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Documentation / filesystems / autofs4-mount-control.txt




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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	
2	Miscellaneous Device control operations for the autofs4 kernel module
3	====================================================================
4	
5	The problem
6	===========
7	
8	There is a problem with active restarts in autofs (that is to say
9	restarting autofs when there are busy mounts).
10	
11	During normal operation autofs uses a file descriptor opened on the
12	directory that is being managed in order to be able to issue control
13	operations. Using a file descriptor gives ioctl operations access to
14	autofs specific information stored in the super block. The operations
15	are things such as setting an autofs mount catatonic, setting the
16	expire timeout and requesting expire checks. As is explained below,
17	certain types of autofs triggered mounts can end up covering an autofs
18	mount itself which prevents us being able to use open(2) to obtain a
19	file descriptor for these operations if we don't already have one open.
20	
21	Currently autofs uses "umount -l" (lazy umount) to clear active mounts
22	at restart. While using lazy umount works for most cases, anything that
23	needs to walk back up the mount tree to construct a path, such as
24	getcwd(2) and the proc file system /proc/<pid>/cwd, no longer works
25	because the point from which the path is constructed has been detached
26	from the mount tree.
27	
28	The actual problem with autofs is that it can't reconnect to existing
29	mounts. Immediately one thinks of just adding the ability to remount
30	autofs file systems would solve it, but alas, that can't work. This is
31	because autofs direct mounts and the implementation of "on demand mount
32	and expire" of nested mount trees have the file system mounted directly
33	on top of the mount trigger directory dentry.
34	
35	For example, there are two types of automount maps, direct (in the kernel
36	module source you will see a third type called an offset, which is just
37	a direct mount in disguise) and indirect.
38	
39	Here is a master map with direct and indirect map entries:
40	
41	/-      /etc/auto.direct
42	/test   /etc/auto.indirect
43	
44	and the corresponding map files:
45	
46	/etc/auto.direct:
47	
48	/automount/dparse/g6  budgie:/autofs/export1
49	/automount/dparse/g1  shark:/autofs/export1
50	and so on.
51	
52	/etc/auto.indirect:
53	
54	g1    shark:/autofs/export1
55	g6    budgie:/autofs/export1
56	and so on.
57	
58	For the above indirect map an autofs file system is mounted on /test and
59	mounts are triggered for each sub-directory key by the inode lookup
60	operation. So we see a mount of shark:/autofs/export1 on /test/g1, for
61	example.
62	
63	The way that direct mounts are handled is by making an autofs mount on
64	each full path, such as /automount/dparse/g1, and using it as a mount
65	trigger. So when we walk on the path we mount shark:/autofs/export1 "on
66	top of this mount point". Since these are always directories we can
67	use the follow_link inode operation to trigger the mount.
68	
69	But, each entry in direct and indirect maps can have offsets (making
70	them multi-mount map entries).
71	
72	For example, an indirect mount map entry could also be:
73	
74	g1  \
75	   /        shark:/autofs/export5/testing/test \
76	   /s1      shark:/autofs/export/testing/test/s1 \
77	   /s2      shark:/autofs/export5/testing/test/s2 \
78	   /s1/ss1  shark:/autofs/export1 \
79	   /s2/ss2  shark:/autofs/export2
80	
81	and a similarly a direct mount map entry could also be:
82	
83	/automount/dparse/g1 \
84	    /       shark:/autofs/export5/testing/test \
85	    /s1     shark:/autofs/export/testing/test/s1 \
86	    /s2     shark:/autofs/export5/testing/test/s2 \
87	    /s1/ss1 shark:/autofs/export2 \
88	    /s2/ss2 shark:/autofs/export2
89	
90	One of the issues with version 4 of autofs was that, when mounting an
91	entry with a large number of offsets, possibly with nesting, we needed
92	to mount and umount all of the offsets as a single unit. Not really a
93	problem, except for people with a large number of offsets in map entries.
94	This mechanism is used for the well known "hosts" map and we have seen
95	cases (in 2.4) where the available number of mounts are exhausted or
96	where the number of privileged ports available is exhausted.
97	
98	In version 5 we mount only as we go down the tree of offsets and
99	similarly for expiring them which resolves the above problem. There is
100	somewhat more detail to the implementation but it isn't needed for the
101	sake of the problem explanation. The one important detail is that these
102	offsets are implemented using the same mechanism as the direct mounts
103	above and so the mount points can be covered by a mount.
104	
105	The current autofs implementation uses an ioctl file descriptor opened
106	on the mount point for control operations. The references held by the
107	descriptor are accounted for in checks made to determine if a mount is
108	in use and is also used to access autofs file system information held
109	in the mount super block. So the use of a file handle needs to be
110	retained.
111	
112	
113	The Solution
114	============
115	
116	To be able to restart autofs leaving existing direct, indirect and
117	offset mounts in place we need to be able to obtain a file handle
118	for these potentially covered autofs mount points. Rather than just
119	implement an isolated operation it was decided to re-implement the
120	existing ioctl interface and add new operations to provide this
121	functionality.
122	
123	In addition, to be able to reconstruct a mount tree that has busy mounts,
124	the uid and gid of the last user that triggered the mount needs to be
125	available because these can be used as macro substitution variables in
126	autofs maps. They are recorded at mount request time and an operation
127	has been added to retrieve them.
128	
129	Since we're re-implementing the control interface, a couple of other
130	problems with the existing interface have been addressed. First, when
131	a mount or expire operation completes a status is returned to the
132	kernel by either a "send ready" or a "send fail" operation. The
133	"send fail" operation of the ioctl interface could only ever send
134	ENOENT so the re-implementation allows user space to send an actual
135	status. Another expensive operation in user space, for those using
136	very large maps, is discovering if a mount is present. Usually this
137	involves scanning /proc/mounts and since it needs to be done quite
138	often it can introduce significant overhead when there are many entries
139	in the mount table. An operation to lookup the mount status of a mount
140	point dentry (covered or not) has also been added.
141	
142	Current kernel development policy recommends avoiding the use of the
143	ioctl mechanism in favor of systems such as Netlink. An implementation
144	using this system was attempted to evaluate its suitability and it was
145	found to be inadequate, in this case. The Generic Netlink system was
146	used for this as raw Netlink would lead to a significant increase in
147	complexity. There's no question that the Generic Netlink system is an
148	elegant solution for common case ioctl functions but it's not a complete
149	replacement probably because its primary purpose in life is to be a
150	message bus implementation rather than specifically an ioctl replacement.
151	While it would be possible to work around this there is one concern
152	that lead to the decision to not use it. This is that the autofs
153	expire in the daemon has become far to complex because umount
154	candidates are enumerated, almost for no other reason than to "count"
155	the number of times to call the expire ioctl. This involves scanning
156	the mount table which has proved to be a big overhead for users with
157	large maps. The best way to improve this is try and get back to the
158	way the expire was done long ago. That is, when an expire request is
159	issued for a mount (file handle) we should continually call back to
160	the daemon until we can't umount any more mounts, then return the
161	appropriate status to the daemon. At the moment we just expire one
162	mount at a time. A Generic Netlink implementation would exclude this
163	possibility for future development due to the requirements of the
164	message bus architecture.
165	
166	
167	autofs4 Miscellaneous Device mount control interface
168	====================================================
169	
170	The control interface is opening a device node, typically /dev/autofs.
171	
172	All the ioctls use a common structure to pass the needed parameter
173	information and return operation results:
174	
175	struct autofs_dev_ioctl {
176		__u32 ver_major;
177		__u32 ver_minor;
178		__u32 size;             /* total size of data passed in
179					 * including this struct */
180		__s32 ioctlfd;          /* automount command fd */
181	
182		/* Command parameters */
183		union {
184			struct args_protover		protover;
185			struct args_protosubver		protosubver;
186			struct args_openmount		openmount;
187			struct args_ready		ready;
188			struct args_fail		fail;
189			struct args_setpipefd		setpipefd;
190			struct args_timeout		timeout;
191			struct args_requester		requester;
192			struct args_expire		expire;
193			struct args_askumount		askumount;
194			struct args_ismountpoint	ismountpoint;
195		};
196	
197		char path[0];
198	};
199	
200	The ioctlfd field is a mount point file descriptor of an autofs mount
201	point. It is returned by the open call and is used by all calls except
202	the check for whether a given path is a mount point, where it may
203	optionally be used to check a specific mount corresponding to a given
204	mount point file descriptor, and when requesting the uid and gid of the
205	last successful mount on a directory within the autofs file system.
206	
207	The union is used to communicate parameters and results of calls made
208	as described below.
209	
210	The path field is used to pass a path where it is needed and the size field
211	is used account for the increased structure length when translating the
212	structure sent from user space.
213	
214	This structure can be initialized before setting specific fields by using
215	the void function call init_autofs_dev_ioctl(struct autofs_dev_ioctl *).
216	
217	All of the ioctls perform a copy of this structure from user space to
218	kernel space and return -EINVAL if the size parameter is smaller than
219	the structure size itself, -ENOMEM if the kernel memory allocation fails
220	or -EFAULT if the copy itself fails. Other checks include a version check
221	of the compiled in user space version against the module version and a
222	mismatch results in a -EINVAL return. If the size field is greater than
223	the structure size then a path is assumed to be present and is checked to
224	ensure it begins with a "/" and is NULL terminated, otherwise -EINVAL is
225	returned. Following these checks, for all ioctl commands except
226	AUTOFS_DEV_IOCTL_VERSION_CMD, AUTOFS_DEV_IOCTL_OPENMOUNT_CMD and
227	AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD the ioctlfd is validated and if it is
228	not a valid descriptor or doesn't correspond to an autofs mount point
229	an error of -EBADF, -ENOTTY or -EINVAL (not an autofs descriptor) is
230	returned.
231	
232	
233	The ioctls
234	==========
235	
236	An example of an implementation which uses this interface can be seen
237	in autofs version 5.0.4 and later in file lib/dev-ioctl-lib.c of the
238	distribution tar available for download from kernel.org in directory
239	/pub/linux/daemons/autofs/v5.
240	
241	The device node ioctl operations implemented by this interface are:
242	
243	
244	AUTOFS_DEV_IOCTL_VERSION
245	------------------------
246	
247	Get the major and minor version of the autofs4 device ioctl kernel module
248	implementation. It requires an initialized struct autofs_dev_ioctl as an
249	input parameter and sets the version information in the passed in structure.
250	It returns 0 on success or the error -EINVAL if a version mismatch is
251	detected.
252	
253	
254	AUTOFS_DEV_IOCTL_PROTOVER_CMD and AUTOFS_DEV_IOCTL_PROTOSUBVER_CMD
255	------------------------------------------------------------------
256	
257	Get the major and minor version of the autofs4 protocol version understood
258	by loaded module. This call requires an initialized struct autofs_dev_ioctl
259	with the ioctlfd field set to a valid autofs mount point descriptor
260	and sets the requested version number in version field of struct args_protover
261	or sub_version field of struct args_protosubver. These commands return
262	0 on success or one of the negative error codes if validation fails.
263	
264	
265	AUTOFS_DEV_IOCTL_OPENMOUNT and AUTOFS_DEV_IOCTL_CLOSEMOUNT
266	----------------------------------------------------------
267	
268	Obtain and release a file descriptor for an autofs managed mount point
269	path. The open call requires an initialized struct autofs_dev_ioctl with
270	the path field set and the size field adjusted appropriately as well
271	as the devid field of struct args_openmount set to the device number of
272	the autofs mount. The device number can be obtained from the mount options
273	shown in /proc/mounts. The close call requires an initialized struct
274	autofs_dev_ioct with the ioctlfd field set to the descriptor obtained
275	from the open call. The release of the file descriptor can also be done
276	with close(2) so any open descriptors will also be closed at process exit.
277	The close call is included in the implemented operations largely for
278	completeness and to provide for a consistent user space implementation.
279	
280	
281	AUTOFS_DEV_IOCTL_READY_CMD and AUTOFS_DEV_IOCTL_FAIL_CMD
282	--------------------------------------------------------
283	
284	Return mount and expire result status from user space to the kernel.
285	Both of these calls require an initialized struct autofs_dev_ioctl
286	with the ioctlfd field set to the descriptor obtained from the open
287	call and the token field of struct args_ready or struct args_fail set
288	to the wait queue token number, received by user space in the foregoing
289	mount or expire request. The status field of struct args_fail is set to
290	the errno of the operation. It is set to 0 on success.
291	
292	
293	AUTOFS_DEV_IOCTL_SETPIPEFD_CMD
294	------------------------------
295	
296	Set the pipe file descriptor used for kernel communication to the daemon.
297	Normally this is set at mount time using an option but when reconnecting
298	to a existing mount we need to use this to tell the autofs mount about
299	the new kernel pipe descriptor. In order to protect mounts against
300	incorrectly setting the pipe descriptor we also require that the autofs
301	mount be catatonic (see next call).
302	
303	The call requires an initialized struct autofs_dev_ioctl with the
304	ioctlfd field set to the descriptor obtained from the open call and
305	the pipefd field of struct args_setpipefd set to descriptor of the pipe.
306	On success the call also sets the process group id used to identify the
307	controlling process (eg. the owning automount(8) daemon) to the process
308	group of the caller.
309	
310	
311	AUTOFS_DEV_IOCTL_CATATONIC_CMD
312	------------------------------
313	
314	Make the autofs mount point catatonic. The autofs mount will no longer
315	issue mount requests, the kernel communication pipe descriptor is released
316	and any remaining waits in the queue released.
317	
318	The call requires an initialized struct autofs_dev_ioctl with the
319	ioctlfd field set to the descriptor obtained from the open call.
320	
321	
322	AUTOFS_DEV_IOCTL_TIMEOUT_CMD
323	----------------------------
324	
325	Set the expire timeout for mounts within an autofs mount point.
326	
327	The call requires an initialized struct autofs_dev_ioctl with the
328	ioctlfd field set to the descriptor obtained from the open call.
329	
330	
331	AUTOFS_DEV_IOCTL_REQUESTER_CMD
332	------------------------------
333	
334	Return the uid and gid of the last process to successfully trigger a the
335	mount on the given path dentry.
336	
337	The call requires an initialized struct autofs_dev_ioctl with the path
338	field set to the mount point in question and the size field adjusted
339	appropriately. Upon return the uid field of struct args_requester contains
340	the uid and gid field the gid.
341	
342	When reconstructing an autofs mount tree with active mounts we need to
343	re-connect to mounts that may have used the original process uid and
344	gid (or string variations of them) for mount lookups within the map entry.
345	This call provides the ability to obtain this uid and gid so they may be
346	used by user space for the mount map lookups.
347	
348	
349	AUTOFS_DEV_IOCTL_EXPIRE_CMD
350	---------------------------
351	
352	Issue an expire request to the kernel for an autofs mount. Typically
353	this ioctl is called until no further expire candidates are found.
354	
355	The call requires an initialized struct autofs_dev_ioctl with the
356	ioctlfd field set to the descriptor obtained from the open call. In
357	addition an immediate expire, independent of the mount timeout, can be
358	requested by setting the how field of struct args_expire to 1. If no
359	expire candidates can be found the ioctl returns -1 with errno set to
360	EAGAIN.
361	
362	This call causes the kernel module to check the mount corresponding
363	to the given ioctlfd for mounts that can be expired, issues an expire
364	request back to the daemon and waits for completion.
365	
366	AUTOFS_DEV_IOCTL_ASKUMOUNT_CMD
367	------------------------------
368	
369	Checks if an autofs mount point is in use.
370	
371	The call requires an initialized struct autofs_dev_ioctl with the
372	ioctlfd field set to the descriptor obtained from the open call and
373	it returns the result in the may_umount field of struct args_askumount,
374	1 for busy and 0 otherwise.
375	
376	
377	AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD
378	---------------------------------
379	
380	Check if the given path is a mountpoint.
381	
382	The call requires an initialized struct autofs_dev_ioctl. There are two
383	possible variations. Both use the path field set to the path of the mount
384	point to check and the size field adjusted appropriately. One uses the
385	ioctlfd field to identify a specific mount point to check while the other
386	variation uses the path and optionally in.type field of struct args_ismountpoint
387	set to an autofs mount type. The call returns 1 if this is a mount point
388	and sets out.devid field to the device number of the mount and out.magic
389	field to the relevant super block magic number (described below) or 0 if
390	it isn't a mountpoint. In both cases the the device number (as returned
391	by new_encode_dev()) is returned in out.devid field.
392	
393	If supplied with a file descriptor we're looking for a specific mount,
394	not necessarily at the top of the mounted stack. In this case the path
395	the descriptor corresponds to is considered a mountpoint if it is itself
396	a mountpoint or contains a mount, such as a multi-mount without a root
397	mount. In this case we return 1 if the descriptor corresponds to a mount
398	point and and also returns the super magic of the covering mount if there
399	is one or 0 if it isn't a mountpoint.
400	
401	If a path is supplied (and the ioctlfd field is set to -1) then the path
402	is looked up and is checked to see if it is the root of a mount. If a
403	type is also given we are looking for a particular autofs mount and if
404	a match isn't found a fail is returned. If the the located path is the
405	root of a mount 1 is returned along with the super magic of the mount
406	or 0 otherwise.
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