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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1				 ============================
2				 KERNEL KEY RETENTION SERVICE
3				 ============================
4	
5	This service allows cryptographic keys, authentication tokens, cross-domain
6	user mappings, and similar to be cached in the kernel for the use of
7	filesystems and other kernel services.
8	
9	Keyrings are permitted; these are a special type of key that can hold links to
10	other keys. Processes each have three standard keyring subscriptions that a
11	kernel service can search for relevant keys.
12	
13	The key service can be configured on by enabling:
14	
15		"Security options"/"Enable access key retention support" (CONFIG_KEYS)
16	
17	This document has the following sections:
18	
19		- Key overview
20		- Key service overview
21		- Key access permissions
22		- SELinux support
23		- New procfs files
24		- Userspace system call interface
25		- Kernel services
26		- Notes on accessing payload contents
27		- Defining a key type
28		- Request-key callback service
29		- Garbage collection
30	
31	
32	============
33	KEY OVERVIEW
34	============
35	
36	In this context, keys represent units of cryptographic data, authentication
37	tokens, keyrings, etc.. These are represented in the kernel by struct key.
38	
39	Each key has a number of attributes:
40	
41		- A serial number.
42		- A type.
43		- A description (for matching a key in a search).
44		- Access control information.
45		- An expiry time.
46		- A payload.
47		- State.
48	
49	
50	 (*) Each key is issued a serial number of type key_serial_t that is unique for
51	     the lifetime of that key. All serial numbers are positive non-zero 32-bit
52	     integers.
53	
54	     Userspace programs can use a key's serial numbers as a way to gain access
55	     to it, subject to permission checking.
56	
57	 (*) Each key is of a defined "type". Types must be registered inside the
58	     kernel by a kernel service (such as a filesystem) before keys of that type
59	     can be added or used. Userspace programs cannot define new types directly.
60	
61	     Key types are represented in the kernel by struct key_type. This defines a
62	     number of operations that can be performed on a key of that type.
63	
64	     Should a type be removed from the system, all the keys of that type will
65	     be invalidated.
66	
67	 (*) Each key has a description. This should be a printable string. The key
68	     type provides an operation to perform a match between the description on a
69	     key and a criterion string.
70	
71	 (*) Each key has an owner user ID, a group ID and a permissions mask. These
72	     are used to control what a process may do to a key from userspace, and
73	     whether a kernel service will be able to find the key.
74	
75	 (*) Each key can be set to expire at a specific time by the key type's
76	     instantiation function. Keys can also be immortal.
77	
78	 (*) Each key can have a payload. This is a quantity of data that represent the
79	     actual "key". In the case of a keyring, this is a list of keys to which
80	     the keyring links; in the case of a user-defined key, it's an arbitrary
81	     blob of data.
82	
83	     Having a payload is not required; and the payload can, in fact, just be a
84	     value stored in the struct key itself.
85	
86	     When a key is instantiated, the key type's instantiation function is
87	     called with a blob of data, and that then creates the key's payload in
88	     some way.
89	
90	     Similarly, when userspace wants to read back the contents of the key, if
91	     permitted, another key type operation will be called to convert the key's
92	     attached payload back into a blob of data.
93	
94	 (*) Each key can be in one of a number of basic states:
95	
96	     (*) Uninstantiated. The key exists, but does not have any data attached.
97	     	 Keys being requested from userspace will be in this state.
98	
99	     (*) Instantiated. This is the normal state. The key is fully formed, and
100		 has data attached.
101	
102	     (*) Negative. This is a relatively short-lived state. The key acts as a
103		 note saying that a previous call out to userspace failed, and acts as
104		 a throttle on key lookups. A negative key can be updated to a normal
105		 state.
106	
107	     (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,
108		 they traverse to this state. An expired key can be updated back to a
109		 normal state.
110	
111	     (*) Revoked. A key is put in this state by userspace action. It can't be
112		 found or operated upon (apart from by unlinking it).
113	
114	     (*) Dead. The key's type was unregistered, and so the key is now useless.
115	
116	Keys in the last three states are subject to garbage collection.  See the
117	section on "Garbage collection".
118	
119	
120	====================
121	KEY SERVICE OVERVIEW
122	====================
123	
124	The key service provides a number of features besides keys:
125	
126	 (*) The key service defines three special key types:
127	
128	     (+) "keyring"
129	
130		 Keyrings are special keys that contain a list of other keys. Keyring
131		 lists can be modified using various system calls. Keyrings should not
132		 be given a payload when created.
133	
134	     (+) "user"
135	
136		 A key of this type has a description and a payload that are arbitrary
137		 blobs of data. These can be created, updated and read by userspace,
138		 and aren't intended for use by kernel services.
139	
140	     (+) "logon"
141	
142		 Like a "user" key, a "logon" key has a payload that is an arbitrary
143		 blob of data. It is intended as a place to store secrets which are
144		 accessible to the kernel but not to userspace programs.
145	
146		 The description can be arbitrary, but must be prefixed with a non-zero
147		 length string that describes the key "subclass". The subclass is
148		 separated from the rest of the description by a ':'. "logon" keys can
149		 be created and updated from userspace, but the payload is only
150		 readable from kernel space.
151	
152	 (*) Each process subscribes to three keyrings: a thread-specific keyring, a
153	     process-specific keyring, and a session-specific keyring.
154	
155	     The thread-specific keyring is discarded from the child when any sort of
156	     clone, fork, vfork or execve occurs. A new keyring is created only when
157	     required.
158	
159	     The process-specific keyring is replaced with an empty one in the child on
160	     clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
161	     shared. execve also discards the process's process keyring and creates a
162	     new one.
163	
164	     The session-specific keyring is persistent across clone, fork, vfork and
165	     execve, even when the latter executes a set-UID or set-GID binary. A
166	     process can, however, replace its current session keyring with a new one
167	     by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
168	     new one, or to attempt to create or join one of a specific name.
169	
170	     The ownership of the thread keyring changes when the real UID and GID of
171	     the thread changes.
172	
173	 (*) Each user ID resident in the system holds two special keyrings: a user
174	     specific keyring and a default user session keyring. The default session
175	     keyring is initialised with a link to the user-specific keyring.
176	
177	     When a process changes its real UID, if it used to have no session key, it
178	     will be subscribed to the default session key for the new UID.
179	
180	     If a process attempts to access its session key when it doesn't have one,
181	     it will be subscribed to the default for its current UID.
182	
183	 (*) Each user has two quotas against which the keys they own are tracked. One
184	     limits the total number of keys and keyrings, the other limits the total
185	     amount of description and payload space that can be consumed.
186	
187	     The user can view information on this and other statistics through procfs
188	     files.  The root user may also alter the quota limits through sysctl files
189	     (see the section "New procfs files").
190	
191	     Process-specific and thread-specific keyrings are not counted towards a
192	     user's quota.
193	
194	     If a system call that modifies a key or keyring in some way would put the
195	     user over quota, the operation is refused and error EDQUOT is returned.
196	
197	 (*) There's a system call interface by which userspace programs can create and
198	     manipulate keys and keyrings.
199	
200	 (*) There's a kernel interface by which services can register types and search
201	     for keys.
202	
203	 (*) There's a way for the a search done from the kernel to call back to
204	     userspace to request a key that can't be found in a process's keyrings.
205	
206	 (*) An optional filesystem is available through which the key database can be
207	     viewed and manipulated.
208	
209	
210	======================
211	KEY ACCESS PERMISSIONS
212	======================
213	
214	Keys have an owner user ID, a group access ID, and a permissions mask. The mask
215	has up to eight bits each for possessor, user, group and other access. Only
216	six of each set of eight bits are defined. These permissions granted are:
217	
218	 (*) View
219	
220	     This permits a key or keyring's attributes to be viewed - including key
221	     type and description.
222	
223	 (*) Read
224	
225	     This permits a key's payload to be viewed or a keyring's list of linked
226	     keys.
227	
228	 (*) Write
229	
230	     This permits a key's payload to be instantiated or updated, or it allows a
231	     link to be added to or removed from a keyring.
232	
233	 (*) Search
234	
235	     This permits keyrings to be searched and keys to be found. Searches can
236	     only recurse into nested keyrings that have search permission set.
237	
238	 (*) Link
239	
240	     This permits a key or keyring to be linked to. To create a link from a
241	     keyring to a key, a process must have Write permission on the keyring and
242	     Link permission on the key.
243	
244	 (*) Set Attribute
245	
246	     This permits a key's UID, GID and permissions mask to be changed.
247	
248	For changing the ownership, group ID or permissions mask, being the owner of
249	the key or having the sysadmin capability is sufficient.
250	
251	
252	===============
253	SELINUX SUPPORT
254	===============
255	
256	The security class "key" has been added to SELinux so that mandatory access
257	controls can be applied to keys created within various contexts.  This support
258	is preliminary, and is likely to change quite significantly in the near future.
259	Currently, all of the basic permissions explained above are provided in SELinux
260	as well; SELinux is simply invoked after all basic permission checks have been
261	performed.
262	
263	The value of the file /proc/self/attr/keycreate influences the labeling of
264	newly-created keys.  If the contents of that file correspond to an SELinux
265	security context, then the key will be assigned that context.  Otherwise, the
266	key will be assigned the current context of the task that invoked the key
267	creation request.  Tasks must be granted explicit permission to assign a
268	particular context to newly-created keys, using the "create" permission in the
269	key security class.
270	
271	The default keyrings associated with users will be labeled with the default
272	context of the user if and only if the login programs have been instrumented to
273	properly initialize keycreate during the login process.  Otherwise, they will
274	be labeled with the context of the login program itself.
275	
276	Note, however, that the default keyrings associated with the root user are
277	labeled with the default kernel context, since they are created early in the
278	boot process, before root has a chance to log in.
279	
280	The keyrings associated with new threads are each labeled with the context of
281	their associated thread, and both session and process keyrings are handled
282	similarly.
283	
284	
285	================
286	NEW PROCFS FILES
287	================
288	
289	Two files have been added to procfs by which an administrator can find out
290	about the status of the key service:
291	
292	 (*) /proc/keys
293	
294	     This lists the keys that are currently viewable by the task reading the
295	     file, giving information about their type, description and permissions.
296	     It is not possible to view the payload of the key this way, though some
297	     information about it may be given.
298	
299	     The only keys included in the list are those that grant View permission to
300	     the reading process whether or not it possesses them.  Note that LSM
301	     security checks are still performed, and may further filter out keys that
302	     the current process is not authorised to view.
303	
304	     The contents of the file look like this:
305	
306		SERIAL   FLAGS  USAGE EXPY PERM     UID   GID   TYPE      DESCRIPTION: SUMMARY
307		00000001 I-----    39 perm 1f3f0000     0     0 keyring   _uid_ses.0: 1/4
308		00000002 I-----     2 perm 1f3f0000     0     0 keyring   _uid.0: empty
309		00000007 I-----     1 perm 1f3f0000     0     0 keyring   _pid.1: empty
310		0000018d I-----     1 perm 1f3f0000     0     0 keyring   _pid.412: empty
311		000004d2 I--Q--     1 perm 1f3f0000    32    -1 keyring   _uid.32: 1/4
312		000004d3 I--Q--     3 perm 1f3f0000    32    -1 keyring   _uid_ses.32: empty
313		00000892 I--QU-     1 perm 1f000000     0     0 user      metal:copper: 0
314		00000893 I--Q-N     1  35s 1f3f0000     0     0 user      metal:silver: 0
315		00000894 I--Q--     1  10h 003f0000     0     0 user      metal:gold: 0
316	
317	     The flags are:
318	
319		I	Instantiated
320		R	Revoked
321		D	Dead
322		Q	Contributes to user's quota
323		U	Under construction by callback to userspace
324		N	Negative key
325	
326	     This file must be enabled at kernel configuration time as it allows anyone
327	     to list the keys database.
328	
329	 (*) /proc/key-users
330	
331	     This file lists the tracking data for each user that has at least one key
332	     on the system.  Such data includes quota information and statistics:
333	
334		[root@andromeda root]# cat /proc/key-users
335		0:     46 45/45 1/100 13/10000
336		29:     2 2/2 2/100 40/10000
337		32:     2 2/2 2/100 40/10000
338		38:     2 2/2 2/100 40/10000
339	
340	     The format of each line is
341		<UID>:			User ID to which this applies
342		<usage>			Structure refcount
343		<inst>/<keys>		Total number of keys and number instantiated
344		<keys>/<max>		Key count quota
345		<bytes>/<max>		Key size quota
346	
347	
348	Four new sysctl files have been added also for the purpose of controlling the
349	quota limits on keys:
350	
351	 (*) /proc/sys/kernel/keys/root_maxkeys
352	     /proc/sys/kernel/keys/root_maxbytes
353	
354	     These files hold the maximum number of keys that root may have and the
355	     maximum total number of bytes of data that root may have stored in those
356	     keys.
357	
358	 (*) /proc/sys/kernel/keys/maxkeys
359	     /proc/sys/kernel/keys/maxbytes
360	
361	     These files hold the maximum number of keys that each non-root user may
362	     have and the maximum total number of bytes of data that each of those
363	     users may have stored in their keys.
364	
365	Root may alter these by writing each new limit as a decimal number string to
366	the appropriate file.
367	
368	
369	===============================
370	USERSPACE SYSTEM CALL INTERFACE
371	===============================
372	
373	Userspace can manipulate keys directly through three new syscalls: add_key,
374	request_key and keyctl. The latter provides a number of functions for
375	manipulating keys.
376	
377	When referring to a key directly, userspace programs should use the key's
378	serial number (a positive 32-bit integer). However, there are some special
379	values available for referring to special keys and keyrings that relate to the
380	process making the call:
381	
382		CONSTANT			VALUE	KEY REFERENCED
383		==============================	======	===========================
384		KEY_SPEC_THREAD_KEYRING		-1	thread-specific keyring
385		KEY_SPEC_PROCESS_KEYRING	-2	process-specific keyring
386		KEY_SPEC_SESSION_KEYRING	-3	session-specific keyring
387		KEY_SPEC_USER_KEYRING		-4	UID-specific keyring
388		KEY_SPEC_USER_SESSION_KEYRING	-5	UID-session keyring
389		KEY_SPEC_GROUP_KEYRING		-6	GID-specific keyring
390		KEY_SPEC_REQKEY_AUTH_KEY	-7	assumed request_key()
391							  authorisation key
392	
393	
394	The main syscalls are:
395	
396	 (*) Create a new key of given type, description and payload and add it to the
397	     nominated keyring:
398	
399		key_serial_t add_key(const char *type, const char *desc,
400				     const void *payload, size_t plen,
401				     key_serial_t keyring);
402	
403	     If a key of the same type and description as that proposed already exists
404	     in the keyring, this will try to update it with the given payload, or it
405	     will return error EEXIST if that function is not supported by the key
406	     type. The process must also have permission to write to the key to be able
407	     to update it. The new key will have all user permissions granted and no
408	     group or third party permissions.
409	
410	     Otherwise, this will attempt to create a new key of the specified type and
411	     description, and to instantiate it with the supplied payload and attach it
412	     to the keyring. In this case, an error will be generated if the process
413	     does not have permission to write to the keyring.
414	
415	     If the key type supports it, if the description is NULL or an empty
416	     string, the key type will try and generate a description from the content
417	     of the payload.
418	
419	     The payload is optional, and the pointer can be NULL if not required by
420	     the type. The payload is plen in size, and plen can be zero for an empty
421	     payload.
422	
423	     A new keyring can be generated by setting type "keyring", the keyring name
424	     as the description (or NULL) and setting the payload to NULL.
425	
426	     User defined keys can be created by specifying type "user". It is
427	     recommended that a user defined key's description by prefixed with a type
428	     ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
429	     ticket.
430	
431	     Any other type must have been registered with the kernel in advance by a
432	     kernel service such as a filesystem.
433	
434	     The ID of the new or updated key is returned if successful.
435	
436	
437	 (*) Search the process's keyrings for a key, potentially calling out to
438	     userspace to create it.
439	
440		key_serial_t request_key(const char *type, const char *description,
441					 const char *callout_info,
442					 key_serial_t dest_keyring);
443	
444	     This function searches all the process's keyrings in the order thread,
445	     process, session for a matching key. This works very much like
446	     KEYCTL_SEARCH, including the optional attachment of the discovered key to
447	     a keyring.
448	
449	     If a key cannot be found, and if callout_info is not NULL, then
450	     /sbin/request-key will be invoked in an attempt to obtain a key. The
451	     callout_info string will be passed as an argument to the program.
452	
453	     See also Documentation/security/keys-request-key.txt.
454	
455	
456	The keyctl syscall functions are:
457	
458	 (*) Map a special key ID to a real key ID for this process:
459	
460		key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
461				    int create);
462	
463	     The special key specified by "id" is looked up (with the key being created
464	     if necessary) and the ID of the key or keyring thus found is returned if
465	     it exists.
466	
467	     If the key does not yet exist, the key will be created if "create" is
468	     non-zero; and the error ENOKEY will be returned if "create" is zero.
469	
470	
471	 (*) Replace the session keyring this process subscribes to with a new one:
472	
473		key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
474	
475	     If name is NULL, an anonymous keyring is created attached to the process
476	     as its session keyring, displacing the old session keyring.
477	
478	     If name is not NULL, if a keyring of that name exists, the process
479	     attempts to attach it as the session keyring, returning an error if that
480	     is not permitted; otherwise a new keyring of that name is created and
481	     attached as the session keyring.
482	
483	     To attach to a named keyring, the keyring must have search permission for
484	     the process's ownership.
485	
486	     The ID of the new session keyring is returned if successful.
487	
488	
489	 (*) Update the specified key:
490	
491		long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
492			    size_t plen);
493	
494	     This will try to update the specified key with the given payload, or it
495	     will return error EOPNOTSUPP if that function is not supported by the key
496	     type. The process must also have permission to write to the key to be able
497	     to update it.
498	
499	     The payload is of length plen, and may be absent or empty as for
500	     add_key().
501	
502	
503	 (*) Revoke a key:
504	
505		long keyctl(KEYCTL_REVOKE, key_serial_t key);
506	
507	     This makes a key unavailable for further operations. Further attempts to
508	     use the key will be met with error EKEYREVOKED, and the key will no longer
509	     be findable.
510	
511	
512	 (*) Change the ownership of a key:
513	
514		long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
515	
516	     This function permits a key's owner and group ID to be changed. Either one
517	     of uid or gid can be set to -1 to suppress that change.
518	
519	     Only the superuser can change a key's owner to something other than the
520	     key's current owner. Similarly, only the superuser can change a key's
521	     group ID to something other than the calling process's group ID or one of
522	     its group list members.
523	
524	
525	 (*) Change the permissions mask on a key:
526	
527		long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
528	
529	     This function permits the owner of a key or the superuser to change the
530	     permissions mask on a key.
531	
532	     Only bits the available bits are permitted; if any other bits are set,
533	     error EINVAL will be returned.
534	
535	
536	 (*) Describe a key:
537	
538		long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
539			    size_t buflen);
540	
541	     This function returns a summary of the key's attributes (but not its
542	     payload data) as a string in the buffer provided.
543	
544	     Unless there's an error, it always returns the amount of data it could
545	     produce, even if that's too big for the buffer, but it won't copy more
546	     than requested to userspace. If the buffer pointer is NULL then no copy
547	     will take place.
548	
549	     A process must have view permission on the key for this function to be
550	     successful.
551	
552	     If successful, a string is placed in the buffer in the following format:
553	
554		<type>;<uid>;<gid>;<perm>;<description>
555	
556	     Where type and description are strings, uid and gid are decimal, and perm
557	     is hexadecimal. A NUL character is included at the end of the string if
558	     the buffer is sufficiently big.
559	
560	     This can be parsed with
561	
562		sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
563	
564	
565	 (*) Clear out a keyring:
566	
567		long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
568	
569	     This function clears the list of keys attached to a keyring. The calling
570	     process must have write permission on the keyring, and it must be a
571	     keyring (or else error ENOTDIR will result).
572	
573	     This function can also be used to clear special kernel keyrings if they
574	     are appropriately marked if the user has CAP_SYS_ADMIN capability.  The
575	     DNS resolver cache keyring is an example of this.
576	
577	
578	 (*) Link a key into a keyring:
579	
580		long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
581	
582	     This function creates a link from the keyring to the key. The process must
583	     have write permission on the keyring and must have link permission on the
584	     key.
585	
586	     Should the keyring not be a keyring, error ENOTDIR will result; and if the
587	     keyring is full, error ENFILE will result.
588	
589	     The link procedure checks the nesting of the keyrings, returning ELOOP if
590	     it appears too deep or EDEADLK if the link would introduce a cycle.
591	
592	     Any links within the keyring to keys that match the new key in terms of
593	     type and description will be discarded from the keyring as the new one is
594	     added.
595	
596	
597	 (*) Unlink a key or keyring from another keyring:
598	
599		long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
600	
601	     This function looks through the keyring for the first link to the
602	     specified key, and removes it if found. Subsequent links to that key are
603	     ignored. The process must have write permission on the keyring.
604	
605	     If the keyring is not a keyring, error ENOTDIR will result; and if the key
606	     is not present, error ENOENT will be the result.
607	
608	
609	 (*) Search a keyring tree for a key:
610	
611		key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
612				    const char *type, const char *description,
613				    key_serial_t dest_keyring);
614	
615	     This searches the keyring tree headed by the specified keyring until a key
616	     is found that matches the type and description criteria. Each keyring is
617	     checked for keys before recursion into its children occurs.
618	
619	     The process must have search permission on the top level keyring, or else
620	     error EACCES will result. Only keyrings that the process has search
621	     permission on will be recursed into, and only keys and keyrings for which
622	     a process has search permission can be matched. If the specified keyring
623	     is not a keyring, ENOTDIR will result.
624	
625	     If the search succeeds, the function will attempt to link the found key
626	     into the destination keyring if one is supplied (non-zero ID). All the
627	     constraints applicable to KEYCTL_LINK apply in this case too.
628	
629	     Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
630	     fails. On success, the resulting key ID will be returned.
631	
632	
633	 (*) Read the payload data from a key:
634	
635		long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
636			    size_t buflen);
637	
638	     This function attempts to read the payload data from the specified key
639	     into the buffer. The process must have read permission on the key to
640	     succeed.
641	
642	     The returned data will be processed for presentation by the key type. For
643	     instance, a keyring will return an array of key_serial_t entries
644	     representing the IDs of all the keys to which it is subscribed. The user
645	     defined key type will return its data as is. If a key type does not
646	     implement this function, error EOPNOTSUPP will result.
647	
648	     As much of the data as can be fitted into the buffer will be copied to
649	     userspace if the buffer pointer is not NULL.
650	
651	     On a successful return, the function will always return the amount of data
652	     available rather than the amount copied.
653	
654	
655	 (*) Instantiate a partially constructed key.
656	
657		long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
658			    const void *payload, size_t plen,
659			    key_serial_t keyring);
660		long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
661			    const struct iovec *payload_iov, unsigned ioc,
662			    key_serial_t keyring);
663	
664	     If the kernel calls back to userspace to complete the instantiation of a
665	     key, userspace should use this call to supply data for the key before the
666	     invoked process returns, or else the key will be marked negative
667	     automatically.
668	
669	     The process must have write access on the key to be able to instantiate
670	     it, and the key must be uninstantiated.
671	
672	     If a keyring is specified (non-zero), the key will also be linked into
673	     that keyring, however all the constraints applying in KEYCTL_LINK apply in
674	     this case too.
675	
676	     The payload and plen arguments describe the payload data as for add_key().
677	
678	     The payload_iov and ioc arguments describe the payload data in an iovec
679	     array instead of a single buffer.
680	
681	
682	 (*) Negatively instantiate a partially constructed key.
683	
684		long keyctl(KEYCTL_NEGATE, key_serial_t key,
685			    unsigned timeout, key_serial_t keyring);
686		long keyctl(KEYCTL_REJECT, key_serial_t key,
687			    unsigned timeout, unsigned error, key_serial_t keyring);
688	
689	     If the kernel calls back to userspace to complete the instantiation of a
690	     key, userspace should use this call mark the key as negative before the
691	     invoked process returns if it is unable to fulfill the request.
692	
693	     The process must have write access on the key to be able to instantiate
694	     it, and the key must be uninstantiated.
695	
696	     If a keyring is specified (non-zero), the key will also be linked into
697	     that keyring, however all the constraints applying in KEYCTL_LINK apply in
698	     this case too.
699	
700	     If the key is rejected, future searches for it will return the specified
701	     error code until the rejected key expires.  Negating the key is the same
702	     as rejecting the key with ENOKEY as the error code.
703	
704	
705	 (*) Set the default request-key destination keyring.
706	
707		long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
708	
709	     This sets the default keyring to which implicitly requested keys will be
710	     attached for this thread. reqkey_defl should be one of these constants:
711	
712		CONSTANT				VALUE	NEW DEFAULT KEYRING
713		======================================	======	=======================
714		KEY_REQKEY_DEFL_NO_CHANGE		-1	No change
715		KEY_REQKEY_DEFL_DEFAULT			0	Default[1]
716		KEY_REQKEY_DEFL_THREAD_KEYRING		1	Thread keyring
717		KEY_REQKEY_DEFL_PROCESS_KEYRING		2	Process keyring
718		KEY_REQKEY_DEFL_SESSION_KEYRING		3	Session keyring
719		KEY_REQKEY_DEFL_USER_KEYRING		4	User keyring
720		KEY_REQKEY_DEFL_USER_SESSION_KEYRING	5	User session keyring
721		KEY_REQKEY_DEFL_GROUP_KEYRING		6	Group keyring
722	
723	     The old default will be returned if successful and error EINVAL will be
724	     returned if reqkey_defl is not one of the above values.
725	
726	     The default keyring can be overridden by the keyring indicated to the
727	     request_key() system call.
728	
729	     Note that this setting is inherited across fork/exec.
730	
731	     [1] The default is: the thread keyring if there is one, otherwise
732	     the process keyring if there is one, otherwise the session keyring if
733	     there is one, otherwise the user default session keyring.
734	
735	
736	 (*) Set the timeout on a key.
737	
738		long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
739	
740	     This sets or clears the timeout on a key. The timeout can be 0 to clear
741	     the timeout or a number of seconds to set the expiry time that far into
742	     the future.
743	
744	     The process must have attribute modification access on a key to set its
745	     timeout. Timeouts may not be set with this function on negative, revoked
746	     or expired keys.
747	
748	
749	 (*) Assume the authority granted to instantiate a key
750	
751		long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
752	
753	     This assumes or divests the authority required to instantiate the
754	     specified key. Authority can only be assumed if the thread has the
755	     authorisation key associated with the specified key in its keyrings
756	     somewhere.
757	
758	     Once authority is assumed, searches for keys will also search the
759	     requester's keyrings using the requester's security label, UID, GID and
760	     groups.
761	
762	     If the requested authority is unavailable, error EPERM will be returned,
763	     likewise if the authority has been revoked because the target key is
764	     already instantiated.
765	
766	     If the specified key is 0, then any assumed authority will be divested.
767	
768	     The assumed authoritative key is inherited across fork and exec.
769	
770	
771	 (*) Get the LSM security context attached to a key.
772	
773		long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
774			    size_t buflen)
775	
776	     This function returns a string that represents the LSM security context
777	     attached to a key in the buffer provided.
778	
779	     Unless there's an error, it always returns the amount of data it could
780	     produce, even if that's too big for the buffer, but it won't copy more
781	     than requested to userspace. If the buffer pointer is NULL then no copy
782	     will take place.
783	
784	     A NUL character is included at the end of the string if the buffer is
785	     sufficiently big.  This is included in the returned count.  If no LSM is
786	     in force then an empty string will be returned.
787	
788	     A process must have view permission on the key for this function to be
789	     successful.
790	
791	
792	 (*) Install the calling process's session keyring on its parent.
793	
794		long keyctl(KEYCTL_SESSION_TO_PARENT);
795	
796	     This functions attempts to install the calling process's session keyring
797	     on to the calling process's parent, replacing the parent's current session
798	     keyring.
799	
800	     The calling process must have the same ownership as its parent, the
801	     keyring must have the same ownership as the calling process, the calling
802	     process must have LINK permission on the keyring and the active LSM module
803	     mustn't deny permission, otherwise error EPERM will be returned.
804	
805	     Error ENOMEM will be returned if there was insufficient memory to complete
806	     the operation, otherwise 0 will be returned to indicate success.
807	
808	     The keyring will be replaced next time the parent process leaves the
809	     kernel and resumes executing userspace.
810	
811	
812	 (*) Invalidate a key.
813	
814		long keyctl(KEYCTL_INVALIDATE, key_serial_t key);
815	
816	     This function marks a key as being invalidated and then wakes up the
817	     garbage collector.  The garbage collector immediately removes invalidated
818	     keys from all keyrings and deletes the key when its reference count
819	     reaches zero.
820	
821	     Keys that are marked invalidated become invisible to normal key operations
822	     immediately, though they are still visible in /proc/keys until deleted
823	     (they're marked with an 'i' flag).
824	
825	     A process must have search permission on the key for this function to be
826	     successful.
827	
828	
829	===============
830	KERNEL SERVICES
831	===============
832	
833	The kernel services for key management are fairly simple to deal with. They can
834	be broken down into two areas: keys and key types.
835	
836	Dealing with keys is fairly straightforward. Firstly, the kernel service
837	registers its type, then it searches for a key of that type. It should retain
838	the key as long as it has need of it, and then it should release it. For a
839	filesystem or device file, a search would probably be performed during the open
840	call, and the key released upon close. How to deal with conflicting keys due to
841	two different users opening the same file is left to the filesystem author to
842	solve.
843	
844	To access the key manager, the following header must be #included:
845	
846		<linux/key.h>
847	
848	Specific key types should have a header file under include/keys/ that should be
849	used to access that type.  For keys of type "user", for example, that would be:
850	
851		<keys/user-type.h>
852	
853	Note that there are two different types of pointers to keys that may be
854	encountered:
855	
856	 (*) struct key *
857	
858	     This simply points to the key structure itself. Key structures will be at
859	     least four-byte aligned.
860	
861	 (*) key_ref_t
862	
863	     This is equivalent to a struct key *, but the least significant bit is set
864	     if the caller "possesses" the key. By "possession" it is meant that the
865	     calling processes has a searchable link to the key from one of its
866	     keyrings. There are three functions for dealing with these:
867	
868		key_ref_t make_key_ref(const struct key *key, bool possession);
869	
870		struct key *key_ref_to_ptr(const key_ref_t key_ref);
871	
872		bool is_key_possessed(const key_ref_t key_ref);
873	
874	     The first function constructs a key reference from a key pointer and
875	     possession information (which must be true or false).
876	
877	     The second function retrieves the key pointer from a reference and the
878	     third retrieves the possession flag.
879	
880	When accessing a key's payload contents, certain precautions must be taken to
881	prevent access vs modification races. See the section "Notes on accessing
882	payload contents" for more information.
883	
884	(*) To search for a key, call:
885	
886		struct key *request_key(const struct key_type *type,
887					const char *description,
888					const char *callout_info);
889	
890	    This is used to request a key or keyring with a description that matches
891	    the description specified according to the key type's match function. This
892	    permits approximate matching to occur. If callout_string is not NULL, then
893	    /sbin/request-key will be invoked in an attempt to obtain the key from
894	    userspace. In that case, callout_string will be passed as an argument to
895	    the program.
896	
897	    Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
898	    returned.
899	
900	    If successful, the key will have been attached to the default keyring for
901	    implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
902	
903	    See also Documentation/security/keys-request-key.txt.
904	
905	
906	(*) To search for a key, passing auxiliary data to the upcaller, call:
907	
908		struct key *request_key_with_auxdata(const struct key_type *type,
909						     const char *description,
910						     const void *callout_info,
911						     size_t callout_len,
912						     void *aux);
913	
914	    This is identical to request_key(), except that the auxiliary data is
915	    passed to the key_type->request_key() op if it exists, and the callout_info
916	    is a blob of length callout_len, if given (the length may be 0).
917	
918	
919	(*) A key can be requested asynchronously by calling one of:
920	
921		struct key *request_key_async(const struct key_type *type,
922					      const char *description,
923					      const void *callout_info,
924					      size_t callout_len);
925	
926	    or:
927	
928		struct key *request_key_async_with_auxdata(const struct key_type *type,
929							   const char *description,
930							   const char *callout_info,
931						     	   size_t callout_len,
932						     	   void *aux);
933	
934	    which are asynchronous equivalents of request_key() and
935	    request_key_with_auxdata() respectively.
936	
937	    These two functions return with the key potentially still under
938	    construction.  To wait for construction completion, the following should be
939	    called:
940	
941		int wait_for_key_construction(struct key *key, bool intr);
942	
943	    The function will wait for the key to finish being constructed and then
944	    invokes key_validate() to return an appropriate value to indicate the state
945	    of the key (0 indicates the key is usable).
946	
947	    If intr is true, then the wait can be interrupted by a signal, in which
948	    case error ERESTARTSYS will be returned.
949	
950	
951	(*) When it is no longer required, the key should be released using:
952	
953		void key_put(struct key *key);
954	
955	    Or:
956	
957		void key_ref_put(key_ref_t key_ref);
958	
959	    These can be called from interrupt context. If CONFIG_KEYS is not set then
960	    the argument will not be parsed.
961	
962	
963	(*) Extra references can be made to a key by calling one of the following
964	    functions:
965	
966		struct key *__key_get(struct key *key);
967		struct key *key_get(struct key *key);
968	
969	    Keys so references will need to be disposed of by calling key_put() when
970	    they've been finished with.  The key pointer passed in will be returned.
971	
972	    In the case of key_get(), if the pointer is NULL or CONFIG_KEYS is not set
973	    then the key will not be dereferenced and no increment will take place.
974	
975	
976	(*) A key's serial number can be obtained by calling:
977	
978		key_serial_t key_serial(struct key *key);
979	
980	    If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
981	    latter case without parsing the argument).
982	
983	
984	(*) If a keyring was found in the search, this can be further searched by:
985	
986		key_ref_t keyring_search(key_ref_t keyring_ref,
987					 const struct key_type *type,
988					 const char *description)
989	
990	    This searches the keyring tree specified for a matching key. Error ENOKEY
991	    is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
992	    the returned key will need to be released.
993	
994	    The possession attribute from the keyring reference is used to control
995	    access through the permissions mask and is propagated to the returned key
996	    reference pointer if successful.
997	
998	
999	(*) A keyring can be created by:
1000	
1001		struct key *keyring_alloc(const char *description, uid_t uid, gid_t gid,
1002					  const struct cred *cred,
1003					  key_perm_t perm,
1004					  unsigned long flags,
1005					  struct key *dest);
1006	
1007	    This creates a keyring with the given attributes and returns it.  If dest
1008	    is not NULL, the new keyring will be linked into the keyring to which it
1009	    points.  No permission checks are made upon the destination keyring.
1010	
1011	    Error EDQUOT can be returned if the keyring would overload the quota (pass
1012	    KEY_ALLOC_NOT_IN_QUOTA in flags if the keyring shouldn't be accounted
1013	    towards the user's quota).  Error ENOMEM can also be returned.
1014	
1015	
1016	(*) To check the validity of a key, this function can be called:
1017	
1018		int validate_key(struct key *key);
1019	
1020	    This checks that the key in question hasn't expired or and hasn't been
1021	    revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
1022	    be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
1023	    returned (in the latter case without parsing the argument).
1024	
1025	
1026	(*) To register a key type, the following function should be called:
1027	
1028		int register_key_type(struct key_type *type);
1029	
1030	    This will return error EEXIST if a type of the same name is already
1031	    present.
1032	
1033	
1034	(*) To unregister a key type, call:
1035	
1036		void unregister_key_type(struct key_type *type);
1037	
1038	
1039	Under some circumstances, it may be desirable to deal with a bundle of keys.
1040	The facility provides access to the keyring type for managing such a bundle:
1041	
1042		struct key_type key_type_keyring;
1043	
1044	This can be used with a function such as request_key() to find a specific
1045	keyring in a process's keyrings.  A keyring thus found can then be searched
1046	with keyring_search().  Note that it is not possible to use request_key() to
1047	search a specific keyring, so using keyrings in this way is of limited utility.
1048	
1049	
1050	===================================
1051	NOTES ON ACCESSING PAYLOAD CONTENTS
1052	===================================
1053	
1054	The simplest payload is just a number in key->payload.value. In this case,
1055	there's no need to indulge in RCU or locking when accessing the payload.
1056	
1057	More complex payload contents must be allocated and a pointer to them set in
1058	key->payload.data. One of the following ways must be selected to access the
1059	data:
1060	
1061	 (1) Unmodifiable key type.
1062	
1063	     If the key type does not have a modify method, then the key's payload can
1064	     be accessed without any form of locking, provided that it's known to be
1065	     instantiated (uninstantiated keys cannot be "found").
1066	
1067	 (2) The key's semaphore.
1068	
1069	     The semaphore could be used to govern access to the payload and to control
1070	     the payload pointer. It must be write-locked for modifications and would
1071	     have to be read-locked for general access. The disadvantage of doing this
1072	     is that the accessor may be required to sleep.
1073	
1074	 (3) RCU.
1075	
1076	     RCU must be used when the semaphore isn't already held; if the semaphore
1077	     is held then the contents can't change under you unexpectedly as the
1078	     semaphore must still be used to serialise modifications to the key. The
1079	     key management code takes care of this for the key type.
1080	
1081	     However, this means using:
1082	
1083		rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
1084	
1085	     to read the pointer, and:
1086	
1087		rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
1088	
1089	     to set the pointer and dispose of the old contents after a grace period.
1090	     Note that only the key type should ever modify a key's payload.
1091	
1092	     Furthermore, an RCU controlled payload must hold a struct rcu_head for the
1093	     use of call_rcu() and, if the payload is of variable size, the length of
1094	     the payload. key->datalen cannot be relied upon to be consistent with the
1095	     payload just dereferenced if the key's semaphore is not held.
1096	
1097	
1098	===================
1099	DEFINING A KEY TYPE
1100	===================
1101	
1102	A kernel service may want to define its own key type. For instance, an AFS
1103	filesystem might want to define a Kerberos 5 ticket key type. To do this, it
1104	author fills in a key_type struct and registers it with the system.
1105	
1106	Source files that implement key types should include the following header file:
1107	
1108		<linux/key-type.h>
1109	
1110	The structure has a number of fields, some of which are mandatory:
1111	
1112	 (*) const char *name
1113	
1114	     The name of the key type. This is used to translate a key type name
1115	     supplied by userspace into a pointer to the structure.
1116	
1117	
1118	 (*) size_t def_datalen
1119	
1120	     This is optional - it supplies the default payload data length as
1121	     contributed to the quota. If the key type's payload is always or almost
1122	     always the same size, then this is a more efficient way to do things.
1123	
1124	     The data length (and quota) on a particular key can always be changed
1125	     during instantiation or update by calling:
1126	
1127		int key_payload_reserve(struct key *key, size_t datalen);
1128	
1129	     With the revised data length. Error EDQUOT will be returned if this is not
1130	     viable.
1131	
1132	
1133	 (*) int (*vet_description)(const char *description);
1134	
1135	     This optional method is called to vet a key description.  If the key type
1136	     doesn't approve of the key description, it may return an error, otherwise
1137	     it should return 0.
1138	
1139	
1140	 (*) int (*preparse)(struct key_preparsed_payload *prep);
1141	
1142	     This optional method permits the key type to attempt to parse payload
1143	     before a key is created (add key) or the key semaphore is taken (update or
1144	     instantiate key).  The structure pointed to by prep looks like:
1145	
1146		struct key_preparsed_payload {
1147			char		*description;
1148			void		*type_data[2];
1149			void		*payload;
1150			const void	*data;
1151			size_t		datalen;
1152			size_t		quotalen;
1153		};
1154	
1155	     Before calling the method, the caller will fill in data and datalen with
1156	     the payload blob parameters; quotalen will be filled in with the default
1157	     quota size from the key type and the rest will be cleared.
1158	
1159	     If a description can be proposed from the payload contents, that should be
1160	     attached as a string to the description field.  This will be used for the
1161	     key description if the caller of add_key() passes NULL or "".
1162	
1163	     The method can attach anything it likes to type_data[] and payload.  These
1164	     are merely passed along to the instantiate() or update() operations.
1165	
1166	     The method should return 0 if success ful or a negative error code
1167	     otherwise.
1168	
1169	     
1170	 (*) void (*free_preparse)(struct key_preparsed_payload *prep);
1171	
1172	     This method is only required if the preparse() method is provided,
1173	     otherwise it is unused.  It cleans up anything attached to the
1174	     description, type_data and payload fields of the key_preparsed_payload
1175	     struct as filled in by the preparse() method.
1176	
1177	
1178	 (*) int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);
1179	
1180	     This method is called to attach a payload to a key during construction.
1181	     The payload attached need not bear any relation to the data passed to this
1182	     function.
1183	
1184	     The prep->data and prep->datalen fields will define the original payload
1185	     blob.  If preparse() was supplied then other fields may be filled in also.
1186	
1187	     If the amount of data attached to the key differs from the size in
1188	     keytype->def_datalen, then key_payload_reserve() should be called.
1189	
1190	     This method does not have to lock the key in order to attach a payload.
1191	     The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1192	     anything else from gaining access to the key.
1193	
1194	     It is safe to sleep in this method.
1195	
1196	
1197	 (*) int (*update)(struct key *key, const void *data, size_t datalen);
1198	
1199	     If this type of key can be updated, then this method should be provided.
1200	     It is called to update a key's payload from the blob of data provided.
1201	
1202	     The prep->data and prep->datalen fields will define the original payload
1203	     blob.  If preparse() was supplied then other fields may be filled in also.
1204	
1205	     key_payload_reserve() should be called if the data length might change
1206	     before any changes are actually made. Note that if this succeeds, the type
1207	     is committed to changing the key because it's already been altered, so all
1208	     memory allocation must be done first.
1209	
1210	     The key will have its semaphore write-locked before this method is called,
1211	     but this only deters other writers; any changes to the key's payload must
1212	     be made under RCU conditions, and call_rcu() must be used to dispose of
1213	     the old payload.
1214	
1215	     key_payload_reserve() should be called before the changes are made, but
1216	     after all allocations and other potentially failing function calls are
1217	     made.
1218	
1219	     It is safe to sleep in this method.
1220	
1221	
1222	 (*) int (*match)(const struct key *key, const void *desc);
1223	
1224	     This method is called to match a key against a description. It should
1225	     return non-zero if the two match, zero if they don't.
1226	
1227	     This method should not need to lock the key in any way. The type and
1228	     description can be considered invariant, and the payload should not be
1229	     accessed (the key may not yet be instantiated).
1230	
1231	     It is not safe to sleep in this method; the caller may hold spinlocks.
1232	
1233	
1234	 (*) void (*revoke)(struct key *key);
1235	
1236	     This method is optional.  It is called to discard part of the payload
1237	     data upon a key being revoked.  The caller will have the key semaphore
1238	     write-locked.
1239	
1240	     It is safe to sleep in this method, though care should be taken to avoid
1241	     a deadlock against the key semaphore.
1242	
1243	
1244	 (*) void (*destroy)(struct key *key);
1245	
1246	     This method is optional. It is called to discard the payload data on a key
1247	     when it is being destroyed.
1248	
1249	     This method does not need to lock the key to access the payload; it can
1250	     consider the key as being inaccessible at this time. Note that the key's
1251	     type may have been changed before this function is called.
1252	
1253	     It is not safe to sleep in this method; the caller may hold spinlocks.
1254	
1255	
1256	 (*) void (*describe)(const struct key *key, struct seq_file *p);
1257	
1258	     This method is optional. It is called during /proc/keys reading to
1259	     summarise a key's description and payload in text form.
1260	
1261	     This method will be called with the RCU read lock held. rcu_dereference()
1262	     should be used to read the payload pointer if the payload is to be
1263	     accessed. key->datalen cannot be trusted to stay consistent with the
1264	     contents of the payload.
1265	
1266	     The description will not change, though the key's state may.
1267	
1268	     It is not safe to sleep in this method; the RCU read lock is held by the
1269	     caller.
1270	
1271	
1272	 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
1273	
1274	     This method is optional. It is called by KEYCTL_READ to translate the
1275	     key's payload into something a blob of data for userspace to deal with.
1276	     Ideally, the blob should be in the same format as that passed in to the
1277	     instantiate and update methods.
1278	
1279	     If successful, the blob size that could be produced should be returned
1280	     rather than the size copied.
1281	
1282	     This method will be called with the key's semaphore read-locked. This will
1283	     prevent the key's payload changing. It is not necessary to use RCU locking
1284	     when accessing the key's payload. It is safe to sleep in this method, such
1285	     as might happen when the userspace buffer is accessed.
1286	
1287	
1288	 (*) int (*request_key)(struct key_construction *cons, const char *op,
1289				void *aux);
1290	
1291	     This method is optional.  If provided, request_key() and friends will
1292	     invoke this function rather than upcalling to /sbin/request-key to operate
1293	     upon a key of this type.
1294	
1295	     The aux parameter is as passed to request_key_async_with_auxdata() and
1296	     similar or is NULL otherwise.  Also passed are the construction record for
1297	     the key to be operated upon and the operation type (currently only
1298	     "create").
1299	
1300	     This method is permitted to return before the upcall is complete, but the
1301	     following function must be called under all circumstances to complete the
1302	     instantiation process, whether or not it succeeds, whether or not there's
1303	     an error:
1304	
1305		void complete_request_key(struct key_construction *cons, int error);
1306	
1307	     The error parameter should be 0 on success, -ve on error.  The
1308	     construction record is destroyed by this action and the authorisation key
1309	     will be revoked.  If an error is indicated, the key under construction
1310	     will be negatively instantiated if it wasn't already instantiated.
1311	
1312	     If this method returns an error, that error will be returned to the
1313	     caller of request_key*().  complete_request_key() must be called prior to
1314	     returning.
1315	
1316	     The key under construction and the authorisation key can be found in the
1317	     key_construction struct pointed to by cons:
1318	
1319	     (*) struct key *key;
1320	
1321	     	 The key under construction.
1322	
1323	     (*) struct key *authkey;
1324	
1325	     	 The authorisation key.
1326	
1327	
1328	============================
1329	REQUEST-KEY CALLBACK SERVICE
1330	============================
1331	
1332	To create a new key, the kernel will attempt to execute the following command
1333	line:
1334	
1335		/sbin/request-key create <key> <uid> <gid> \
1336			<threadring> <processring> <sessionring> <callout_info>
1337	
1338	<key> is the key being constructed, and the three keyrings are the process
1339	keyrings from the process that caused the search to be issued. These are
1340	included for two reasons:
1341	
1342	  (1) There may be an authentication token in one of the keyrings that is
1343	      required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1344	
1345	  (2) The new key should probably be cached in one of these rings.
1346	
1347	This program should set it UID and GID to those specified before attempting to
1348	access any more keys. It may then look around for a user specific process to
1349	hand the request off to (perhaps a path held in placed in another key by, for
1350	example, the KDE desktop manager).
1351	
1352	The program (or whatever it calls) should finish construction of the key by
1353	calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
1354	cache the key in one of the keyrings (probably the session ring) before
1355	returning.  Alternatively, the key can be marked as negative with KEYCTL_NEGATE
1356	or KEYCTL_REJECT; this also permits the key to be cached in one of the
1357	keyrings.
1358	
1359	If it returns with the key remaining in the unconstructed state, the key will
1360	be marked as being negative, it will be added to the session keyring, and an
1361	error will be returned to the key requestor.
1362	
1363	Supplementary information may be provided from whoever or whatever invoked this
1364	service. This will be passed as the <callout_info> parameter. If no such
1365	information was made available, then "-" will be passed as this parameter
1366	instead.
1367	
1368	
1369	Similarly, the kernel may attempt to update an expired or a soon to expire key
1370	by executing:
1371	
1372		/sbin/request-key update <key> <uid> <gid> \
1373			<threadring> <processring> <sessionring>
1374	
1375	In this case, the program isn't required to actually attach the key to a ring;
1376	the rings are provided for reference.
1377	
1378	
1379	==================
1380	GARBAGE COLLECTION
1381	==================
1382	
1383	Dead keys (for which the type has been removed) will be automatically unlinked
1384	from those keyrings that point to them and deleted as soon as possible by a
1385	background garbage collector.
1386	
1387	Similarly, revoked and expired keys will be garbage collected, but only after a
1388	certain amount of time has passed.  This time is set as a number of seconds in:
1389	
1390		/proc/sys/kernel/keys/gc_delay
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