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1 | <head> |
2 | <style> p { max-width:50em} ol, ul {max-width: 40em}</style> | |
3 | </head> | |
4 | ||
5 | autofs - how it works | |
6 | ===================== | |
7 | ||
8 | Purpose | |
9 | ------- | |
10 | ||
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: | |
14 | ||
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. | |
21 | ||
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. | |
29 | ||
30 | Context | |
31 | ------- | |
32 | ||
33 | The "autofs4" 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 "autofs4". | |
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". | |
40 | ||
41 | "autofs4" 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. | |
44 | ||
45 | Content | |
46 | ------- | |
47 | ||
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. | |
51 | ||
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. | |
58 | ||
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. | |
66 | ||
67 | Directories created in the root directory are mount traps only if the | |
d0846dd5 | 68 | filesystem is mounted *indirect* and they are empty. |
87d672cb N |
69 | |
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. | |
76 | ||
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. | |
81 | ||
82 | Mount Traps | |
83 | --------------- | |
84 | ||
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. | |
89 | ||
90 | **DCACHE_NEED_AUTOMOUNT** | |
91 | ||
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. | |
100 | ||
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. | |
107 | ||
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. | |
115 | ||
116 | **DCACHE_MANAGE_TRANSIT** | |
117 | ||
118 | If a dentry has DCACHE_MANAGE_TRANSIT set then two very different but | |
119 | related behaviors are invoked, both using the `d_op->d_manage()` | |
120 | dentry operation. | |
121 | ||
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: | |
125 | ||
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. | |
129 | ||
130 | autofs normally returns zero, but first waits for any | |
131 | expiry (automatic unmounting of the mounted filesystem) to | |
132 | complete. This avoids races. | |
133 | ||
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. | |
138 | ||
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. | |
144 | ||
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. | |
152 | ||
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. | |
156 | ||
157 | - Any other negative value is treated as an error and returned | |
158 | to the caller. | |
159 | ||
160 | autofs can return | |
161 | ||
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. | |
167 | ||
168 | ||
169 | The second use case only occurs during an "RCU-walk" and so `rcu_walk` | |
170 | will be set. | |
171 | ||
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. | |
176 | ||
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`. | |
181 | ||
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. | |
187 | ||
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`. | |
194 | ||
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 end the | |
197 | mounted filesystem, and otherwise should return 0. | |
198 | ||
199 | autofs will return `-ECHILD` if an expiry of the filesystem has been | |
200 | initiated or is being considered, otherwise it returns 0. | |
201 | ||
202 | ||
203 | Mountpoint expiry | |
204 | ----------------- | |
205 | ||
e662145f | 206 | The VFS has a mechanism for automatically expiring unused mounts, |
87d672cb | 207 | much as it can expire any unused dentry information from the dcache. |
e662145f | 208 | This is guided by the MNT_SHRINKABLE flag. This only applies to |
87d672cb N |
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 | of expiry. | |
214 | ||
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. autofs4 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. | |
222 | ||
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 particlar, `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. | |
232 | ||
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. | |
238 | ||
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 a slight | |
244 | misnomer. It actually records the last time that autofs checked if | |
245 | the directory or one of its descendents was busy and found that it | |
246 | was. This is just as useful and doesn't require updating the field so | |
247 | often. | |
248 | ||
249 | The daemon is able to ask autofs if anything is due to be expired, | |
250 | using an `ioctl` as discussed later. For a *direct* mount, autofs | |
251 | considers if the entire mount-tree can be unmounted or not. For an | |
252 | *indirect* mount, autofs considers each of the names in the top level | |
253 | directory to determine if any of those can be unmounted and cleaned | |
254 | up. | |
255 | ||
256 | There is an option with indirect mounts to consider each of the leaves | |
257 | that has been mounted on instead of considering the top-level names. | |
258 | This is intended for compatability with version 4 of autofs and should | |
259 | be considered as deprecated. | |
260 | ||
261 | When autofs considers a directory it checks the `last_used` time and | |
262 | compares it with the "timeout" value set when the filesystem was | |
263 | mounted, though this check is ignored in some cases. It also checks if | |
264 | the directory or anything below it is in use. For symbolic links, | |
265 | only the `last_used` time is ever considered. | |
266 | ||
267 | If both appear to support expiring the directory or symlink, an action | |
268 | is taken. | |
269 | ||
270 | There are two ways to ask autofs to consider expiry. The first is to | |
271 | use the **AUTOFS_IOC_EXPIRE** ioctl. This only works for indirect | |
272 | mounts. If it finds something in the root directory to expire it will | |
273 | return the name of that thing. Once a name has been returned the | |
274 | automount daemon needs to unmount any filesystems mounted below the | |
275 | name normally. As described above, this is unsafe for non-toplevel | |
276 | mounts in a version-5 autofs. For this reason the current `automountd` | |
277 | does not use this ioctl. | |
278 | ||
279 | The second mechanism uses either the **AUTOFS_DEV_IOCTL_EXPIRE_CMD** or | |
280 | the **AUTOFS_IOC_EXPIRE_MULTI** ioctl. This will work for both direct and | |
281 | indirect mounts. If it selects an object to expire, it will notify | |
282 | the daemon using the notification mechanism described below. This | |
283 | will block until the daemon acknowledges the expiry notification. | |
284 | This implies that the "`EXPIRE`" ioctl must be sent from a different | |
285 | thread than the one which handles notification. | |
286 | ||
287 | While the ioctl is blocking, the entry is marked as "expiring" and | |
288 | `d_manage` will block until the daemon affirms that the unmount has | |
289 | completed (together with removing any directories that might have been | |
290 | necessary), or has been aborted. | |
291 | ||
292 | Communicating with autofs: detecting the daemon | |
293 | ----------------------------------------------- | |
294 | ||
295 | There are several forms of communication between the automount daemon | |
296 | and the filesystem. As we have already seen, the daemon can create and | |
297 | remove directories and symlinks using normal filesystem operations. | |
298 | autofs knows whether a process requesting some operation is the daemon | |
299 | or not based on its process-group id number (see getpgid(1)). | |
300 | ||
e662145f | 301 | When an autofs filesystem is mounted the pgid of the mounting |
87d672cb N |
302 | processes is recorded unless the "pgrp=" option is given, in which |
303 | case that number is recorded instead. Any request arriving from a | |
304 | process in that process group is considered to come from the daemon. | |
305 | If the daemon ever has to be stopped and restarted a new pgid can be | |
306 | provided through an ioctl as will be described below. | |
307 | ||
308 | Communicating with autofs: the event pipe | |
309 | ----------------------------------------- | |
310 | ||
311 | When an autofs filesystem is mounted, the 'write' end of a pipe must | |
312 | be passed using the 'fd=' mount option. autofs will write | |
313 | notification messages to this pipe for the daemon to respond to. | |
314 | For version 5, the format of the message is: | |
315 | ||
316 | struct autofs_v5_packet { | |
317 | int proto_version; /* Protocol version */ | |
318 | int type; /* Type of packet */ | |
7cee9384 | 319 | autofs_wqt_t wait_queue_token; |
87d672cb N |
320 | __u32 dev; |
321 | __u64 ino; | |
322 | __u32 uid; | |
323 | __u32 gid; | |
324 | __u32 pid; | |
325 | __u32 tgid; | |
326 | __u32 len; | |
327 | char name[NAME_MAX+1]; | |
328 | }; | |
329 | ||
330 | where the type is one of | |
331 | ||
332 | autofs_ptype_missing_indirect | |
333 | autofs_ptype_expire_indirect | |
334 | autofs_ptype_missing_direct | |
335 | autofs_ptype_expire_direct | |
336 | ||
337 | so messages can indicate that a name is missing (something tried to | |
338 | access it but it isn't there) or that it has been selected for expiry. | |
339 | ||
340 | The pipe will be set to "packet mode" (equivalent to passing | |
341 | `O_DIRECT`) to _pipe2(2)_ so that a read from the pipe will return at | |
342 | most one packet, and any unread portion of a packet will be discarded. | |
343 | ||
7cee9384 | 344 | The `wait_queue_token` is a unique number which can identify a |
87d672cb N |
345 | particular request to be acknowledged. When a message is sent over |
346 | the pipe the affected dentry is marked as either "active" or | |
347 | "expiring" and other accesses to it block until the message is | |
348 | acknowledged using one of the ioctls below and the relevant | |
7cee9384 | 349 | `wait_queue_token`. |
87d672cb N |
350 | |
351 | Communicating with autofs: root directory ioctls | |
352 | ------------------------------------------------ | |
353 | ||
354 | The root directory of an autofs filesystem will respond to a number of | |
d0846dd5 | 355 | ioctls. The process issuing the ioctl must have the CAP_SYS_ADMIN |
87d672cb N |
356 | capability, or must be the automount daemon. |
357 | ||
358 | The available ioctl commands are: | |
359 | ||
360 | - **AUTOFS_IOC_READY**: a notification has been handled. The argument | |
7cee9384 | 361 | to the ioctl command is the "wait_queue_token" number |
87d672cb N |
362 | corresponding to the notification being acknowledged. |
363 | - **AUTOFS_IOC_FAIL**: similar to above, but indicates failure with | |
364 | the error code `ENOENT`. | |
365 | - **AUTOFS_IOC_CATATONIC**: Causes the autofs to enter "catatonic" | |
366 | mode meaning that it stops sending notifications to the daemon. | |
367 | This mode is also entered if a write to the pipe fails. | |
368 | - **AUTOFS_IOC_PROTOVER**: This returns the protocol version in use. | |
369 | - **AUTOFS_IOC_PROTOSUBVER**: Returns the protocol sub-version which | |
370 | is really a version number for the implementation. It is | |
371 | currently 2. | |
372 | - **AUTOFS_IOC_SETTIMEOUT**: This passes a pointer to an unsigned | |
373 | long. The value is used to set the timeout for expiry, and | |
374 | the current timeout value is stored back through the pointer. | |
375 | - **AUTOFS_IOC_ASKUMOUNT**: Returns, in the pointed-to `int`, 1 if | |
376 | the filesystem could be unmounted. This is only a hint as | |
377 | the situation could change at any instant. This call can be | |
378 | use to avoid a more expensive full unmount attempt. | |
379 | - **AUTOFS_IOC_EXPIRE**: as described above, this asks if there is | |
380 | anything suitable to expire. A pointer to a packet: | |
381 | ||
382 | struct autofs_packet_expire_multi { | |
383 | int proto_version; /* Protocol version */ | |
384 | int type; /* Type of packet */ | |
7cee9384 | 385 | autofs_wqt_t wait_queue_token; |
87d672cb N |
386 | int len; |
387 | char name[NAME_MAX+1]; | |
388 | }; | |
389 | ||
390 | is required. This is filled in with the name of something | |
391 | that can be unmounted or removed. If nothing can be expired, | |
7cee9384 | 392 | `errno` is set to `EAGAIN`. Even though a `wait_queue_token` |
87d672cb N |
393 | is present in the structure, no "wait queue" is established |
394 | and no acknowledgment is needed. | |
395 | - **AUTOFS_IOC_EXPIRE_MULTI**: This is similar to | |
396 | **AUTOFS_IOC_EXPIRE** except that it causes notification to be | |
397 | sent to the daemon, and it blocks until the daemon acknowledges. | |
398 | The argument is an integer which can contain two different flags. | |
399 | ||
400 | **AUTOFS_EXP_IMMEDIATE** causes `last_used` time to be ignored | |
401 | and objects are expired if the are not in use. | |
402 | ||
403 | **AUTOFS_EXP_LEAVES** will select a leaf rather than a top-level | |
404 | name to expire. This is only safe when *maxproto* is 4. | |
405 | ||
406 | Communicating with autofs: char-device ioctls | |
407 | --------------------------------------------- | |
408 | ||
409 | It is not always possible to open the root of an autofs filesystem, | |
410 | particularly a *direct* mounted filesystem. If the automount daemon | |
411 | is restarted there is no way for it to regain control of existing | |
412 | mounts using any of the above communication channels. To address this | |
413 | need there is a "miscellaneous" character device (major 10, minor 235) | |
414 | which can be used to communicate directly with the autofs filesystem. | |
415 | It requires CAP_SYS_ADMIN for access. | |
416 | ||
417 | The `ioctl`s that can be used on this device are described in a separate | |
418 | document `autofs4-mount-control.txt`, and are summarized briefly here. | |
419 | Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure: | |
420 | ||
421 | struct autofs_dev_ioctl { | |
422 | __u32 ver_major; | |
423 | __u32 ver_minor; | |
424 | __u32 size; /* total size of data passed in | |
425 | * including this struct */ | |
426 | __s32 ioctlfd; /* automount command fd */ | |
427 | ||
88488080 TK |
428 | /* Command parameters */ |
429 | union { | |
430 | struct args_protover protover; | |
431 | struct args_protosubver protosubver; | |
432 | struct args_openmount openmount; | |
433 | struct args_ready ready; | |
434 | struct args_fail fail; | |
435 | struct args_setpipefd setpipefd; | |
436 | struct args_timeout timeout; | |
437 | struct args_requester requester; | |
438 | struct args_expire expire; | |
439 | struct args_askumount askumount; | |
440 | struct args_ismountpoint ismountpoint; | |
441 | }; | |
87d672cb N |
442 | |
443 | char path[0]; | |
444 | }; | |
445 | ||
446 | For the **OPEN_MOUNT** and **IS_MOUNTPOINT** commands, the target | |
447 | filesystem is identified by the `path`. All other commands identify | |
448 | the filesystem by the `ioctlfd` which is a file descriptor open on the | |
449 | root, and which can be returned by **OPEN_MOUNT**. | |
450 | ||
451 | The `ver_major` and `ver_minor` are in/out parameters which check that | |
452 | the requested version is supported, and report the maximum version | |
453 | that the kernel module can support. | |
454 | ||
455 | Commands are: | |
456 | ||
457 | - **AUTOFS_DEV_IOCTL_VERSION_CMD**: does nothing, except validate and | |
458 | set version numbers. | |
459 | - **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD**: return an open file descriptor | |
460 | on the root of an autofs filesystem. The filesystem is identified | |
88488080 TK |
461 | by name and device number, which is stored in `openmount.devid`. |
462 | Device numbers for existing filesystems can be found in | |
87d672cb N |
463 | `/proc/self/mountinfo`. |
464 | - **AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD**: same as `close(ioctlfd)`. | |
e662145f | 465 | - **AUTOFS_DEV_IOCTL_SETPIPEFD_CMD**: if the filesystem is in |
87d672cb | 466 | catatonic mode, this can provide the write end of a new pipe |
88488080 TK |
467 | in `setpipefd.pipefd` to re-establish communication with a daemon. |
468 | The process group of the calling process is used to identify the | |
87d672cb N |
469 | daemon. |
470 | - **AUTOFS_DEV_IOCTL_REQUESTER_CMD**: `path` should be a | |
471 | name within the filesystem that has been auto-mounted on. | |
88488080 TK |
472 | On successful return, `requester.uid` and `requester.gid` will be |
473 | the UID and GID of the process which triggered that mount. | |
87d672cb N |
474 | - **AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD**: Check if path is a |
475 | mountpoint of a particular type - see separate documentation for | |
476 | details. | |
87d672cb N |
477 | - **AUTOFS_DEV_IOCTL_PROTOVER_CMD**: |
478 | - **AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD**: | |
479 | - **AUTOFS_DEV_IOCTL_READY_CMD**: | |
480 | - **AUTOFS_DEV_IOCTL_FAIL_CMD**: | |
481 | - **AUTOFS_DEV_IOCTL_CATATONIC_CMD**: | |
482 | - **AUTOFS_DEV_IOCTL_TIMEOUT_CMD**: | |
483 | - **AUTOFS_DEV_IOCTL_EXPIRE_CMD**: | |
484 | - **AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD**: These all have the same | |
485 | function as the similarly named **AUTOFS_IOC** ioctls, except | |
88488080 | 486 | that **FAIL** can be given an explicit error number in `fail.status` |
87d672cb N |
487 | instead of assuming `ENOENT`, and this **EXPIRE** command |
488 | corresponds to **AUTOFS_IOC_EXPIRE_MULTI**. | |
489 | ||
490 | Catatonic mode | |
491 | -------------- | |
492 | ||
493 | As mentioned, an autofs mount can enter "catatonic" mode. This | |
494 | happens if a write to the notification pipe fails, or if it is | |
495 | explicitly requested by an `ioctl`. | |
496 | ||
497 | When entering catatonic mode, the pipe is closed and any pending | |
498 | notifications are acknowledged with the error `ENOENT`. | |
499 | ||
500 | Once in catatonic mode attempts to access non-existing names will | |
501 | result in `ENOENT` while attempts to access existing directories will | |
502 | be treated in the same way as if they came from the daemon, so mount | |
503 | traps will not fire. | |
504 | ||
505 | When the filesystem is mounted a _uid_ and _gid_ can be given which | |
506 | set the ownership of directories and symbolic links. When the | |
507 | filesystem is in catatonic mode, any process with a matching UID can | |
508 | create directories or symlinks in the root directory, but not in other | |
509 | directories. | |
510 | ||
511 | Catatonic mode can only be left via the | |
512 | **AUTOFS_DEV_IOCTL_OPENMOUNT_CMD** ioctl on the `/dev/autofs`. | |
513 | ||
514 | autofs, name spaces, and shared mounts | |
515 | -------------------------------------- | |
516 | ||
517 | With bind mounts and name spaces it is possible for an autofs | |
518 | filesystem to appear at multiple places in one or more filesystem | |
519 | name spaces. For this to work sensibly, the autofs filesystem should | |
520 | always be mounted "shared". e.g. | |
521 | ||
522 | > `mount --make-shared /autofs/mount/point` | |
523 | ||
d0846dd5 | 524 | The automount daemon is only able to manage a single mount location for |
87d672cb N |
525 | an autofs filesystem and if mounts on that are not 'shared', other |
526 | locations will not behave as expected. In particular access to those | |
527 | other locations will likely result in the `ELOOP` error | |
528 | ||
529 | > Too many levels of symbolic links |