xref: /openbmc/linux/ipc/util.c (revision e6dec923)
1 /*
2  * linux/ipc/util.c
3  * Copyright (C) 1992 Krishna Balasubramanian
4  *
5  * Sep 1997 - Call suser() last after "normal" permission checks so we
6  *            get BSD style process accounting right.
7  *            Occurs in several places in the IPC code.
8  *            Chris Evans, <chris@ferret.lmh.ox.ac.uk>
9  * Nov 1999 - ipc helper functions, unified SMP locking
10  *	      Manfred Spraul <manfred@colorfullife.com>
11  * Oct 2002 - One lock per IPC id. RCU ipc_free for lock-free grow_ary().
12  *            Mingming Cao <cmm@us.ibm.com>
13  * Mar 2006 - support for audit of ipc object properties
14  *            Dustin Kirkland <dustin.kirkland@us.ibm.com>
15  * Jun 2006 - namespaces ssupport
16  *            OpenVZ, SWsoft Inc.
17  *            Pavel Emelianov <xemul@openvz.org>
18  *
19  * General sysv ipc locking scheme:
20  *	rcu_read_lock()
21  *          obtain the ipc object (kern_ipc_perm) by looking up the id in an idr
22  *	    tree.
23  *	    - perform initial checks (capabilities, auditing and permission,
24  *	      etc).
25  *	    - perform read-only operations, such as STAT, INFO commands.
26  *	      acquire the ipc lock (kern_ipc_perm.lock) through
27  *	      ipc_lock_object()
28  *		- perform data updates, such as SET, RMID commands and
29  *		  mechanism-specific operations (semop/semtimedop,
30  *		  msgsnd/msgrcv, shmat/shmdt).
31  *	    drop the ipc lock, through ipc_unlock_object().
32  *	rcu_read_unlock()
33  *
34  *  The ids->rwsem must be taken when:
35  *	- creating, removing and iterating the existing entries in ipc
36  *	  identifier sets.
37  *	- iterating through files under /proc/sysvipc/
38  *
39  *  Note that sems have a special fast path that avoids kern_ipc_perm.lock -
40  *  see sem_lock().
41  */
42 
43 #include <linux/mm.h>
44 #include <linux/shm.h>
45 #include <linux/init.h>
46 #include <linux/msg.h>
47 #include <linux/vmalloc.h>
48 #include <linux/slab.h>
49 #include <linux/notifier.h>
50 #include <linux/capability.h>
51 #include <linux/highuid.h>
52 #include <linux/security.h>
53 #include <linux/rcupdate.h>
54 #include <linux/workqueue.h>
55 #include <linux/seq_file.h>
56 #include <linux/proc_fs.h>
57 #include <linux/audit.h>
58 #include <linux/nsproxy.h>
59 #include <linux/rwsem.h>
60 #include <linux/memory.h>
61 #include <linux/ipc_namespace.h>
62 
63 #include <asm/unistd.h>
64 
65 #include "util.h"
66 
67 struct ipc_proc_iface {
68 	const char *path;
69 	const char *header;
70 	int ids;
71 	int (*show)(struct seq_file *, void *);
72 };
73 
74 /**
75  * ipc_init - initialise ipc subsystem
76  *
77  * The various sysv ipc resources (semaphores, messages and shared
78  * memory) are initialised.
79  *
80  * A callback routine is registered into the memory hotplug notifier
81  * chain: since msgmni scales to lowmem this callback routine will be
82  * called upon successful memory add / remove to recompute msmgni.
83  */
84 static int __init ipc_init(void)
85 {
86 	sem_init();
87 	msg_init();
88 	shm_init();
89 	return 0;
90 }
91 device_initcall(ipc_init);
92 
93 /**
94  * ipc_init_ids	- initialise ipc identifiers
95  * @ids: ipc identifier set
96  *
97  * Set up the sequence range to use for the ipc identifier range (limited
98  * below IPCMNI) then initialise the ids idr.
99  */
100 void ipc_init_ids(struct ipc_ids *ids)
101 {
102 	ids->in_use = 0;
103 	ids->seq = 0;
104 	ids->next_id = -1;
105 	init_rwsem(&ids->rwsem);
106 	idr_init(&ids->ipcs_idr);
107 }
108 
109 #ifdef CONFIG_PROC_FS
110 static const struct file_operations sysvipc_proc_fops;
111 /**
112  * ipc_init_proc_interface -  create a proc interface for sysipc types using a seq_file interface.
113  * @path: Path in procfs
114  * @header: Banner to be printed at the beginning of the file.
115  * @ids: ipc id table to iterate.
116  * @show: show routine.
117  */
118 void __init ipc_init_proc_interface(const char *path, const char *header,
119 		int ids, int (*show)(struct seq_file *, void *))
120 {
121 	struct proc_dir_entry *pde;
122 	struct ipc_proc_iface *iface;
123 
124 	iface = kmalloc(sizeof(*iface), GFP_KERNEL);
125 	if (!iface)
126 		return;
127 	iface->path	= path;
128 	iface->header	= header;
129 	iface->ids	= ids;
130 	iface->show	= show;
131 
132 	pde = proc_create_data(path,
133 			       S_IRUGO,        /* world readable */
134 			       NULL,           /* parent dir */
135 			       &sysvipc_proc_fops,
136 			       iface);
137 	if (!pde)
138 		kfree(iface);
139 }
140 #endif
141 
142 /**
143  * ipc_findkey	- find a key in an ipc identifier set
144  * @ids: ipc identifier set
145  * @key: key to find
146  *
147  * Returns the locked pointer to the ipc structure if found or NULL
148  * otherwise. If key is found ipc points to the owning ipc structure
149  *
150  * Called with ipc_ids.rwsem held.
151  */
152 static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
153 {
154 	struct kern_ipc_perm *ipc;
155 	int next_id;
156 	int total;
157 
158 	for (total = 0, next_id = 0; total < ids->in_use; next_id++) {
159 		ipc = idr_find(&ids->ipcs_idr, next_id);
160 
161 		if (ipc == NULL)
162 			continue;
163 
164 		if (ipc->key != key) {
165 			total++;
166 			continue;
167 		}
168 
169 		rcu_read_lock();
170 		ipc_lock_object(ipc);
171 		return ipc;
172 	}
173 
174 	return NULL;
175 }
176 
177 /**
178  * ipc_get_maxid - get the last assigned id
179  * @ids: ipc identifier set
180  *
181  * Called with ipc_ids.rwsem held.
182  */
183 int ipc_get_maxid(struct ipc_ids *ids)
184 {
185 	struct kern_ipc_perm *ipc;
186 	int max_id = -1;
187 	int total, id;
188 
189 	if (ids->in_use == 0)
190 		return -1;
191 
192 	if (ids->in_use == IPCMNI)
193 		return IPCMNI - 1;
194 
195 	/* Look for the last assigned id */
196 	total = 0;
197 	for (id = 0; id < IPCMNI && total < ids->in_use; id++) {
198 		ipc = idr_find(&ids->ipcs_idr, id);
199 		if (ipc != NULL) {
200 			max_id = id;
201 			total++;
202 		}
203 	}
204 	return max_id;
205 }
206 
207 /**
208  * ipc_addid - add an ipc identifier
209  * @ids: ipc identifier set
210  * @new: new ipc permission set
211  * @size: limit for the number of used ids
212  *
213  * Add an entry 'new' to the ipc ids idr. The permissions object is
214  * initialised and the first free entry is set up and the id assigned
215  * is returned. The 'new' entry is returned in a locked state on success.
216  * On failure the entry is not locked and a negative err-code is returned.
217  *
218  * Called with writer ipc_ids.rwsem held.
219  */
220 int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
221 {
222 	kuid_t euid;
223 	kgid_t egid;
224 	int id;
225 	int next_id = ids->next_id;
226 
227 	if (size > IPCMNI)
228 		size = IPCMNI;
229 
230 	if (ids->in_use >= size)
231 		return -ENOSPC;
232 
233 	idr_preload(GFP_KERNEL);
234 
235 	atomic_set(&new->refcount, 1);
236 	spin_lock_init(&new->lock);
237 	new->deleted = false;
238 	rcu_read_lock();
239 	spin_lock(&new->lock);
240 
241 	current_euid_egid(&euid, &egid);
242 	new->cuid = new->uid = euid;
243 	new->gid = new->cgid = egid;
244 
245 	id = idr_alloc(&ids->ipcs_idr, new,
246 		       (next_id < 0) ? 0 : ipcid_to_idx(next_id), 0,
247 		       GFP_NOWAIT);
248 	idr_preload_end();
249 	if (id < 0) {
250 		spin_unlock(&new->lock);
251 		rcu_read_unlock();
252 		return id;
253 	}
254 
255 	ids->in_use++;
256 
257 	if (next_id < 0) {
258 		new->seq = ids->seq++;
259 		if (ids->seq > IPCID_SEQ_MAX)
260 			ids->seq = 0;
261 	} else {
262 		new->seq = ipcid_to_seqx(next_id);
263 		ids->next_id = -1;
264 	}
265 
266 	new->id = ipc_buildid(id, new->seq);
267 	return id;
268 }
269 
270 /**
271  * ipcget_new -	create a new ipc object
272  * @ns: ipc namespace
273  * @ids: ipc identifier set
274  * @ops: the actual creation routine to call
275  * @params: its parameters
276  *
277  * This routine is called by sys_msgget, sys_semget() and sys_shmget()
278  * when the key is IPC_PRIVATE.
279  */
280 static int ipcget_new(struct ipc_namespace *ns, struct ipc_ids *ids,
281 		const struct ipc_ops *ops, struct ipc_params *params)
282 {
283 	int err;
284 
285 	down_write(&ids->rwsem);
286 	err = ops->getnew(ns, params);
287 	up_write(&ids->rwsem);
288 	return err;
289 }
290 
291 /**
292  * ipc_check_perms - check security and permissions for an ipc object
293  * @ns: ipc namespace
294  * @ipcp: ipc permission set
295  * @ops: the actual security routine to call
296  * @params: its parameters
297  *
298  * This routine is called by sys_msgget(), sys_semget() and sys_shmget()
299  * when the key is not IPC_PRIVATE and that key already exists in the
300  * ds IDR.
301  *
302  * On success, the ipc id is returned.
303  *
304  * It is called with ipc_ids.rwsem and ipcp->lock held.
305  */
306 static int ipc_check_perms(struct ipc_namespace *ns,
307 			   struct kern_ipc_perm *ipcp,
308 			   const struct ipc_ops *ops,
309 			   struct ipc_params *params)
310 {
311 	int err;
312 
313 	if (ipcperms(ns, ipcp, params->flg))
314 		err = -EACCES;
315 	else {
316 		err = ops->associate(ipcp, params->flg);
317 		if (!err)
318 			err = ipcp->id;
319 	}
320 
321 	return err;
322 }
323 
324 /**
325  * ipcget_public - get an ipc object or create a new one
326  * @ns: ipc namespace
327  * @ids: ipc identifier set
328  * @ops: the actual creation routine to call
329  * @params: its parameters
330  *
331  * This routine is called by sys_msgget, sys_semget() and sys_shmget()
332  * when the key is not IPC_PRIVATE.
333  * It adds a new entry if the key is not found and does some permission
334  * / security checkings if the key is found.
335  *
336  * On success, the ipc id is returned.
337  */
338 static int ipcget_public(struct ipc_namespace *ns, struct ipc_ids *ids,
339 		const struct ipc_ops *ops, struct ipc_params *params)
340 {
341 	struct kern_ipc_perm *ipcp;
342 	int flg = params->flg;
343 	int err;
344 
345 	/*
346 	 * Take the lock as a writer since we are potentially going to add
347 	 * a new entry + read locks are not "upgradable"
348 	 */
349 	down_write(&ids->rwsem);
350 	ipcp = ipc_findkey(ids, params->key);
351 	if (ipcp == NULL) {
352 		/* key not used */
353 		if (!(flg & IPC_CREAT))
354 			err = -ENOENT;
355 		else
356 			err = ops->getnew(ns, params);
357 	} else {
358 		/* ipc object has been locked by ipc_findkey() */
359 
360 		if (flg & IPC_CREAT && flg & IPC_EXCL)
361 			err = -EEXIST;
362 		else {
363 			err = 0;
364 			if (ops->more_checks)
365 				err = ops->more_checks(ipcp, params);
366 			if (!err)
367 				/*
368 				 * ipc_check_perms returns the IPC id on
369 				 * success
370 				 */
371 				err = ipc_check_perms(ns, ipcp, ops, params);
372 		}
373 		ipc_unlock(ipcp);
374 	}
375 	up_write(&ids->rwsem);
376 
377 	return err;
378 }
379 
380 
381 /**
382  * ipc_rmid - remove an ipc identifier
383  * @ids: ipc identifier set
384  * @ipcp: ipc perm structure containing the identifier to remove
385  *
386  * ipc_ids.rwsem (as a writer) and the spinlock for this ID are held
387  * before this function is called, and remain locked on the exit.
388  */
389 void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)
390 {
391 	int lid = ipcid_to_idx(ipcp->id);
392 
393 	idr_remove(&ids->ipcs_idr, lid);
394 	ids->in_use--;
395 	ipcp->deleted = true;
396 }
397 
398 int ipc_rcu_getref(struct kern_ipc_perm *ptr)
399 {
400 	return atomic_inc_not_zero(&ptr->refcount);
401 }
402 
403 void ipc_rcu_putref(struct kern_ipc_perm *ptr,
404 			void (*func)(struct rcu_head *head))
405 {
406 	if (!atomic_dec_and_test(&ptr->refcount))
407 		return;
408 
409 	call_rcu(&ptr->rcu, func);
410 }
411 
412 /**
413  * ipcperms - check ipc permissions
414  * @ns: ipc namespace
415  * @ipcp: ipc permission set
416  * @flag: desired permission set
417  *
418  * Check user, group, other permissions for access
419  * to ipc resources. return 0 if allowed
420  *
421  * @flag will most probably be 0 or ``S_...UGO`` from <linux/stat.h>
422  */
423 int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flag)
424 {
425 	kuid_t euid = current_euid();
426 	int requested_mode, granted_mode;
427 
428 	audit_ipc_obj(ipcp);
429 	requested_mode = (flag >> 6) | (flag >> 3) | flag;
430 	granted_mode = ipcp->mode;
431 	if (uid_eq(euid, ipcp->cuid) ||
432 	    uid_eq(euid, ipcp->uid))
433 		granted_mode >>= 6;
434 	else if (in_group_p(ipcp->cgid) || in_group_p(ipcp->gid))
435 		granted_mode >>= 3;
436 	/* is there some bit set in requested_mode but not in granted_mode? */
437 	if ((requested_mode & ~granted_mode & 0007) &&
438 	    !ns_capable(ns->user_ns, CAP_IPC_OWNER))
439 		return -1;
440 
441 	return security_ipc_permission(ipcp, flag);
442 }
443 
444 /*
445  * Functions to convert between the kern_ipc_perm structure and the
446  * old/new ipc_perm structures
447  */
448 
449 /**
450  * kernel_to_ipc64_perm	- convert kernel ipc permissions to user
451  * @in: kernel permissions
452  * @out: new style ipc permissions
453  *
454  * Turn the kernel object @in into a set of permissions descriptions
455  * for returning to userspace (@out).
456  */
457 void kernel_to_ipc64_perm(struct kern_ipc_perm *in, struct ipc64_perm *out)
458 {
459 	out->key	= in->key;
460 	out->uid	= from_kuid_munged(current_user_ns(), in->uid);
461 	out->gid	= from_kgid_munged(current_user_ns(), in->gid);
462 	out->cuid	= from_kuid_munged(current_user_ns(), in->cuid);
463 	out->cgid	= from_kgid_munged(current_user_ns(), in->cgid);
464 	out->mode	= in->mode;
465 	out->seq	= in->seq;
466 }
467 
468 /**
469  * ipc64_perm_to_ipc_perm - convert new ipc permissions to old
470  * @in: new style ipc permissions
471  * @out: old style ipc permissions
472  *
473  * Turn the new style permissions object @in into a compatibility
474  * object and store it into the @out pointer.
475  */
476 void ipc64_perm_to_ipc_perm(struct ipc64_perm *in, struct ipc_perm *out)
477 {
478 	out->key	= in->key;
479 	SET_UID(out->uid, in->uid);
480 	SET_GID(out->gid, in->gid);
481 	SET_UID(out->cuid, in->cuid);
482 	SET_GID(out->cgid, in->cgid);
483 	out->mode	= in->mode;
484 	out->seq	= in->seq;
485 }
486 
487 /**
488  * ipc_obtain_object
489  * @ids: ipc identifier set
490  * @id: ipc id to look for
491  *
492  * Look for an id in the ipc ids idr and return associated ipc object.
493  *
494  * Call inside the RCU critical section.
495  * The ipc object is *not* locked on exit.
496  */
497 struct kern_ipc_perm *ipc_obtain_object_idr(struct ipc_ids *ids, int id)
498 {
499 	struct kern_ipc_perm *out;
500 	int lid = ipcid_to_idx(id);
501 
502 	out = idr_find(&ids->ipcs_idr, lid);
503 	if (!out)
504 		return ERR_PTR(-EINVAL);
505 
506 	return out;
507 }
508 
509 /**
510  * ipc_lock - lock an ipc structure without rwsem held
511  * @ids: ipc identifier set
512  * @id: ipc id to look for
513  *
514  * Look for an id in the ipc ids idr and lock the associated ipc object.
515  *
516  * The ipc object is locked on successful exit.
517  */
518 struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)
519 {
520 	struct kern_ipc_perm *out;
521 
522 	rcu_read_lock();
523 	out = ipc_obtain_object_idr(ids, id);
524 	if (IS_ERR(out))
525 		goto err;
526 
527 	spin_lock(&out->lock);
528 
529 	/*
530 	 * ipc_rmid() may have already freed the ID while ipc_lock()
531 	 * was spinning: here verify that the structure is still valid.
532 	 * Upon races with RMID, return -EIDRM, thus indicating that
533 	 * the ID points to a removed identifier.
534 	 */
535 	if (ipc_valid_object(out))
536 		return out;
537 
538 	spin_unlock(&out->lock);
539 	out = ERR_PTR(-EIDRM);
540 err:
541 	rcu_read_unlock();
542 	return out;
543 }
544 
545 /**
546  * ipc_obtain_object_check
547  * @ids: ipc identifier set
548  * @id: ipc id to look for
549  *
550  * Similar to ipc_obtain_object_idr() but also checks
551  * the ipc object reference counter.
552  *
553  * Call inside the RCU critical section.
554  * The ipc object is *not* locked on exit.
555  */
556 struct kern_ipc_perm *ipc_obtain_object_check(struct ipc_ids *ids, int id)
557 {
558 	struct kern_ipc_perm *out = ipc_obtain_object_idr(ids, id);
559 
560 	if (IS_ERR(out))
561 		goto out;
562 
563 	if (ipc_checkid(out, id))
564 		return ERR_PTR(-EINVAL);
565 out:
566 	return out;
567 }
568 
569 /**
570  * ipcget - Common sys_*get() code
571  * @ns: namespace
572  * @ids: ipc identifier set
573  * @ops: operations to be called on ipc object creation, permission checks
574  *       and further checks
575  * @params: the parameters needed by the previous operations.
576  *
577  * Common routine called by sys_msgget(), sys_semget() and sys_shmget().
578  */
579 int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
580 			const struct ipc_ops *ops, struct ipc_params *params)
581 {
582 	if (params->key == IPC_PRIVATE)
583 		return ipcget_new(ns, ids, ops, params);
584 	else
585 		return ipcget_public(ns, ids, ops, params);
586 }
587 
588 /**
589  * ipc_update_perm - update the permissions of an ipc object
590  * @in:  the permission given as input.
591  * @out: the permission of the ipc to set.
592  */
593 int ipc_update_perm(struct ipc64_perm *in, struct kern_ipc_perm *out)
594 {
595 	kuid_t uid = make_kuid(current_user_ns(), in->uid);
596 	kgid_t gid = make_kgid(current_user_ns(), in->gid);
597 	if (!uid_valid(uid) || !gid_valid(gid))
598 		return -EINVAL;
599 
600 	out->uid = uid;
601 	out->gid = gid;
602 	out->mode = (out->mode & ~S_IRWXUGO)
603 		| (in->mode & S_IRWXUGO);
604 
605 	return 0;
606 }
607 
608 /**
609  * ipcctl_pre_down_nolock - retrieve an ipc and check permissions for some IPC_XXX cmd
610  * @ns:  ipc namespace
611  * @ids:  the table of ids where to look for the ipc
612  * @id:   the id of the ipc to retrieve
613  * @cmd:  the cmd to check
614  * @perm: the permission to set
615  * @extra_perm: one extra permission parameter used by msq
616  *
617  * This function does some common audit and permissions check for some IPC_XXX
618  * cmd and is called from semctl_down, shmctl_down and msgctl_down.
619  * It must be called without any lock held and:
620  *
621  *   - retrieves the ipc with the given id in the given table.
622  *   - performs some audit and permission check, depending on the given cmd
623  *   - returns a pointer to the ipc object or otherwise, the corresponding
624  *     error.
625  *
626  * Call holding the both the rwsem and the rcu read lock.
627  */
628 struct kern_ipc_perm *ipcctl_pre_down_nolock(struct ipc_namespace *ns,
629 					struct ipc_ids *ids, int id, int cmd,
630 					struct ipc64_perm *perm, int extra_perm)
631 {
632 	kuid_t euid;
633 	int err = -EPERM;
634 	struct kern_ipc_perm *ipcp;
635 
636 	ipcp = ipc_obtain_object_check(ids, id);
637 	if (IS_ERR(ipcp)) {
638 		err = PTR_ERR(ipcp);
639 		goto err;
640 	}
641 
642 	audit_ipc_obj(ipcp);
643 	if (cmd == IPC_SET)
644 		audit_ipc_set_perm(extra_perm, perm->uid,
645 				   perm->gid, perm->mode);
646 
647 	euid = current_euid();
648 	if (uid_eq(euid, ipcp->cuid) || uid_eq(euid, ipcp->uid)  ||
649 	    ns_capable(ns->user_ns, CAP_SYS_ADMIN))
650 		return ipcp; /* successful lookup */
651 err:
652 	return ERR_PTR(err);
653 }
654 
655 #ifdef CONFIG_ARCH_WANT_IPC_PARSE_VERSION
656 
657 
658 /**
659  * ipc_parse_version - ipc call version
660  * @cmd: pointer to command
661  *
662  * Return IPC_64 for new style IPC and IPC_OLD for old style IPC.
663  * The @cmd value is turned from an encoding command and version into
664  * just the command code.
665  */
666 int ipc_parse_version(int *cmd)
667 {
668 	if (*cmd & IPC_64) {
669 		*cmd ^= IPC_64;
670 		return IPC_64;
671 	} else {
672 		return IPC_OLD;
673 	}
674 }
675 
676 #endif /* CONFIG_ARCH_WANT_IPC_PARSE_VERSION */
677 
678 #ifdef CONFIG_PROC_FS
679 struct ipc_proc_iter {
680 	struct ipc_namespace *ns;
681 	struct ipc_proc_iface *iface;
682 };
683 
684 /*
685  * This routine locks the ipc structure found at least at position pos.
686  */
687 static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
688 					      loff_t *new_pos)
689 {
690 	struct kern_ipc_perm *ipc;
691 	int total, id;
692 
693 	total = 0;
694 	for (id = 0; id < pos && total < ids->in_use; id++) {
695 		ipc = idr_find(&ids->ipcs_idr, id);
696 		if (ipc != NULL)
697 			total++;
698 	}
699 
700 	if (total >= ids->in_use)
701 		return NULL;
702 
703 	for (; pos < IPCMNI; pos++) {
704 		ipc = idr_find(&ids->ipcs_idr, pos);
705 		if (ipc != NULL) {
706 			*new_pos = pos + 1;
707 			rcu_read_lock();
708 			ipc_lock_object(ipc);
709 			return ipc;
710 		}
711 	}
712 
713 	/* Out of range - return NULL to terminate iteration */
714 	return NULL;
715 }
716 
717 static void *sysvipc_proc_next(struct seq_file *s, void *it, loff_t *pos)
718 {
719 	struct ipc_proc_iter *iter = s->private;
720 	struct ipc_proc_iface *iface = iter->iface;
721 	struct kern_ipc_perm *ipc = it;
722 
723 	/* If we had an ipc id locked before, unlock it */
724 	if (ipc && ipc != SEQ_START_TOKEN)
725 		ipc_unlock(ipc);
726 
727 	return sysvipc_find_ipc(&iter->ns->ids[iface->ids], *pos, pos);
728 }
729 
730 /*
731  * File positions: pos 0 -> header, pos n -> ipc id = n - 1.
732  * SeqFile iterator: iterator value locked ipc pointer or SEQ_TOKEN_START.
733  */
734 static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
735 {
736 	struct ipc_proc_iter *iter = s->private;
737 	struct ipc_proc_iface *iface = iter->iface;
738 	struct ipc_ids *ids;
739 
740 	ids = &iter->ns->ids[iface->ids];
741 
742 	/*
743 	 * Take the lock - this will be released by the corresponding
744 	 * call to stop().
745 	 */
746 	down_read(&ids->rwsem);
747 
748 	/* pos < 0 is invalid */
749 	if (*pos < 0)
750 		return NULL;
751 
752 	/* pos == 0 means header */
753 	if (*pos == 0)
754 		return SEQ_START_TOKEN;
755 
756 	/* Find the (pos-1)th ipc */
757 	return sysvipc_find_ipc(ids, *pos - 1, pos);
758 }
759 
760 static void sysvipc_proc_stop(struct seq_file *s, void *it)
761 {
762 	struct kern_ipc_perm *ipc = it;
763 	struct ipc_proc_iter *iter = s->private;
764 	struct ipc_proc_iface *iface = iter->iface;
765 	struct ipc_ids *ids;
766 
767 	/* If we had a locked structure, release it */
768 	if (ipc && ipc != SEQ_START_TOKEN)
769 		ipc_unlock(ipc);
770 
771 	ids = &iter->ns->ids[iface->ids];
772 	/* Release the lock we took in start() */
773 	up_read(&ids->rwsem);
774 }
775 
776 static int sysvipc_proc_show(struct seq_file *s, void *it)
777 {
778 	struct ipc_proc_iter *iter = s->private;
779 	struct ipc_proc_iface *iface = iter->iface;
780 
781 	if (it == SEQ_START_TOKEN) {
782 		seq_puts(s, iface->header);
783 		return 0;
784 	}
785 
786 	return iface->show(s, it);
787 }
788 
789 static const struct seq_operations sysvipc_proc_seqops = {
790 	.start = sysvipc_proc_start,
791 	.stop  = sysvipc_proc_stop,
792 	.next  = sysvipc_proc_next,
793 	.show  = sysvipc_proc_show,
794 };
795 
796 static int sysvipc_proc_open(struct inode *inode, struct file *file)
797 {
798 	struct ipc_proc_iter *iter;
799 
800 	iter = __seq_open_private(file, &sysvipc_proc_seqops, sizeof(*iter));
801 	if (!iter)
802 		return -ENOMEM;
803 
804 	iter->iface = PDE_DATA(inode);
805 	iter->ns    = get_ipc_ns(current->nsproxy->ipc_ns);
806 
807 	return 0;
808 }
809 
810 static int sysvipc_proc_release(struct inode *inode, struct file *file)
811 {
812 	struct seq_file *seq = file->private_data;
813 	struct ipc_proc_iter *iter = seq->private;
814 	put_ipc_ns(iter->ns);
815 	return seq_release_private(inode, file);
816 }
817 
818 static const struct file_operations sysvipc_proc_fops = {
819 	.open    = sysvipc_proc_open,
820 	.read    = seq_read,
821 	.llseek  = seq_lseek,
822 	.release = sysvipc_proc_release,
823 };
824 #endif /* CONFIG_PROC_FS */
825