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Documentation / filesystems / autofs4-mount-control.txt

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Based on kernel version 4.8. Page generated on 2016-10-06 23:15 EST.

2	Miscellaneous Device control operations for the autofs4 kernel module
3	====================================================================
5	The problem
6	===========
8	There is a problem with active restarts in autofs (that is to say
9	restarting autofs when there are busy mounts).
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.
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.
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.
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.
39	Here is a master map with direct and indirect map entries:
41	/-      /etc/auto.direct
42	/test   /etc/auto.indirect
44	and the corresponding map files:
46	/etc/auto.direct:
48	/automount/dparse/g6  budgie:/autofs/export1
49	/automount/dparse/g1  shark:/autofs/export1
50	and so on.
52	/etc/auto.indirect:
54	g1    shark:/autofs/export1
55	g6    budgie:/autofs/export1
56	and so on.
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.
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.
69	But, each entry in direct and indirect maps can have offsets (making
70	them multi-mount map entries).
72	For example, an indirect mount map entry could also be:
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
81	and a similarly a direct mount map entry could also be:
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
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.
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.
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.
113	The Solution
114	============
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.
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.
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.
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.
167	autofs4 Miscellaneous Device mount control interface
168	====================================================
170	The control interface is opening a device node, typically /dev/autofs.
172	All the ioctls use a common structure to pass the needed parameter
173	information and return operation results:
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 */
182		__u32 arg1;             /* Command parameters */
183		__u32 arg2;
185		char path[0];
186	};
188	The ioctlfd field is a mount point file descriptor of an autofs mount
189	point. It is returned by the open call and is used by all calls except
190	the check for whether a given path is a mount point, where it may
191	optionally be used to check a specific mount corresponding to a given
192	mount point file descriptor, and when requesting the uid and gid of the
193	last successful mount on a directory within the autofs file system.
195	The fields arg1 and arg2 are used to communicate parameters and results of
196	calls made as described below.
198	The path field is used to pass a path where it is needed and the size field
199	is used account for the increased structure length when translating the
200	structure sent from user space.
202	This structure can be initialized before setting specific fields by using
203	the void function call init_autofs_dev_ioctl(struct autofs_dev_ioctl *).
205	All of the ioctls perform a copy of this structure from user space to
206	kernel space and return -EINVAL if the size parameter is smaller than
207	the structure size itself, -ENOMEM if the kernel memory allocation fails
208	or -EFAULT if the copy itself fails. Other checks include a version check
209	of the compiled in user space version against the module version and a
210	mismatch results in a -EINVAL return. If the size field is greater than
211	the structure size then a path is assumed to be present and is checked to
212	ensure it begins with a "/" and is NULL terminated, otherwise -EINVAL is
213	returned. Following these checks, for all ioctl commands except
215	AUTOFS_DEV_IOCTL_CLOSEMOUNT_CMD the ioctlfd is validated and if it is
216	not a valid descriptor or doesn't correspond to an autofs mount point
217	an error of -EBADF, -ENOTTY or -EINVAL (not an autofs descriptor) is
218	returned.
221	The ioctls
222	==========
224	An example of an implementation which uses this interface can be seen
225	in autofs version 5.0.4 and later in file lib/dev-ioctl-lib.c of the
226	distribution tar available for download from kernel.org in directory
227	/pub/linux/daemons/autofs/v5.
229	The device node ioctl operations implemented by this interface are:
233	------------------------
235	Get the major and minor version of the autofs4 device ioctl kernel module
236	implementation. It requires an initialized struct autofs_dev_ioctl as an
237	input parameter and sets the version information in the passed in structure.
238	It returns 0 on success or the error -EINVAL if a version mismatch is
239	detected.
243	------------------------------------------------------------------
245	Get the major and minor version of the autofs4 protocol version understood
246	by loaded module. This call requires an initialized struct autofs_dev_ioctl
247	with the ioctlfd field set to a valid autofs mount point descriptor
248	and sets the requested version number in structure field arg1. These
249	commands return 0 on success or one of the negative error codes if
250	validation fails.
254	----------------------------------------------------------
256	Obtain and release a file descriptor for an autofs managed mount point
257	path. The open call requires an initialized struct autofs_dev_ioctl with
258	the path field set and the size field adjusted appropriately as well
259	as the arg1 field set to the device number of the autofs mount. The
260	device number can be obtained from the mount options shown in
261	/proc/mounts. The close call requires an initialized struct
262	autofs_dev_ioct with the ioctlfd field set to the descriptor obtained
263	from the open call. The release of the file descriptor can also be done
264	with close(2) so any open descriptors will also be closed at process exit.
265	The close call is included in the implemented operations largely for
266	completeness and to provide for a consistent user space implementation.
270	--------------------------------------------------------
272	Return mount and expire result status from user space to the kernel.
273	Both of these calls require an initialized struct autofs_dev_ioctl
274	with the ioctlfd field set to the descriptor obtained from the open
275	call and the arg1 field set to the wait queue token number, received
276	by user space in the foregoing mount or expire request. The arg2 field
277	is set to the status to be returned. For the ready call this is always
278	0 and for the fail call it is set to the errno of the operation.
282	------------------------------
284	Set the pipe file descriptor used for kernel communication to the daemon.
285	Normally this is set at mount time using an option but when reconnecting
286	to a existing mount we need to use this to tell the autofs mount about
287	the new kernel pipe descriptor. In order to protect mounts against
288	incorrectly setting the pipe descriptor we also require that the autofs
289	mount be catatonic (see next call).
291	The call requires an initialized struct autofs_dev_ioctl with the
292	ioctlfd field set to the descriptor obtained from the open call and
293	the arg1 field set to descriptor of the pipe. On success the call
294	also sets the process group id used to identify the controlling process
295	(eg. the owning automount(8) daemon) to the process group of the caller.
299	------------------------------
301	Make the autofs mount point catatonic. The autofs mount will no longer
302	issue mount requests, the kernel communication pipe descriptor is released
303	and any remaining waits in the queue released.
305	The call requires an initialized struct autofs_dev_ioctl with the
306	ioctlfd field set to the descriptor obtained from the open call.
310	----------------------------
312	Set the expire timeout for mounts within an autofs mount point.
314	The call requires an initialized struct autofs_dev_ioctl with the
315	ioctlfd field set to the descriptor obtained from the open call.
319	------------------------------
321	Return the uid and gid of the last process to successfully trigger a the
322	mount on the given path dentry.
324	The call requires an initialized struct autofs_dev_ioctl with the path
325	field set to the mount point in question and the size field adjusted
326	appropriately as well as the arg1 field set to the device number of the
327	containing autofs mount. Upon return the struct field arg1 contains the
328	uid and arg2 the gid.
330	When reconstructing an autofs mount tree with active mounts we need to
331	re-connect to mounts that may have used the original process uid and
332	gid (or string variations of them) for mount lookups within the map entry.
333	This call provides the ability to obtain this uid and gid so they may be
334	used by user space for the mount map lookups.
338	---------------------------
340	Issue an expire request to the kernel for an autofs mount. Typically
341	this ioctl is called until no further expire candidates are found.
343	The call requires an initialized struct autofs_dev_ioctl with the
344	ioctlfd field set to the descriptor obtained from the open call. In
345	addition an immediate expire, independent of the mount timeout, can be
346	requested by setting the arg1 field to 1. If no expire candidates can
347	be found the ioctl returns -1 with errno set to EAGAIN.
349	This call causes the kernel module to check the mount corresponding
350	to the given ioctlfd for mounts that can be expired, issues an expire
351	request back to the daemon and waits for completion.
354	------------------------------
356	Checks if an autofs mount point is in use.
358	The call requires an initialized struct autofs_dev_ioctl with the
359	ioctlfd field set to the descriptor obtained from the open call and
360	it returns the result in the arg1 field, 1 for busy and 0 otherwise.
364	---------------------------------
366	Check if the given path is a mountpoint.
368	The call requires an initialized struct autofs_dev_ioctl. There are two
369	possible variations. Both use the path field set to the path of the mount
370	point to check and the size field adjusted appropriately. One uses the
371	ioctlfd field to identify a specific mount point to check while the other
372	variation uses the path and optionally arg1 set to an autofs mount type.
373	The call returns 1 if this is a mount point and sets arg1 to the device
374	number of the mount and field arg2 to the relevant super block magic
375	number (described below) or 0 if it isn't a mountpoint. In both cases
376	the the device number (as returned by new_encode_dev()) is returned
377	in field arg1.
379	If supplied with a file descriptor we're looking for a specific mount,
380	not necessarily at the top of the mounted stack. In this case the path
381	the descriptor corresponds to is considered a mountpoint if it is itself
382	a mountpoint or contains a mount, such as a multi-mount without a root
383	mount. In this case we return 1 if the descriptor corresponds to a mount
384	point and and also returns the super magic of the covering mount if there
385	is one or 0 if it isn't a mountpoint.
387	If a path is supplied (and the ioctlfd field is set to -1) then the path
388	is looked up and is checked to see if it is the root of a mount. If a
389	type is also given we are looking for a particular autofs mount and if
390	a match isn't found a fail is returned. If the the located path is the
391	root of a mount 1 is returned along with the super magic of the mount
392	or 0 otherwise.
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