
743
744``dentry->d_seq`` and ``nd->seq``
745~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
746
747In place of taking a count or lock on ``d_reflock``, RCU-walk samples
748the per-dentry ``d_seq`` seqlock, and stores the sequence number in the
749``seq`` field of the nameidata structure, so ``nd->seq`` should always be
750the current sequence number of ``nd->dentry``. This number needs to be
751revalidated after copying, and before using, the name, parent, or
752inode of the dentry.
753
754The handling of the name we have already looked at, and the parent is
755only accessed in ``follow_dotdot_rcu()`` which fairly trivially follows
756the required pattern, though it does so for three different cases.
757
758When not at a mount point, ``d_parent`` is followed and its ``d_seq`` is
759collected. When we are at a mount point, we instead follow the
760``mnt->mnt_mountpoint`` link to get a new dentry and collect its
761``d_seq``. Then, after finally finding a ``d_parent`` to follow, we must
762check if we have landed on a mount point and, if so, must find that
763mount point and follow the ``mnt->mnt_root`` link. This would imply a
764somewhat unusual, but certainly possible, circumstance where the
765starting point of the path lookup was in part of the filesystem that
766was mounted on, and so not visible from the root.
767
768The inode pointer, stored in ``->d_inode``, is a little more
769interesting. The inode will always need to be accessed at least
770twice, once to determine if it is NULL and once to verify access
771permissions. Symlink handling requires a validated inode pointer too.
772Rather than revalidating on each access, a copy is made on the first
773access and it is stored in the ``inode`` field of ``nameidata`` from where
774it can be safely accessed without further validation.
775
776``lookup_fast()`` is the only lookup routine that is used in RCU-mode,
777``lookup_slow()`` being too slow and requiring locks. It is in
778``lookup_fast()`` that we find the important "hand over hand" tracking
779of the current dentry.
780
781The current ``dentry`` and current ``seq`` number are passed to
782``__d_lookup_rcu()`` which, on success, returns a new ``dentry`` and a
783new ``seq`` number. ``lookup_fast()`` then copies the inode pointer and
784revalidates the new ``seq`` number. It then validates the old ``dentry``
785with the old ``seq`` number one last time and only then continues. This
786process of getting the ``seq`` number of the new dentry and then
787checking the ``seq`` number of the old exactly mirrors the process of
788getting a counted reference to the new dentry before dropping that for
789the old dentry which we saw in REF-walk.
790
791No ``inode->i_rwsem`` or even ``rename_lock``
792~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
793
794A semaphore is a fairly heavyweight lock that can only be taken when it is
795permissible to sleep. As ``rcu_read_lock()`` forbids sleeping,
796``inode->i_rwsem`` plays no role in RCU-walk. If some other thread does
797take ``i_rwsem`` and modifies the directory in a way that RCU-walk needs
798to notice, the result will be either that RCU-walk fails to find the
799dentry that it is looking for, or it will find a dentry which
800``read_seqretry()`` won't validate. In either case it will drop down to
801REF-walk mode which can take whatever locks are needed.
802
803Though ``rename_lock`` could be used by RCU-walk as it doesn't require
804any sleeping, RCU-walk doesn't bother. REF-walk uses ``rename_lock`` to
805protect against the possibility of hash chains in the dcache changing
806while they are being searched. This can result in failing to find
807something that actually is there. When RCU-walk fails to find
808something in the dentry cache, whether it is really there or not, it
809already drops down to REF-walk and tries again with appropriate
810locking. This neatly handles all cases, so adding extra checks on
811rename_lock would bring no significant value.
812
813``unlazy walk()`` and ``complete_walk()``
814-----------------------------------------
815
816That "dropping down to REF-walk" typically involves a call to
817``unlazy_walk()``, so named because "RCU-walk" is also sometimes
818referred to as "lazy walk". ``unlazy_walk()`` is called when
819following the path down to the current vfsmount/dentry pair seems to
820have proceeded successfully, but the next step is problematic. This
821can happen if the next name cannot be found in the dcache, if
822permission checking or name revalidation couldn't be achieved while
823the ``rcu_read_lock()`` is held (which forbids sleeping), if an
824automount point is found, or in a couple of cases involving symlinks.
825It is also called from ``complete_walk()`` when the lookup has reached
826the final component, or the very end of the path, depending on which
827particular flavor of lookup is used.
828
829Other reasons for dropping out of RCU-walk that do not trigger a call
830to ``unlazy_walk()`` are when some inconsistency is found that cannot be
831handled immediately, such as ``mount_lock`` or one of the ``d_seq``
832seqlocks reporting a change. In these cases the relevant function
833will return ``-ECHILD`` which will percolate up until it triggers a new
834attempt from the top using REF-walk.
835
836For those cases where ``unlazy_walk()`` is an option, it essentially
837takes a reference on each of the pointers that it holds (vfsmount,
838dentry, and possibly some symbolic links) and then verifies that the
839relevant seqlocks have not been changed. If there have been changes,
840it, too, aborts with ``-ECHILD``, otherwise the transition to REF-walk
841has been a success and the lookup process continues.
842
843Taking a reference on those pointers is not quite as simple as just
844incrementing a counter. That works to take a second reference if you
845already have one (often indirectly through another object), but it
846isn't sufficient if you don't actually have a counted reference at
847all. For ``dentry->d_lockref``, it is safe to increment the reference
848counter to get a reference unless it has been explicitly marked as
849"dead" which involves setting the counter to ``-128``.
850``lockref_get_not_dead()`` achieves this.
851
852For ``mnt->mnt_count`` it is safe to take a reference as long as
853``mount_lock`` is then used to validate the reference. If that
854validation fails, it may *not* be safe to just drop that reference in
855the standard way of calling ``mnt_put()`` - an unmount may have
856progressed too far. So the code in ``legitimize_mnt()``, when it
857finds that the reference it got might not be safe, checks the
858``MNT_SYNC_UMOUNT`` flag to determine if a simple ``mnt_put()`` is
859correct, or if it should just decrement the count and pretend none of
860this ever happened.
861
862Taking care in filesystems
863--------------------------
864
865RCU-walk depends almost entirely on cached information and often will
866not call into the filesystem at all. However there are two places,
867besides the already-mentioned component-name comparison, where the
868file system might be included in RCU-walk, and it must know to be
869careful.
870
871If the filesystem has non-standard permission-checking requirements -
872such as a networked filesystem which may need to check with the server
873- the ``i_op->permission`` interface might be called during RCU-walk.
874In this case an extra "``MAY_NOT_BLOCK``" flag is passed so that it
875knows not to sleep, but to return ``-ECHILD`` if it cannot complete
876promptly. ``i_op->permission`` is given the inode pointer, not the
877dentry, so it doesn't need to worry about further consistency checks.
878However if it accesses any other filesystem data structures, it must
879ensure they are safe to be accessed with only the ``rcu_read_lock()``
880held. This typically means they must be freed using ``kfree_rcu()`` or
881similar.
882
883.. _READ_ONCE: https://lwn.net/Articles/624126/
884
885If the filesystem may need to revalidate dcache entries, then
886``d_op->d_revalidate`` may be called in RCU-walk too. This interface
887*is* passed the dentry but does not have access to the ``inode`` or the
888``seq`` number from the ``nameidata``, so it needs to be extra careful
889when accessing fields in the dentry. This "extra care" typically
890involves using `READ_ONCE() <READ_ONCE_>`_ to access fields, and verifying the
891result is not NULL before using it. This pattern can be seen in
892``nfs_lookup_revalidate()``.
893
894A pair of patterns
895------------------
896
897In various places in the details of REF-walk and RCU-walk, and also in
898the big picture, there are a couple of related patterns that are worth
899being aware of.
900
901The first is "try quickly and check, if that fails try slowly". We
902can see that in the high-level approach of first trying RCU-walk and
903then trying REF-walk, and in places where ``unlazy_walk()`` is used to
904switch to REF-walk for the rest of the path. We also saw it earlier
905in ``dget_parent()`` when following a "``..``" link. It tries a quick way
906to get a reference, then falls back to taking locks if needed.
907
908The second pattern is "try quickly and check, if that fails try
909again - repeatedly". This is seen with the use of ``rename_lock`` and
910``mount_lock`` in REF-walk. RCU-walk doesn't make use of this pattern -
911if anything goes wrong it is much safer to just abort and try a more
912sedate approach.
913
914The emphasis here is "try quickly and check". It should probably be
915"try quickly _and carefully,_ then check". The fact that checking is
916needed is a reminder that the system is dynamic and only a limited
917number of things are safe at all. The most likely cause of errors in
918this whole process is assuming something is safe when in reality it
919isn't. Careful consideration of what exactly guarantees the safety of
920each access is sometimes necessary.
921
922A walk among the symlinks
923=========================
924
925There are several basic issues that we will examine to understand the
926handling of symbolic links: the symlink stack, together with cache
927lifetimes, will help us understand the overall recursive handling of
928symlinks and lead to the special care needed for the final component.
929Then a consideration of access-time updates and summary of the various
930flags controlling lookup will finish the story.
931
932The symlink stack
933-----------------
934
935There are only two sorts of filesystem objects that can usefully
936appear in a path prior to the final component: directories and symlinks.
937Handling directories is quite straightforward: the new directory
938simply becomes the starting point at which to interpret the next
939component on the path. Handling symbolic links requires a bit more
940work.
941
942Conceptually, symbolic links could be handled by editing the path. If
943a component name refers to a symbolic link, then that component is
944replaced by the body of the link and, if that body starts with a '/',
945then all preceding parts of the path are discarded. This is what the
946"``readlink -f``" command does, though it also edits out "``.``" and
947"``..``" components.
948
949Directly editing the path string is not really necessary when looking
950up a path, and discarding early components is pointless as they aren't
951looked at anyway. Keeping track of all remaining components is
952important, but they can of course be kept separately; there is no need
953to concatenate them. As one symlink may easily refer to another,
954which in turn can refer to a third, we may need to keep the remaining
955components of several paths, each to be processed when the preceding
956ones are completed. These path remnants are kept on a stack of
957limited size.
958
959There are two reasons for placing limits on how many symlinks can
960occur in a single path lookup. The most obvious is to avoid loops.
961If a symlink referred to itself either directly or through
962intermediaries, then following the symlink can never complete
963successfully - the error ``ELOOP`` must be returned. Loops can be
964detected without imposing limits, but limits are the simplest solution
965and, given the second reason for restriction, quite sufficient.
966
967.. _outlined recently: http://thread.gmane.org/gmane.linux.kernel/1934390/focus=1934550
968
969The second reason was `outlined recently`_ by Linus:
970
971 Because it's a latency and DoS issue too. We need to react well to
972 true loops, but also to "very deep" non-loops. It's not about memory
973 use, it's about users triggering unreasonable CPU resources.
974
975Linux imposes a limit on the length of any pathname: ``PATH_MAX``, which
976is 4096. There are a number of reasons for this limit; not letting the
977kernel spend too much time on just one path is one of them. With
978symbolic links you can effectively generate much longer paths so some
979sort of limit is needed for the same reason. Linux imposes a limit of
980at most 40 symlinks in any one path lookup. It previously imposed a
981further limit of eight on the maximum depth of recursion, but that was
982raised to 40 when a separate stack was implemented, so there is now
983just the one limit.
984
985The ``nameidata`` structure that we met in an earlier article contains a
986small stack that can be used to store the remaining part of up to two
987symlinks. In many cases this will be sufficient. If it isn't, a
988separate stack is allocated with room for 40 symlinks. Pathname
989lookup will never exceed that stack as, once the 40th symlink is
990detected, an error is returned.
991
992It might seem that the name remnants are all that needs to be stored on
993this stack, but we need a bit more. To see that, we need to move on to
994cache lifetimes.
995
996Storage and lifetime of cached symlinks
997---------------------------------------
998
999Like other filesystem resources, such as inodes and directory
1000entries, symlinks are cached by Linux to avoid repeated costly access
1001to external storage. It is particularly important for RCU-walk to be
1002able to find and temporarily hold onto these cached entries, so that
1003it doesn't need to drop down into REF-walk.
1004
1005.. _object-oriented design pattern: https://lwn.net/Articles/446317/
1006
1007While each filesystem is free to make its own choice, symlinks are
1008typically stored in one of two places. Short symlinks are often
1009stored directly in the inode. When a filesystem allocates a ``struct
1010inode`` it typically allocates extra space to store private data (a
1011common `object-oriented design pattern`_ in the kernel). This will
1012sometimes include space for a symlink. The other common location is
1013in the page cache, which normally stores the content of files. The
1014pathname in a symlink can be seen as the content of that symlink and
1015can easily be stored in the page cache just like file content.
1016
1017When neither of these is suitable, the next most likely scenario is
1018that the filesystem will allocate some temporary memory and copy or
1019construct the symlink content into that memory whenever it is needed.
1020
1021When the symlink is stored in the inode, it has the same lifetime as
1022the inode which, itself, is protected by RCU or by a counted reference
1023on the dentry. This means that the mechanisms that pathname lookup
1024uses to access the dcache and icache (inode cache) safely are quite
1025sufficient for accessing some cached symlinks safely. In these cases,
1026the ``i_link`` pointer in the inode is set to point to wherever the
1027symlink is stored and it can be accessed directly whenever needed.
1028
1029When the symlink is stored in the page cache or elsewhere, the
1030situation is not so straightforward. A reference on a dentry or even
1031on an inode does not imply any reference on cached pages of that
1032inode, and even an ``rcu_read_lock()`` is not sufficient to ensure that
1033a page will not disappear. So for these symlinks the pathname lookup
1034code needs to ask the filesystem to provide a stable reference and,
1035significantly, needs to release that reference when it is finished
1036with it.
1037
1038Taking a reference to a cache page is often possible even in RCU-walk
1039mode. It does require making changes to memory, which is best avoided,
1040but that isn't necessarily a big cost and it is better than dropping
1041out of RCU-walk mode completely. Even filesystems that allocate
1042space to copy the symlink into can use ``GFP_ATOMIC`` to often successfully
1043allocate memory without the need to drop out of RCU-walk. If a
1044filesystem cannot successfully get a reference in RCU-walk mode, it
1045must return ``-ECHILD`` and ``unlazy_walk()`` will be called to return to
1046REF-walk mode in which the filesystem is allowed to sleep.
1047
1048The place for all this to happen is the ``i_op->follow_link()`` inode
1049method. In the present mainline code this is never actually called in
1050RCU-walk mode as the rewrite is not quite complete. It is likely that
1051in a future release this method will be passed an ``inode`` pointer when
1052called in RCU-walk mode so it both (1) knows to be careful, and (2) has the
1053validated pointer. Much like the ``i_op->permission()`` method we
1054looked at previously, ``->follow_link()`` would need to be careful that
1055all the data structures it references are safe to be accessed while
1056holding no counted reference, only the RCU lock. Though getting a
1057reference with ``->follow_link()`` is not yet done in RCU-walk mode, the
1058code is ready to release the reference when that does happen.
1059
1060This need to drop the reference to a symlink adds significant
1061complexity. It requires a reference to the inode so that the
1062``i_op->put_link()`` inode operation can be called. In REF-walk, that
1063reference is kept implicitly through a reference to the dentry, so
1064keeping the ``struct path`` of the symlink is easiest. For RCU-walk,
1065the pointer to the inode is kept separately. To allow switching from
1066RCU-walk back to REF-walk in the middle of processing nested symlinks
1067we also need the seq number for the dentry so we can confirm that
1068switching back was safe.
1069
1070Finally, when providing a reference to a symlink, the filesystem also
1071provides an opaque "cookie" that must be passed to ``->put_link()`` so that it
1072knows what to free. This might be the allocated memory area, or a
1073pointer to the ``struct page`` in the page cache, or something else
1074completely. Only the filesystem knows what it is.
1075
1076In order for the reference to each symlink to be dropped when the walk completes,
1077whether in RCU-walk or REF-walk, the symlink stack needs to contain,
1078along with the path remnants:
1079
1080- the ``struct path`` to provide a reference to the inode in REF-walk
1081- the ``struct inode *`` to provide a reference to the inode in RCU-walk
1082- the ``seq`` to allow the path to be safely switched from RCU-walk to REF-walk
1083- the ``cookie`` that tells ``->put_path()`` what to put.
1084
1085This means that each entry in the symlink stack needs to hold five
1086pointers and an integer instead of just one pointer (the path
1087remnant). On a 64-bit system, this is about 40 bytes per entry;
1088with 40 entries it adds up to 1600 bytes total, which is less than
1089half a page. So it might seem like a lot, but is by no means
1090excessive.
1091
1092Note that, in a given stack frame, the path remnant (``name``) is not
1093part of the symlink that the other fields refer to. It is the remnant
1094to be followed once that symlink has been fully parsed.
1095
1096Following the symlink
1097---------------------
1098
1099The main loop in ``link_path_walk()`` iterates seamlessly over all
1100components in the path and all of the non-final symlinks. As symlinks
1101are processed, the ``name`` pointer is adjusted to point to a new
1102symlink, or is restored from the stack, so that much of the loop
1103doesn't need to notice. Getting this ``name`` variable on and off the
1104stack is very straightforward; pushing and popping the references is
1105a little more complex.
1106
1107When a symlink is found, ``walk_component()`` returns the value ``1``
1108(``0`` is returned for any other sort of success, and a negative number
1109is, as usual, an error indicator). This causes ``get_link()`` to be
1110called; it then gets the link from the filesystem. Providing that
1111operation is successful, the old path ``name`` is placed on the stack,
1112and the new value is used as the ``name`` for a while. When the end of
1113the path is found (i.e. ``*name`` is ``'\0'``) the old ``name`` is restored
1114off the stack and path walking continues.
1115
1116Pushing and popping the reference pointers (inode, cookie, etc.) is more
1117complex in part because of the desire to handle tail recursion. When
1118the last component of a symlink itself points to a symlink, we
1119want to pop the symlink-just-completed off the stack before pushing
1120the symlink-just-found to avoid leaving empty path remnants that would
1121just get in the way.
1122
1123It is most convenient to push the new symlink references onto the
1124stack in ``walk_component()`` immediately when the symlink is found;
1125``walk_component()`` is also the last piece of code that needs to look at the
1126old symlink as it walks that last component. So it is quite
1127convenient for ``walk_component()`` to release the old symlink and pop
1128the references just before pushing the reference information for the
1129new symlink. It is guided in this by two flags; ``WALK_GET``, which
1130gives it permission to follow a symlink if it finds one, and
1131``WALK_PUT``, which tells it to release the current symlink after it has been
1132followed. ``WALK_PUT`` is tested first, leading to a call to
1133``put_link()``. ``WALK_GET`` is tested subsequently (by
1134``should_follow_link()``) leading to a call to ``pick_link()`` which sets
1135up the stack frame.
1136
1137Symlinks with no final component
1138~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1139
1140A pair of special-case symlinks deserve a little further explanation.
1141Both result in a new ``struct path`` (with mount and dentry) being set
1142up in the ``nameidata``, and result in ``get_link()`` returning ``NULL``.
1143
1144The more obvious case is a symlink to "``/``". All symlinks starting
1145with "``/``" are detected in ``get_link()`` which resets the ``nameidata``
1146to point to the effective filesystem root. If the symlink only
1147contains "``/``" then there is nothing more to do, no components at all,
1148so ``NULL`` is returned to indicate that the symlink can be released and
1149the stack frame discarded.
1150
1151The other case involves things in ``/proc`` that look like symlinks but
1152aren't really::
1153
1154 $ ls -l /proc/self/fd/1
1155 lrwx------ 1 neilb neilb 64 Jun 13 10:19 /proc/self/fd/1 -> /dev/pts/4
1156
1157Every open file descriptor in any process is represented in ``/proc`` by
1158something that looks like a symlink. It is really a reference to the
1159target file, not just the name of it. When you ``readlink`` these
1160objects you get a name that might refer to the same file - unless it
1161has been unlinked or mounted over. When ``walk_component()`` follows
1162one of these, the ``->follow_link()`` method in "procfs" doesn't return
1163a string name, but instead calls ``nd_jump_link()`` which updates the
1164``nameidata`` in place to point to that target. ``->follow_link()`` then
1165returns ``NULL``. Again there is no final component and ``get_link()``
1166reports this by leaving the ``last_type`` field of ``nameidata`` as
1167``LAST_BIND``.
1168
1169Following the symlink in the final component
1170--------------------------------------------
1171
1172All this leads to ``link_path_walk()`` walking down every component, and
1173following all symbolic links it finds, until it reaches the final
1174component. This is just returned in the ``last`` field of ``nameidata``.
1175For some callers, this is all they need; they want to create that
1176``last`` name if it doesn't exist or give an error if it does. Other
1177callers will want to follow a symlink if one is found, and possibly
1178apply special handling to the last component of that symlink, rather
1179than just the last component of the original file name. These callers
1180potentially need to call ``link_path_walk()`` again and again on
1181successive symlinks until one is found that doesn't point to another
1182symlink.
1183
1184This case is handled by the relevant caller of ``link_path_walk()``, such as
1185``path_lookupat()`` using a loop that calls ``link_path_walk()``, and then
1186handles the final component. If the final component is a symlink
1187that needs to be followed, then ``trailing_symlink()`` is called to set
1188things up properly and the loop repeats, calling ``link_path_walk()``
1189again. This could loop as many as 40 times if the last component of
1190each symlink is another symlink.
1191
1192The various functions that examine the final component and possibly
1193report that it is a symlink are ``lookup_last()``, ``mountpoint_last()``
1194and ``do_last()``, each of which use the same convention as
1195``walk_component()`` of returning ``1`` if a symlink was found that needs
1196to be followed.
1197
1198Of these, ``do_last()`` is the most interesting as it is used for
1199opening a file. Part of ``do_last()`` runs with ``i_rwsem`` held and this
1200part is in a separate function: ``lookup_open()``.
1201
1202Explaining ``do_last()`` completely is beyond the scope of this article,
1203but a few highlights should help those interested in exploring the
1204code.
1205
12061. Rather than just finding the target file, ``do_last()`` needs to open
1207 it. If the file was found in the dcache, then ``vfs_open()`` is used for
1208 this. If not, then ``lookup_open()`` will either call ``atomic_open()`` (if
1209 the filesystem provides it) to combine the final lookup with the open, or
1210 will perform the separate ``lookup_real()`` and ``vfs_create()`` steps
1211 directly. In the later case the actual "open" of this newly found or
1212 created file will be performed by ``vfs_open()``, just as if the name
1213 were found in the dcache.
1214
12152. ``vfs_open()`` can fail with ``-EOPENSTALE`` if the cached information
1216 wasn't quite current enough. Rather than restarting the lookup from
1217 the top with ``LOOKUP_REVAL`` set, ``lookup_open()`` is called instead,
1218 giving the filesystem a chance to resolve small inconsistencies.
1219 If that doesn't work, only then is the lookup restarted from the top.
1220
12213. An open with O_CREAT **does** follow a symlink in the final component,
1222 unlike other creation system calls (like ``mkdir``). So the sequence::
1223
1224 ln -s bar /tmp/foo
1225 echo hello > /tmp/foo
1226
1227 will create a file called ``/tmp/bar``. This is not permitted if
1228 ``O_EXCL`` is set but otherwise is handled for an O_CREAT open much
1229 like for a non-creating open: ``should_follow_link()`` returns ``1``, and
1230 so does ``do_last()`` so that ``trailing_symlink()`` gets called and the
1231 open process continues on the symlink that was found.
1232
1233Updating the access time
1234------------------------
1235
1236We previously said of RCU-walk that it would "take no locks, increment
1237no counts, leave no footprints." We have since seen that some
1238"footprints" can be needed when handling symlinks as a counted
1239reference (or even a memory allocation) may be needed. But these
1240footprints are best kept to a minimum.
1241
1242One other place where walking down a symlink can involve leaving
1243footprints in a way that doesn't affect directories is in updating access times.
1244In Unix (and Linux) every filesystem object has a "last accessed
1245time", or "``atime``". Passing through a directory to access a file
1246within is not considered to be an access for the purposes of
1247``atime``; only listing the contents of a directory can update its ``atime``.
1248Symlinks are different it seems. Both reading a symlink (with ``readlink()``)
1249and looking up a symlink on the way to some other destination can
1250update the atime on that symlink.
1251
1252.. _clearest statement: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_08
1253
1254It is not clear why this is the case; POSIX has little to say on the
1255subject. The `clearest statement`_ is that, if a particular implementation
1256updates a timestamp in a place not specified by POSIX, this must be
1257documented "except that any changes caused by pathname resolution need
1258not be documented". This seems to imply that POSIX doesn't really
1259care about access-time updates during pathname lookup.
1260
1261.. _Linux 1.3.87: https://git.kernel.org/cgit/linux/kernel/git/history/history.git/diff/fs/ext2/symlink.c?id=f806c6db77b8eaa6e00dcfb6b567706feae8dbb8
1262
1263An examination of history shows that prior to `Linux 1.3.87`_, the ext2
1264filesystem, at least, didn't update atime when following a link.
1265Unfortunately we have no record of why that behavior was changed.
1266
1267In any case, access time must now be updated and that operation can be
1268quite complex. Trying to stay in RCU-walk while doing it is best
1269avoided. Fortunately it is often permitted to skip the ``atime``
1270update. Because ``atime`` updates cause performance problems in various
1271areas, Linux supports the ``relatime`` mount option, which generally
1272limits the updates of ``atime`` to once per day on files that aren't
1273being changed (and symlinks never change once created). Even without
1274``relatime``, many filesystems record ``atime`` with a one-second
1275granularity, so only one update per second is required.
1276
1277It is easy to test if an ``atime`` update is needed while in RCU-walk
1278mode and, if it isn't, the update can be skipped and RCU-walk mode
1279continues. Only when an ``atime`` update is actually required does the
1280path walk drop down to REF-walk. All of this is handled in the
1281``get_link()`` function.
1282
1283A few flags
1284-----------
1285
1286A suitable way to wrap up this tour of pathname walking is to list
1287the various flags that can be stored in the ``nameidata`` to guide the
1288lookup process. Many of these are only meaningful on the final
1289component, others reflect the current state of the pathname lookup.
1290And then there is ``LOOKUP_EMPTY``, which doesn't fit conceptually with
1291the others. If this is not set, an empty pathname causes an error
1292very early on. If it is set, empty pathnames are not considered to be
1293an error.
1294
1295Global state flags
1296~~~~~~~~~~~~~~~~~~
1297
1298We have already met two global state flags: ``LOOKUP_RCU`` and
1299``LOOKUP_REVAL``. These select between one of three overall approaches
1300to lookup: RCU-walk, REF-walk, and REF-walk with forced revalidation.
1301
1302``LOOKUP_PARENT`` indicates that the final component hasn't been reached
1303yet. This is primarily used to tell the audit subsystem the full
1304context of a particular access being audited.
1305
1306``LOOKUP_ROOT`` indicates that the ``root`` field in the ``nameidata`` was
1307provided by the caller, so it shouldn't be released when it is no
1308longer needed.
1309
1310``LOOKUP_JUMPED`` means that the current dentry was chosen not because
1311it had the right name but for some other reason. This happens when
1312following "``..``", following a symlink to ``/``, crossing a mount point
1313or accessing a "``/proc/$PID/fd/$FD``" symlink. In this case the
1314filesystem has not been asked to revalidate the name (with
1315``d_revalidate()``). In such cases the inode may still need to be
1316revalidated, so ``d_op->d_weak_revalidate()`` is called if
1317``LOOKUP_JUMPED`` is set when the look completes - which may be at the
1318final component or, when creating, unlinking, or renaming, at the penultimate component.
1319
1320Final-component flags
1321~~~~~~~~~~~~~~~~~~~~~
1322
1323Some of these flags are only set when the final component is being
1324considered. Others are only checked for when considering that final
1325component.
1326
1327``LOOKUP_AUTOMOUNT`` ensures that, if the final component is an automount
1328point, then the mount is triggered. Some operations would trigger it
1329anyway, but operations like ``stat()`` deliberately don't. ``statfs()``
1330needs to trigger the mount but otherwise behaves a lot like ``stat()``, so
1331it sets ``LOOKUP_AUTOMOUNT``, as does "``quotactl()``" and the handling of
1332"``mount --bind``".
1333
1334``LOOKUP_FOLLOW`` has a similar function to ``LOOKUP_AUTOMOUNT`` but for
1335symlinks. Some system calls set or clear it implicitly, while
1336others have API flags such as ``AT_SYMLINK_FOLLOW`` and
1337``UMOUNT_NOFOLLOW`` to control it. Its effect is similar to
1338``WALK_GET`` that we already met, but it is used in a different way.
1339
1340``LOOKUP_DIRECTORY`` insists that the final component is a directory.
1341Various callers set this and it is also set when the final component
1342is found to be followed by a slash.
1343
1344Finally ``LOOKUP_OPEN``, ``LOOKUP_CREATE``, ``LOOKUP_EXCL``, and
1345``LOOKUP_RENAME_TARGET`` are not used directly by the VFS but are made
1346available to the filesystem and particularly the ``->d_revalidate()``
1347method. A filesystem can choose not to bother revalidating too hard
1348if it knows that it will be asked to open or create the file soon.
1349These flags were previously useful for ``->lookup()`` too but with the
1350introduction of ``->atomic_open()`` they are less relevant there.
1351
1352End of the road
1353---------------
1354
1355Despite its complexity, all this pathname lookup code appears to be
1356in good shape - various parts are certainly easier to understand now
1357than even a couple of releases ago. But that doesn't mean it is
1358"finished". As already mentioned, RCU-walk currently only follows
1359symlinks that are stored in the inode so, while it handles many ext4
1360symlinks, it doesn't help with NFS, XFS, or Btrfs. That support
1361is not likely to be long delayed.