/*
   * linux/ipc/util.c
   * Copyright (C) 1992 Krishna Balasubramanian
   *
   * Sep 1997 - Call suser() last after "normal" permission checks so we
   *            get BSD style process accounting right.
   *            Occurs in several places in the IPC code.
   *            Chris Evans, <chris@ferret.lmh.ox.ac.uk>
   * Nov 1999 - ipc helper functions, unified SMP locking

   *	      Manfred Spraul <manfred@colorfullife.com>

   * Oct 2002 - One lock per IPC id. RCU ipc_free for lock-free grow_ary().
   *            Mingming Cao <cmm@us.ibm.com>

   * Mar 2006 - support for audit of ipc object properties
   *            Dustin Kirkland <dustin.kirkland@us.ibm.com>

   * Jun 2006 - namespaces ssupport
   *            OpenVZ, SWsoft Inc.
   *            Pavel Emelianov <xemul@openvz.org>

   *
   * General sysv ipc locking scheme:

   *	rcu_read_lock()
   *          obtain the ipc object (kern_ipc_perm) by looking up the id in an idr
   *	    tree.
   *	    - perform initial checks (capabilities, auditing and permission,
   *	      etc).
   *	    - perform read-only operations, such as STAT, INFO commands.
   *	      acquire the ipc lock (kern_ipc_perm.lock) through
   *	      ipc_lock_object()
   *		- perform data updates, such as SET, RMID commands and
   *		  mechanism-specific operations (semop/semtimedop,
   *		  msgsnd/msgrcv, shmat/shmdt).
   *	    drop the ipc lock, through ipc_unlock_object().
   *	rcu_read_unlock()
   *
   *  The ids->rwsem must be taken when:
   *	- creating, removing and iterating the existing entries in ipc
   *	  identifier sets.
   *	- iterating through files under /proc/sysvipc/
   *
   *  Note that sems have a special fast path that avoids kern_ipc_perm.lock -
   *  see sem_lock().

   */

  #include <linux/mm.h>
  #include <linux/shm.h>
  #include <linux/init.h>
  #include <linux/msg.h>

  #include <linux/vmalloc.h>
  #include <linux/slab.h>

  #include <linux/notifier.h>

  #include <linux/capability.h>

  #include <linux/highuid.h>
  #include <linux/security.h>
  #include <linux/rcupdate.h>
  #include <linux/workqueue.h>

  #include <linux/seq_file.h>
  #include <linux/proc_fs.h>

  #include <linux/audit.h>

  #include <linux/nsproxy.h>

  #include <linux/rwsem.h>

  #include <linux/memory.h>

  #include <linux/ipc_namespace.h>

  
  #include <asm/unistd.h>
  
  #include "util.h"

  struct ipc_proc_iface {
  	const char *path;
  	const char *header;

  	int ids;

  	int (*show)(struct seq_file *, void *);
  };

  /**

   * ipc_init - initialise ipc subsystem

   *

   * The various sysv ipc resources (semaphores, messages and shared
   * memory) are initialised.
   *
   * A callback routine is registered into the memory hotplug notifier
   * chain: since msgmni scales to lowmem this callback routine will be
   * called upon successful memory add / remove to recompute msmgni.

   */

  static int __init ipc_init(void)
  {
  	sem_init();
  	msg_init();
  	shm_init();
  	return 0;
  }

  device_initcall(ipc_init);

  
  /**

   * ipc_init_ids	- initialise ipc identifiers
   * @ids: ipc identifier set

   *

   * Set up the sequence range to use for the ipc identifier range (limited
   * below IPCMNI) then initialise the ids idr.

   */

  void ipc_init_ids(struct ipc_ids *ids)

  {

  	ids->in_use = 0;

  	ids->seq = 0;

  	ids->next_id = -1;

  	init_rwsem(&ids->rwsem);

  	idr_init(&ids->ipcs_idr);

  }

  #ifdef CONFIG_PROC_FS

  static const struct file_operations sysvipc_proc_fops;

  /**

   * ipc_init_proc_interface -  create a proc interface for sysipc types using a seq_file interface.
   * @path: Path in procfs
   * @header: Banner to be printed at the beginning of the file.
   * @ids: ipc id table to iterate.
   * @show: show routine.

   */
  void __init ipc_init_proc_interface(const char *path, const char *header,

  		int ids, int (*show)(struct seq_file *, void *))

  {
  	struct proc_dir_entry *pde;
  	struct ipc_proc_iface *iface;
  
  	iface = kmalloc(sizeof(*iface), GFP_KERNEL);
  	if (!iface)
  		return;
  	iface->path	= path;
  	iface->header	= header;
  	iface->ids	= ids;
  	iface->show	= show;

  	pde = proc_create_data(path,
  			       S_IRUGO,        /* world readable */
  			       NULL,           /* parent dir */
  			       &sysvipc_proc_fops,
  			       iface);

  	if (!pde)

  		kfree(iface);

  }
  #endif

  /**

   * ipc_findkey	- find a key in an ipc identifier set
   * @ids: ipc identifier set
   * @key: key to find

   *

   * Returns the locked pointer to the ipc structure if found or NULL
   * otherwise. If key is found ipc points to the owning ipc structure
   *
   * Called with ipc_ids.rwsem held.

   */

  static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)

  {

  	struct kern_ipc_perm *ipc;
  	int next_id;
  	int total;

  	for (total = 0, next_id = 0; total < ids->in_use; next_id++) {
  		ipc = idr_find(&ids->ipcs_idr, next_id);
  
  		if (ipc == NULL)

  			continue;

  
  		if (ipc->key != key) {
  			total++;
  			continue;
  		}

  		rcu_read_lock();
  		ipc_lock_object(ipc);

  		return ipc;

  	}

  
  	return NULL;

  }

  /**

   * ipc_get_maxid - get the last assigned id
   * @ids: ipc identifier set

   *

   * Called with ipc_ids.rwsem held.

   */

  int ipc_get_maxid(struct ipc_ids *ids)
  {
  	struct kern_ipc_perm *ipc;
  	int max_id = -1;
  	int total, id;
  
  	if (ids->in_use == 0)
  		return -1;

  	if (ids->in_use == IPCMNI)
  		return IPCMNI - 1;
  
  	/* Look for the last assigned id */
  	total = 0;
  	for (id = 0; id < IPCMNI && total < ids->in_use; id++) {
  		ipc = idr_find(&ids->ipcs_idr, id);
  		if (ipc != NULL) {
  			max_id = id;
  			total++;
  		}
  	}
  	return max_id;

  }
  
  /**

   * ipc_addid - add an ipc identifier
   * @ids: ipc identifier set
   * @new: new ipc permission set
   * @size: limit for the number of used ids

   *

   * Add an entry 'new' to the ipc ids idr. The permissions object is
   * initialised and the first free entry is set up and the id assigned
   * is returned. The 'new' entry is returned in a locked state on success.
   * On failure the entry is not locked and a negative err-code is returned.

   *

   * Called with writer ipc_ids.rwsem held.

   */

  int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)

  {

  	kuid_t euid;
  	kgid_t egid;

  	int id;

  	int next_id = ids->next_id;

  	if (size > IPCMNI)
  		size = IPCMNI;
  
  	if (ids->in_use >= size)

  		return -ENOSPC;

  	idr_preload(GFP_KERNEL);

  	spin_lock_init(&new->lock);

  	new->deleted = false;

  	rcu_read_lock();
  	spin_lock(&new->lock);

  	id = idr_alloc(&ids->ipcs_idr, new,
  		       (next_id < 0) ? 0 : ipcid_to_idx(next_id), 0,
  		       GFP_NOWAIT);
  	idr_preload_end();
  	if (id < 0) {

  		spin_unlock(&new->lock);
  		rcu_read_unlock();

  		return id;

  	}

  	ids->in_use++;

  	current_euid_egid(&euid, &egid);
  	new->cuid = new->uid = euid;
  	new->gid = new->cgid = egid;

  	if (next_id < 0) {
  		new->seq = ids->seq++;

  		if (ids->seq > IPCID_SEQ_MAX)

  			ids->seq = 0;
  	} else {
  		new->seq = ipcid_to_seqx(next_id);
  		ids->next_id = -1;
  	}

  	new->id = ipc_buildid(id, new->seq);

  	return id;
  }
  
  /**

   * ipcget_new -	create a new ipc object
   * @ns: ipc namespace

   * @ids: ipc identifier set

   * @ops: the actual creation routine to call
   * @params: its parameters
   *
   * This routine is called by sys_msgget, sys_semget() and sys_shmget()
   * when the key is IPC_PRIVATE.

   */

  static int ipcget_new(struct ipc_namespace *ns, struct ipc_ids *ids,

  		const struct ipc_ops *ops, struct ipc_params *params)

  {
  	int err;

  	down_write(&ids->rwsem);

  	err = ops->getnew(ns, params);

  	up_write(&ids->rwsem);

  	return err;
  }
  
  /**

   * ipc_check_perms - check security and permissions for an ipc object
   * @ns: ipc namespace
   * @ipcp: ipc permission set
   * @ops: the actual security routine to call
   * @params: its parameters

   *

   * This routine is called by sys_msgget(), sys_semget() and sys_shmget()
   * when the key is not IPC_PRIVATE and that key already exists in the
   * ds IDR.

   *

   * On success, the ipc id is returned.

   *

   * It is called with ipc_ids.rwsem and ipcp->lock held.

   */

  static int ipc_check_perms(struct ipc_namespace *ns,
  			   struct kern_ipc_perm *ipcp,

  			   const struct ipc_ops *ops,

  			   struct ipc_params *params)

  {
  	int err;

  	if (ipcperms(ns, ipcp, params->flg))

  		err = -EACCES;
  	else {
  		err = ops->associate(ipcp, params->flg);
  		if (!err)
  			err = ipcp->id;
  	}
  
  	return err;
  }
  
  /**

   * ipcget_public - get an ipc object or create a new one
   * @ns: ipc namespace

   * @ids: ipc identifier set

   * @ops: the actual creation routine to call
   * @params: its parameters
   *
   * This routine is called by sys_msgget, sys_semget() and sys_shmget()
   * when the key is not IPC_PRIVATE.
   * It adds a new entry if the key is not found and does some permission
   * / security checkings if the key is found.
   *
   * On success, the ipc id is returned.

   */

  static int ipcget_public(struct ipc_namespace *ns, struct ipc_ids *ids,

  		const struct ipc_ops *ops, struct ipc_params *params)

  {
  	struct kern_ipc_perm *ipcp;
  	int flg = params->flg;
  	int err;

  	/*
  	 * Take the lock as a writer since we are potentially going to add
  	 * a new entry + read locks are not "upgradable"
  	 */

  	down_write(&ids->rwsem);

  	ipcp = ipc_findkey(ids, params->key);
  	if (ipcp == NULL) {
  		/* key not used */
  		if (!(flg & IPC_CREAT))
  			err = -ENOENT;

  		else
  			err = ops->getnew(ns, params);
  	} else {
  		/* ipc object has been locked by ipc_findkey() */
  
  		if (flg & IPC_CREAT && flg & IPC_EXCL)
  			err = -EEXIST;
  		else {
  			err = 0;
  			if (ops->more_checks)
  				err = ops->more_checks(ipcp, params);
  			if (!err)

  				/*
  				 * ipc_check_perms returns the IPC id on
  				 * success
  				 */

  				err = ipc_check_perms(ns, ipcp, ops, params);

  		}
  		ipc_unlock(ipcp);
  	}

  	up_write(&ids->rwsem);

  
  	return err;
  }
  
  
  /**

   * ipc_rmid - remove an ipc identifier
   * @ids: ipc identifier set
   * @ipcp: ipc perm structure containing the identifier to remove

   *

   * ipc_ids.rwsem (as a writer) and the spinlock for this ID are held
   * before this function is called, and remain locked on the exit.

   */

  void ipc_rmid(struct ipc_ids *ids, struct kern_ipc_perm *ipcp)

  {

  	int lid = ipcid_to_idx(ipcp->id);

  
  	idr_remove(&ids->ipcs_idr, lid);

  	ids->in_use--;

  	ipcp->deleted = true;

  }
  
  /**

   * ipc_alloc -	allocate ipc space
   * @size: size desired

   *

   * Allocate memory from the appropriate pools and return a pointer to it.
   * NULL is returned if the allocation fails

   */

  void *ipc_alloc(int size)

  {

  	void *out;

  	if (size > PAGE_SIZE)

  		out = vmalloc(size);
  	else
  		out = kmalloc(size, GFP_KERNEL);
  	return out;
  }
  
  /**

   * ipc_free - free ipc space
   * @ptr: pointer returned by ipc_alloc
   * @size: size of block

   *

   * Free a block created with ipc_alloc(). The caller must know the size
   * used in the allocation call.

   */

  void ipc_free(void *ptr, int size)

  {

  	if (size > PAGE_SIZE)

  		vfree(ptr);
  	else
  		kfree(ptr);
  }

  /**

   * ipc_rcu_alloc - allocate ipc and rcu space
   * @size: size desired

   *

   * Allocate memory for the rcu header structure +  the object.
   * Returns the pointer to the object or NULL upon failure.

   */

  void *ipc_rcu_alloc(int size)

  {

  	/*

  	 * We prepend the allocation with the rcu struct

  	 */

  	struct ipc_rcu *out = ipc_alloc(sizeof(struct ipc_rcu) + size);
  	if (unlikely(!out))
  		return NULL;
  	atomic_set(&out->refcount, 1);

  	return out + 1;

  }

  int ipc_rcu_getref(void *ptr)

  {

  	struct ipc_rcu *p = ((struct ipc_rcu *)ptr) - 1;
  
  	return atomic_inc_not_zero(&p->refcount);

  }

  void ipc_rcu_putref(void *ptr, void (*func)(struct rcu_head *head))

  {

  	struct ipc_rcu *p = ((struct ipc_rcu *)ptr) - 1;

  
  	if (!atomic_dec_and_test(&p->refcount))

  		return;

  	call_rcu(&p->rcu, func);
  }
  
  void ipc_rcu_free(struct rcu_head *head)
  {
  	struct ipc_rcu *p = container_of(head, struct ipc_rcu, rcu);
  
  	if (is_vmalloc_addr(p))
  		vfree(p);
  	else
  		kfree(p);

  }
  
  /**

   * ipcperms - check ipc permissions
   * @ns: ipc namespace
   * @ipcp: ipc permission set
   * @flag: desired permission set

   *

   * Check user, group, other permissions for access
   * to ipc resources. return 0 if allowed

   *

   * @flag will most probably be 0 or S_...UGO from <linux/stat.h>

   */

  int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flag)
  {

  	kuid_t euid = current_euid();

  	int requested_mode, granted_mode;

  	audit_ipc_obj(ipcp);

  	requested_mode = (flag >> 6) | (flag >> 3) | flag;
  	granted_mode = ipcp->mode;

  	if (uid_eq(euid, ipcp->cuid) ||
  	    uid_eq(euid, ipcp->uid))

  		granted_mode >>= 6;
  	else if (in_group_p(ipcp->cgid) || in_group_p(ipcp->gid))
  		granted_mode >>= 3;
  	/* is there some bit set in requested_mode but not in granted_mode? */

  	if ((requested_mode & ~granted_mode & 0007) &&

  	    !ns_capable(ns->user_ns, CAP_IPC_OWNER))

  		return -1;
  
  	return security_ipc_permission(ipcp, flag);
  }
  
  /*
   * Functions to convert between the kern_ipc_perm structure and the
   * old/new ipc_perm structures
   */
  
  /**

   * kernel_to_ipc64_perm	- convert kernel ipc permissions to user
   * @in: kernel permissions
   * @out: new style ipc permissions

   *

   * Turn the kernel object @in into a set of permissions descriptions
   * for returning to userspace (@out).

   */

  void kernel_to_ipc64_perm(struct kern_ipc_perm *in, struct ipc64_perm *out)

  {
  	out->key	= in->key;

  	out->uid	= from_kuid_munged(current_user_ns(), in->uid);
  	out->gid	= from_kgid_munged(current_user_ns(), in->gid);
  	out->cuid	= from_kuid_munged(current_user_ns(), in->cuid);
  	out->cgid	= from_kgid_munged(current_user_ns(), in->cgid);

  	out->mode	= in->mode;
  	out->seq	= in->seq;
  }
  
  /**

   * ipc64_perm_to_ipc_perm - convert new ipc permissions to old
   * @in: new style ipc permissions
   * @out: old style ipc permissions

   *

   * Turn the new style permissions object @in into a compatibility
   * object and store it into the @out pointer.

   */

  void ipc64_perm_to_ipc_perm(struct ipc64_perm *in, struct ipc_perm *out)

  {
  	out->key	= in->key;
  	SET_UID(out->uid, in->uid);
  	SET_GID(out->gid, in->gid);
  	SET_UID(out->cuid, in->cuid);
  	SET_GID(out->cgid, in->cgid);
  	out->mode	= in->mode;
  	out->seq	= in->seq;
  }

  /**

   * ipc_obtain_object
   * @ids: ipc identifier set
   * @id: ipc id to look for
   *
   * Look for an id in the ipc ids idr and return associated ipc object.
   *
   * Call inside the RCU critical section.
   * The ipc object is *not* locked on exit.
   */
  struct kern_ipc_perm *ipc_obtain_object(struct ipc_ids *ids, int id)
  {
  	struct kern_ipc_perm *out;
  	int lid = ipcid_to_idx(id);
  
  	out = idr_find(&ids->ipcs_idr, lid);
  	if (!out)
  		return ERR_PTR(-EINVAL);
  
  	return out;
  }
  
  /**

   * ipc_lock - lock an ipc structure without rwsem held
   * @ids: ipc identifier set

   * @id: ipc id to look for
   *
   * Look for an id in the ipc ids idr and lock the associated ipc object.
   *

   * The ipc object is locked on successful exit.

   */

  struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)

  {

  	struct kern_ipc_perm *out;

  
  	rcu_read_lock();

  	out = ipc_obtain_object(ids, id);
  	if (IS_ERR(out))
  		goto err1;

  	spin_lock(&out->lock);

  	/* ipc_rmid() may have already freed the ID while ipc_lock
  	 * was spinning: here verify that the structure is still valid
  	 */

  	if (ipc_valid_object(out))

  		return out;
  
  	spin_unlock(&out->lock);
  	out = ERR_PTR(-EINVAL);
  err1:
  	rcu_read_unlock();
  	return out;
  }

  /**
   * ipc_obtain_object_check
   * @ids: ipc identifier set
   * @id: ipc id to look for
   *
   * Similar to ipc_obtain_object() but also checks
   * the ipc object reference counter.
   *
   * Call inside the RCU critical section.
   * The ipc object is *not* locked on exit.
   */
  struct kern_ipc_perm *ipc_obtain_object_check(struct ipc_ids *ids, int id)
  {
  	struct kern_ipc_perm *out = ipc_obtain_object(ids, id);
  
  	if (IS_ERR(out))
  		goto out;
  
  	if (ipc_checkid(out, id))
  		return ERR_PTR(-EIDRM);
  out:

  	return out;
  }

  /**
   * ipcget - Common sys_*get() code

   * @ns: namespace

   * @ids: ipc identifier set
   * @ops: operations to be called on ipc object creation, permission checks
   *       and further checks
   * @params: the parameters needed by the previous operations.

   *
   * Common routine called by sys_msgget(), sys_semget() and sys_shmget().
   */
  int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,

  			const struct ipc_ops *ops, struct ipc_params *params)

  {
  	if (params->key == IPC_PRIVATE)
  		return ipcget_new(ns, ids, ops, params);
  	else
  		return ipcget_public(ns, ids, ops, params);
  }

  /**

   * ipc_update_perm - update the permissions of an ipc object

   * @in:  the permission given as input.
   * @out: the permission of the ipc to set.
   */

  int ipc_update_perm(struct ipc64_perm *in, struct kern_ipc_perm *out)

  {

  	kuid_t uid = make_kuid(current_user_ns(), in->uid);
  	kgid_t gid = make_kgid(current_user_ns(), in->gid);
  	if (!uid_valid(uid) || !gid_valid(gid))
  		return -EINVAL;
  
  	out->uid = uid;
  	out->gid = gid;

  	out->mode = (out->mode & ~S_IRWXUGO)
  		| (in->mode & S_IRWXUGO);

  
  	return 0;

  }

  /**

   * ipcctl_pre_down_nolock - retrieve an ipc and check permissions for some IPC_XXX cmd

   * @ns:  ipc namespace

   * @ids:  the table of ids where to look for the ipc
   * @id:   the id of the ipc to retrieve
   * @cmd:  the cmd to check
   * @perm: the permission to set
   * @extra_perm: one extra permission parameter used by msq
   *
   * This function does some common audit and permissions check for some IPC_XXX
   * cmd and is called from semctl_down, shmctl_down and msgctl_down.
   * It must be called without any lock held and
   *  - retrieves the ipc with the given id in the given table.
   *  - performs some audit and permission check, depending on the given cmd

   *  - returns a pointer to the ipc object or otherwise, the corresponding error.

   *

   * Call holding the both the rwsem and the rcu read lock.

   */

  struct kern_ipc_perm *ipcctl_pre_down_nolock(struct ipc_namespace *ns,

  					struct ipc_ids *ids, int id, int cmd,
  					struct ipc64_perm *perm, int extra_perm)

  {

  	kuid_t euid;

  	int err = -EPERM;
  	struct kern_ipc_perm *ipcp;

  	ipcp = ipc_obtain_object_check(ids, id);

  	if (IS_ERR(ipcp)) {
  		err = PTR_ERR(ipcp);

  		goto err;

  	}

  	audit_ipc_obj(ipcp);

  	if (cmd == IPC_SET)
  		audit_ipc_set_perm(extra_perm, perm->uid,

  				   perm->gid, perm->mode);

  
  	euid = current_euid();

  	if (uid_eq(euid, ipcp->cuid) || uid_eq(euid, ipcp->uid)  ||

  	    ns_capable(ns->user_ns, CAP_SYS_ADMIN))

  		return ipcp; /* successful lookup */
  err:

  	return ERR_PTR(err);
  }

  #ifdef CONFIG_ARCH_WANT_IPC_PARSE_VERSION

  
  
  /**

   * ipc_parse_version - ipc call version
   * @cmd: pointer to command

   *

   * Return IPC_64 for new style IPC and IPC_OLD for old style IPC.
   * The @cmd value is turned from an encoding command and version into
   * just the command code.

   */

  int ipc_parse_version(int *cmd)

  {
  	if (*cmd & IPC_64) {
  		*cmd ^= IPC_64;
  		return IPC_64;
  	} else {
  		return IPC_OLD;
  	}
  }

  #endif /* CONFIG_ARCH_WANT_IPC_PARSE_VERSION */

  
  #ifdef CONFIG_PROC_FS

  struct ipc_proc_iter {
  	struct ipc_namespace *ns;
  	struct ipc_proc_iface *iface;
  };

  /*
   * This routine locks the ipc structure found at least at position pos.
   */

  static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
  					      loff_t *new_pos)

  {

  	struct kern_ipc_perm *ipc;
  	int total, id;

  	total = 0;
  	for (id = 0; id < pos && total < ids->in_use; id++) {
  		ipc = idr_find(&ids->ipcs_idr, id);
  		if (ipc != NULL)
  			total++;
  	}

  	if (total >= ids->in_use)
  		return NULL;

  	for (; pos < IPCMNI; pos++) {

  		ipc = idr_find(&ids->ipcs_idr, pos);
  		if (ipc != NULL) {
  			*new_pos = pos + 1;

  			rcu_read_lock();
  			ipc_lock_object(ipc);

  			return ipc;
  		}
  	}
  
  	/* Out of range - return NULL to terminate iteration */
  	return NULL;
  }

  static void *sysvipc_proc_next(struct seq_file *s, void *it, loff_t *pos)
  {
  	struct ipc_proc_iter *iter = s->private;
  	struct ipc_proc_iface *iface = iter->iface;
  	struct kern_ipc_perm *ipc = it;
  
  	/* If we had an ipc id locked before, unlock it */
  	if (ipc && ipc != SEQ_START_TOKEN)
  		ipc_unlock(ipc);

  	return sysvipc_find_ipc(&iter->ns->ids[iface->ids], *pos, pos);

  }

  /*

   * File positions: pos 0 -> header, pos n -> ipc id = n - 1.
   * SeqFile iterator: iterator value locked ipc pointer or SEQ_TOKEN_START.

   */
  static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
  {

  	struct ipc_proc_iter *iter = s->private;
  	struct ipc_proc_iface *iface = iter->iface;

  	struct ipc_ids *ids;

  	ids = &iter->ns->ids[iface->ids];

  
  	/*
  	 * Take the lock - this will be released by the corresponding
  	 * call to stop().
  	 */

  	down_read(&ids->rwsem);

  
  	/* pos < 0 is invalid */
  	if (*pos < 0)
  		return NULL;
  
  	/* pos == 0 means header */
  	if (*pos == 0)
  		return SEQ_START_TOKEN;
  
  	/* Find the (pos-1)th ipc */

  	return sysvipc_find_ipc(ids, *pos - 1, pos);

  }
  
  static void sysvipc_proc_stop(struct seq_file *s, void *it)
  {
  	struct kern_ipc_perm *ipc = it;

  	struct ipc_proc_iter *iter = s->private;
  	struct ipc_proc_iface *iface = iter->iface;

  	struct ipc_ids *ids;

  	/* If we had a locked structure, release it */

  	if (ipc && ipc != SEQ_START_TOKEN)
  		ipc_unlock(ipc);

  	ids = &iter->ns->ids[iface->ids];

  	/* Release the lock we took in start() */

  	up_read(&ids->rwsem);

  }
  
  static int sysvipc_proc_show(struct seq_file *s, void *it)
  {

  	struct ipc_proc_iter *iter = s->private;
  	struct ipc_proc_iface *iface = iter->iface;

  	if (it == SEQ_START_TOKEN) {
  		seq_puts(s, iface->header);
  		return 0;
  	}

  
  	return iface->show(s, it);
  }

  static const struct seq_operations sysvipc_proc_seqops = {

  	.start = sysvipc_proc_start,
  	.stop  = sysvipc_proc_stop,
  	.next  = sysvipc_proc_next,
  	.show  = sysvipc_proc_show,
  };

  static int sysvipc_proc_open(struct inode *inode, struct file *file)
  {

  	struct ipc_proc_iter *iter;

  	iter = __seq_open_private(file, &sysvipc_proc_seqops, sizeof(*iter));

  	if (!iter)

  		return -ENOMEM;

  	iter->iface = PDE_DATA(inode);

  	iter->ns    = get_ipc_ns(current->nsproxy->ipc_ns);

  
  	return 0;

  }
  
  static int sysvipc_proc_release(struct inode *inode, struct file *file)
  {
  	struct seq_file *seq = file->private_data;
  	struct ipc_proc_iter *iter = seq->private;
  	put_ipc_ns(iter->ns);
  	return seq_release_private(inode, file);

  }

  static const struct file_operations sysvipc_proc_fops = {

  	.open    = sysvipc_proc_open,
  	.read    = seq_read,
  	.llseek  = seq_lseek,

  	.release = sysvipc_proc_release,

  };
  #endif /* CONFIG_PROC_FS */