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5 autofs - how it works
6 =====================
8 Purpose
9 -------
11 The goal of autofs is to provide on-demand mounting and race free
12 automatic unmounting of various other filesystems. This provides two
13 key advantages:
15 1. There is no need to delay boot until all filesystems that
16 might be needed are mounted. Processes that try to access those
17 slow filesystems might be delayed but other processes can
18 continue freely. This is particularly important for
19 network filesystems (e.g. NFS) or filesystems stored on
20 media with a media-changing robot.
22 2. The names and locations of filesystems can be stored in
23 a remote database and can change at any time. The content
24 in that data base at the time of access will be used to provide
25 a target for the access. The interpretation of names in the
26 filesystem can even be programmatic rather than database-backed,
27 allowing wildcards for example, and can vary based on the user who
28 first accessed a name.
30 Context
31 -------
33 The "autofs" filesystem module is only one part of an autofs system.
34 There also needs to be a user-space program which looks up names
35 and mounts filesystems. This will often be the "automount" program,
36 though other tools including "systemd" can make use of "autofs".
37 This document describes only the kernel module and the interactions
38 required with any user-space program. Subsequent text refers to this
39 as the "automount daemon" or simply "the daemon".
41 "autofs" is a Linux kernel module with provides the "autofs"
42 filesystem type. Several "autofs" filesystems can be mounted and they
43 can each be managed separately, or all managed by the same daemon.
45 Content
46 -------
48 An autofs filesystem can contain 3 sorts of objects: directories,
49 symbolic links and mount traps. Mount traps are directories with
50 extra properties as described in the next section.
52 Objects can only be created by the automount daemon: symlinks are
53 created with a regular `symlink` system call, while directories and
54 mount traps are created with `mkdir`. The determination of whether a
55 directory should be a mount trap or not is quite _ad hoc_, largely for
56 historical reasons, and is determined in part by the
57 *direct*/*indirect*/*offset* mount options, and the *maxproto* mount option.
59 If neither the *direct* or *offset* mount options are given (so the
60 mount is considered to be *indirect*), then the root directory is
61 always a regular directory, otherwise it is a mount trap when it is
62 empty and a regular directory when not empty. Note that *direct* and
63 *offset* are treated identically so a concise summary is that the root
64 directory is a mount trap only if the filesystem is mounted *direct*
65 and the root is empty.
67 Directories created in the root directory are mount traps only if the
68 filesystem is mounted *indirect* and they are empty.
70 Directories further down the tree depend on the *maxproto* mount
71 option and particularly whether it is less than five or not.
72 When *maxproto* is five, no directories further down the
73 tree are ever mount traps, they are always regular directories. When
74 the *maxproto* is four (or three), these directories are mount traps
75 precisely when they are empty.
77 So: non-empty (i.e. non-leaf) directories are never mount traps. Empty
78 directories are sometimes mount traps, and sometimes not depending on
79 where in the tree they are (root, top level, or lower), the *maxproto*,
80 and whether the mount was *indirect* or not.
82 Mount Traps
83 ---------------
85 A core element of the implementation of autofs is the Mount Traps
86 which are provided by the Linux VFS. Any directory provided by a
87 filesystem can be designated as a trap. This involves two separate
88 features that work together to allow autofs to do its job.
90 **DCACHE_NEED_AUTOMOUNT**
92 If a dentry has the DCACHE_NEED_AUTOMOUNT flag set (which gets set if
93 the inode has S_AUTOMOUNT set, or can be set directly) then it is
94 (potentially) a mount trap. Any access to this directory beyond a
95 "`stat`" will (normally) cause the `d_op->d_automount()` dentry operation
96 to be called. The task of this method is to find the filesystem that
97 should be mounted on the directory and to return it. The VFS is
98 responsible for actually mounting the root of this filesystem on the
99 directory.
101 autofs doesn't find the filesystem itself but sends a message to the
102 automount daemon asking it to find and mount the filesystem. The
103 autofs `d_automount` method then waits for the daemon to report that
104 everything is ready. It will then return "`NULL`" indicating that the
105 mount has already happened. The VFS doesn't try to mount anything but
106 follows down the mount that is already there.
108 This functionality is sufficient for some users of mount traps such
109 as NFS which creates traps so that mountpoints on the server can be
110 reflected on the client. However it is not sufficient for autofs. As
111 mounting onto a directory is considered to be "beyond a `stat`", the
112 automount daemon would not be able to mount a filesystem on the 'trap'
113 directory without some way to avoid getting caught in the trap. For
114 that purpose there is another flag.
116 **DCACHE_MANAGE_TRANSIT**
118 If a dentry has DCACHE_MANAGE_TRANSIT set then two very different but
119 related behaviours are invoked, both using the `d_op->d_manage()`
120 dentry operation.
122 Firstly, before checking to see if any filesystem is mounted on the
123 directory, d_manage() will be called with the `rcu_walk` parameter set
124 to `false`. It may return one of three things:
126 - A return value of zero indicates that there is nothing special
127 about this dentry and normal checks for mounts and automounts
128 should proceed.
130 autofs normally returns zero, but first waits for any
131 expiry (automatic unmounting of the mounted filesystem) to
132 complete. This avoids races.
134 - A return value of `-EISDIR` tells the VFS to ignore any mounts
135 on the directory and to not consider calling `->d_automount()`.
136 This effectively disables the **DCACHE_NEED_AUTOMOUNT** flag
137 causing the directory not be a mount trap after all.
139 autofs returns this if it detects that the process performing the
140 lookup is the automount daemon and that the mount has been
141 requested but has not yet completed. How it determines this is
142 discussed later. This allows the automount daemon not to get
143 caught in the mount trap.
145 There is a subtlety here. It is possible that a second autofs
146 filesystem can be mounted below the first and for both of them to
147 be managed by the same daemon. For the daemon to be able to mount
148 something on the second it must be able to "walk" down past the
149 first. This means that d_manage cannot *always* return -EISDIR for
150 the automount daemon. It must only return it when a mount has
151 been requested, but has not yet completed.
153 `d_manage` also returns `-EISDIR` if the dentry shouldn't be a
154 mount trap, either because it is a symbolic link or because it is
155 not empty.
157 - Any other negative value is treated as an error and returned
158 to the caller.
160 autofs can return
162 - -ENOENT if the automount daemon failed to mount anything,
163 - -ENOMEM if it ran out of memory,
164 - -EINTR if a signal arrived while waiting for expiry to
165 complete
166 - or any other error sent down by the automount daemon.
169 The second use case only occurs during an "RCU-walk" and so `rcu_walk`
170 will be set.
172 An RCU-walk is a fast and lightweight process for walking down a
173 filename path (i.e. it is like running on tip-toes). RCU-walk cannot
174 cope with all situations so when it finds a difficulty it falls back
175 to "REF-walk", which is slower but more robust.
177 RCU-walk will never call `->d_automount`; the filesystems must already
178 be mounted or RCU-walk cannot handle the path.
179 To determine if a mount-trap is safe for RCU-walk mode it calls
180 `->d_manage()` with `rcu_walk` set to `true`.
182 In this case `d_manage()` must avoid blocking and should avoid taking
183 spinlocks if at all possible. Its sole purpose is to determine if it
184 would be safe to follow down into any mounted directory and the only
185 reason that it might not be is if an expiry of the mount is
186 underway.
188 In the `rcu_walk` case, `d_manage()` cannot return -EISDIR to tell the
189 VFS that this is a directory that doesn't require d_automount. If
190 `rcu_walk` sees a dentry with DCACHE_NEED_AUTOMOUNT set but nothing
191 mounted, it *will* fall back to REF-walk. `d_manage()` cannot make the
192 VFS remain in RCU-walk mode, but can only tell it to get out of
193 RCU-walk mode by returning `-ECHILD`.
195 So `d_manage()`, when called with `rcu_walk` set, should either return
196 -ECHILD if there is any reason to believe it is unsafe to enter the
197 mounted filesystem, otherwise it should return 0.
199 autofs will return `-ECHILD` if an expiry of the filesystem has been
200 initiated or is being considered, otherwise it returns 0.
203 Mountpoint expiry
204 -----------------
206 The VFS has a mechanism for automatically expiring unused mounts,
207 much as it can expire any unused dentry information from the dcache.
208 This is guided by the MNT_SHRINKABLE flag. This only applies to
209 mounts that were created by `d_automount()` returning a filesystem to be
210 mounted. As autofs doesn't return such a filesystem but leaves the
211 mounting to the automount daemon, it must involve the automount daemon
212 in unmounting as well. This also means that autofs has more control
213 over expiry.
215 The VFS also supports "expiry" of mounts using the MNT_EXPIRE flag to
216 the `umount` system call. Unmounting with MNT_EXPIRE will fail unless
217 a previous attempt had been made, and the filesystem has been inactive
218 and untouched since that previous attempt. autofs does not depend on
219 this but has its own internal tracking of whether filesystems were
220 recently used. This allows individual names in the autofs directory
221 to expire separately.
223 With version 4 of the protocol, the automount daemon can try to
224 unmount any filesystems mounted on the autofs filesystem or remove any
225 symbolic links or empty directories any time it likes. If the unmount
226 or removal is successful the filesystem will be returned to the state
227 it was before the mount or creation, so that any access of the name
228 will trigger normal auto-mount processing. In particular, `rmdir` and
229 `unlink` do not leave negative entries in the dcache as a normal
230 filesystem would, so an attempt to access a recently-removed object is
231 passed to autofs for handling.
233 With version 5, this is not safe except for unmounting from top-level
234 directories. As lower-level directories are never mount traps, other
235 processes will see an empty directory as soon as the filesystem is
236 unmounted. So it is generally safest to use the autofs expiry
237 protocol described below.
239 Normally the daemon only wants to remove entries which haven't been
240 used for a while. For this purpose autofs maintains a "`last_used`"
241 time stamp on each directory or symlink. For symlinks it genuinely
242 does record the last time the symlink was "used" or followed to find
243 out where it points to. For directories the field is used slightly
244 differently. The field is updated at mount time and during expire
245 checks if it is found to be in use (ie. open file descriptor or
246 process working directory) and during path walks. The update done
247 during path walks prevents frequent expire and immediate mount of
248 frequently accessed automounts. But in the case where a GUI continually
249 access or an application frequently scans an autofs directory tree
250 there can be an accumulation of mounts that aren't actually being
251 used. To cater for this case the "`strictexpire`" autofs mount option
252 can be used to avoid the "`last_used`" update on path walk thereby
253 preventing this apparent inability to expire mounts that aren't
254 really in use.
256 The daemon is able to ask autofs if anything is due to be expired,
257 using an `ioctl` as discussed later. For a *direct* mount, autofs
258 considers if the entire mount-tree can be unmounted or not. For an
259 *indirect* mount, autofs considers each of the names in the top level
260 directory to determine if any of those can be unmounted and cleaned
261 up.
263 There is an option with indirect mounts to consider each of the leaves
264 that has been mounted on instead of considering the top-level names.
265 This was originally intended for compatibility with version 4 of autofs
266 and should be considered as deprecated for Sun Format automount maps.
267 However, it may be used again for amd format mount maps (which are
268 generally indirect maps) because the amd automounter allows for the
269 setting of an expire timeout for individual mounts. But there are
270 some difficulties in making the needed changes for this.
272 When autofs considers a directory it checks the `last_used` time and
273 compares it with the "timeout" value set when the filesystem was
274 mounted, though this check is ignored in some cases. It also checks if
275 the directory or anything below it is in use. For symbolic links,
276 only the `last_used` time is ever considered.
278 If both appear to support expiring the directory or symlink, an action
279 is taken.
281 There are two ways to ask autofs to consider expiry. The first is to
282 use the **AUTOFS_IOC_EXPIRE** ioctl. This only works for indirect
283 mounts. If it finds something in the root directory to expire it will
284 return the name of that thing. Once a name has been returned the
285 automount daemon needs to unmount any filesystems mounted below the
286 name normally. As described above, this is unsafe for non-toplevel
287 mounts in a version-5 autofs. For this reason the current `automount(8)`
288 does not use this ioctl.
290 The second mechanism uses either the **AUTOFS_DEV_IOCTL_EXPIRE_CMD** or
291 the **AUTOFS_IOC_EXPIRE_MULTI** ioctl. This will work for both direct and
292 indirect mounts. If it selects an object to expire, it will notify
293 the daemon using the notification mechanism described below. This
294 will block until the daemon acknowledges the expiry notification.
295 This implies that the "`EXPIRE`" ioctl must be sent from a different
296 thread than the one which handles notification.
298 While the ioctl is blocking, the entry is marked as "expiring" and
299 `d_manage` will block until the daemon affirms that the unmount has
300 completed (together with removing any directories that might have been
301 necessary), or has been aborted.
303 Communicating with autofs: detecting the daemon
304 -----------------------------------------------
306 There are several forms of communication between the automount daemon
307 and the filesystem. As we have already seen, the daemon can create and
308 remove directories and symlinks using normal filesystem operations.
309 autofs knows whether a process requesting some operation is the daemon
310 or not based on its process-group id number (see getpgid(1)).
312 When an autofs filesystem is mounted the pgid of the mounting
313 processes is recorded unless the "pgrp=" option is given, in which
314 case that number is recorded instead. Any request arriving from a
315 process in that process group is considered to come from the daemon.
316 If the daemon ever has to be stopped and restarted a new pgid can be
317 provided through an ioctl as will be described below.
319 Communicating with autofs: the event pipe
320 -----------------------------------------
322 When an autofs filesystem is mounted, the 'write' end of a pipe must
323 be passed using the 'fd=' mount option. autofs will write
324 notification messages to this pipe for the daemon to respond to.
325 For version 5, the format of the message is:
327 struct autofs_v5_packet {
328 int proto_version; /* Protocol version */
329 int type; /* Type of packet */
330 autofs_wqt_t wait_queue_token;
331 __u32 dev;
332 __u64 ino;
333 __u32 uid;
334 __u32 gid;
335 __u32 pid;
336 __u32 tgid;
337 __u32 len;
338 char name[NAME_MAX+1];
339 };
341 where the type is one of
343 autofs_ptype_missing_indirect
344 autofs_ptype_expire_indirect
345 autofs_ptype_missing_direct
346 autofs_ptype_expire_direct
348 so messages can indicate that a name is missing (something tried to
349 access it but it isn't there) or that it has been selected for expiry.
351 The pipe will be set to "packet mode" (equivalent to passing
352 `O_DIRECT`) to _pipe2(2)_ so that a read from the pipe will return at
353 most one packet, and any unread portion of a packet will be discarded.
355 The `wait_queue_token` is a unique number which can identify a
356 particular request to be acknowledged. When a message is sent over
357 the pipe the affected dentry is marked as either "active" or
358 "expiring" and other accesses to it block until the message is
359 acknowledged using one of the ioctls below with the relevant
360 `wait_queue_token`.
362 Communicating with autofs: root directory ioctls
363 ------------------------------------------------
365 The root directory of an autofs filesystem will respond to a number of
366 ioctls. The process issuing the ioctl must have the CAP_SYS_ADMIN
367 capability, or must be the automount daemon.
369 The available ioctl commands are:
371 - **AUTOFS_IOC_READY**: a notification has been handled. The argument
372 to the ioctl command is the "wait_queue_token" number
373 corresponding to the notification being acknowledged.
374 - **AUTOFS_IOC_FAIL**: similar to above, but indicates failure with
375 the error code `ENOENT`.
376 - **AUTOFS_IOC_CATATONIC**: Causes the autofs to enter "catatonic"
377 mode meaning that it stops sending notifications to the daemon.
378 This mode is also entered if a write to the pipe fails.
379 - **AUTOFS_IOC_PROTOVER**: This returns the protocol version in use.
380 - **AUTOFS_IOC_PROTOSUBVER**: Returns the protocol sub-version which
381 is really a version number for the implementation.
382 - **AUTOFS_IOC_SETTIMEOUT**: This passes a pointer to an unsigned
383 long. The value is used to set the timeout for expiry, and
384 the current timeout value is stored back through the pointer.
385 - **AUTOFS_IOC_ASKUMOUNT**: Returns, in the pointed-to `int`, 1 if
386 the filesystem could be unmounted. This is only a hint as
387 the situation could change at any instant. This call can be
388 used to avoid a more expensive full unmount attempt.
389 - **AUTOFS_IOC_EXPIRE**: as described above, this asks if there is
390 anything suitable to expire. A pointer to a packet:
392 struct autofs_packet_expire_multi {
393 int proto_version; /* Protocol version */
394 int type; /* Type of packet */
395 autofs_wqt_t wait_queue_token;
396 int len;
397 char name[NAME_MAX+1];
398 };
400 is required. This is filled in with the name of something
401 that can be unmounted or removed. If nothing can be expired,
402 `errno` is set to `EAGAIN`. Even though a `wait_queue_token`
403 is present in the structure, no "wait queue" is established
404 and no acknowledgment is needed.
405 - **AUTOFS_IOC_EXPIRE_MULTI**: This is similar to
406 **AUTOFS_IOC_EXPIRE** except that it causes notification to be
407 sent to the daemon, and it blocks until the daemon acknowledges.
408 The argument is an integer which can contain two different flags.
410 **AUTOFS_EXP_IMMEDIATE** causes `last_used` time to be ignored
411 and objects are expired if the are not in use.
413 **AUTOFS_EXP_FORCED** causes the in use status to be ignored
414 and objects are expired ieven if they are in use. This assumes
415 that the daemon has requested this because it is capable of
416 performing the umount.
418 **AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level
419 name to expire. This is only safe when *maxproto* is 4.
421 Communicating with autofs: char-device ioctls
422 ---------------------------------------------
424 It is not always possible to open the root of an autofs filesystem,
425 particularly a *direct* mounted filesystem. If the automount daemon
426 is restarted there is no way for it to regain control of existing
427 mounts using any of the above communication channels. To address this
428 need there is a "miscellaneous" character device (major 10, minor 235)
429 which can be used to communicate directly with the autofs filesystem.
430 It requires CAP_SYS_ADMIN for access.
432 The `ioctl`s that can be used on this device are described in a separate
433 document `autofs-mount-control.txt`, and are summarised briefly here.
434 Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure:
436 struct autofs_dev_ioctl {
437 __u32 ver_major;
438 __u32 ver_minor;
439 __u32 size; /* total size of data passed in
440 * including this struct */
441 __s32 ioctlfd; /* automount command fd */
443 /* Command parameters */
444 union {
445 struct args_protover protover;
446 struct args_protosubver protosubver;
447 struct args_openmount openmount;
448 struct args_ready ready;
449 struct args_fail fail;
450 struct args_setpipefd setpipefd;
451 struct args_timeout timeout;
452 struct args_requester requester;
453 struct args_expire expire;
454 struct args_askumount askumount;
455 struct args_ismountpoint ismountpoint;
456 };
458 char path[0];
459 };
461 For the **OPEN_MOUNT** and **IS_MOUNTPOINT** commands, the target
462 filesystem is identified by the `path`. All other commands identify
463 the filesystem by the `ioctlfd` which is a file descriptor open on the
464 root, and which can be returned by **OPEN_MOUNT**.
466 The `ver_major` and `ver_minor` are in/out parameters which check that
467 the requested version is supported, and report the maximum version
468 that the kernel module can support.
470 Commands are:
472 - **AUTOFS_DEV_IOCTL_VERSION_CMD**: does nothing, except validate and
473 set version numbers.
474 - **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD**: return an open file descriptor
475 on the root of an autofs filesystem. The filesystem is identified
476 by name and device number, which is stored in `openmount.devid`.
477 Device numbers for existing filesystems can be found in
478 `/proc/self/mountinfo`.
479 - **AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD**: same as `close(ioctlfd)`.
480 - **AUTOFS_DEV_IOCTL_SETPIPEFD_CMD**: if the filesystem is in
481 catatonic mode, this can provide the write end of a new pipe
482 in `setpipefd.pipefd` to re-establish communication with a daemon.
483 The process group of the calling process is used to identify the
484 daemon.
485 - **AUTOFS_DEV_IOCTL_REQUESTER_CMD**: `path` should be a
486 name within the filesystem that has been auto-mounted on.
487 On successful return, `requester.uid` and `requester.gid` will be
488 the UID and GID of the process which triggered that mount.
489 - **AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD**: Check if path is a
490 mountpoint of a particular type - see separate documentation for
491 details.
492 - **AUTOFS_DEV_IOCTL_PROTOVER_CMD**:
493 - **AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD**:
494 - **AUTOFS_DEV_IOCTL_READY_CMD**:
495 - **AUTOFS_DEV_IOCTL_FAIL_CMD**:
496 - **AUTOFS_DEV_IOCTL_CATATONIC_CMD**:
497 - **AUTOFS_DEV_IOCTL_TIMEOUT_CMD**:
498 - **AUTOFS_DEV_IOCTL_EXPIRE_CMD**:
499 - **AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD**: These all have the same
500 function as the similarly named **AUTOFS_IOC** ioctls, except
501 that **FAIL** can be given an explicit error number in `fail.status`
502 instead of assuming `ENOENT`, and this **EXPIRE** command
503 corresponds to **AUTOFS_IOC_EXPIRE_MULTI**.
505 Catatonic mode
506 --------------
508 As mentioned, an autofs mount can enter "catatonic" mode. This
509 happens if a write to the notification pipe fails, or if it is
510 explicitly requested by an `ioctl`.
512 When entering catatonic mode, the pipe is closed and any pending
513 notifications are acknowledged with the error `ENOENT`.
515 Once in catatonic mode attempts to access non-existing names will
516 result in `ENOENT` while attempts to access existing directories will
517 be treated in the same way as if they came from the daemon, so mount
518 traps will not fire.
520 When the filesystem is mounted a _uid_ and _gid_ can be given which
521 set the ownership of directories and symbolic links. When the
522 filesystem is in catatonic mode, any process with a matching UID can
523 create directories or symlinks in the root directory, but not in other
524 directories.
526 Catatonic mode can only be left via the
527 **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD** ioctl on the `/dev/autofs`.
529 The "ignore" mount option
530 -------------------------
532 The "ignore" mount option can be used to provide a generic indicator
533 to applications that the mount entry should be ignored when displaying
534 mount information.
536 In other OSes that provide autofs and that provide a mount list to user
537 space based on the kernel mount list a no-op mount option ("ignore" is
538 the one use on the most common OSes) is allowed so that autofs file
539 system users can optionally use it.
541 This is intended to be used by user space programs to exclude autofs
542 mounts from consideration when reading the mounts list.
544 autofs, name spaces, and shared mounts
545 --------------------------------------
547 With bind mounts and name spaces it is possible for an autofs
548 filesystem to appear at multiple places in one or more filesystem
549 name spaces. For this to work sensibly, the autofs filesystem should
550 always be mounted "shared". e.g.
552 > `mount --make-shared /autofs/mount/point`
554 The automount daemon is only able to manage a single mount location for
555 an autofs filesystem and if mounts on that are not 'shared', other
556 locations will not behave as expected. In particular access to those
557 other locations will likely result in the `ELOOP` error
559 > Too many levels of symbolic links