/*
   * linux/ipc/sem.c
   * Copyright (C) 1992 Krishna Balasubramanian
   * Copyright (C) 1995 Eric Schenk, Bruno Haible
   *

   * /proc/sysvipc/sem support (c) 1999 Dragos Acostachioaie <dragos@iname.com>
   *
   * SMP-threaded, sysctl's added

   * (c) 1999 Manfred Spraul <manfred@colorfullife.com>

   * Enforced range limit on SEM_UNDO

   * (c) 2001 Red Hat Inc

   * Lockless wakeup
   * (c) 2003 Manfred Spraul <manfred@colorfullife.com>

   * Further wakeup optimizations, documentation
   * (c) 2010 Manfred Spraul <manfred@colorfullife.com>

   *
   * support for audit of ipc object properties and permission changes
   * Dustin Kirkland <dustin.kirkland@us.ibm.com>

   *
   * namespaces support
   * OpenVZ, SWsoft Inc.
   * Pavel Emelianov <xemul@openvz.org>

   *
   * Implementation notes: (May 2010)
   * This file implements System V semaphores.
   *
   * User space visible behavior:
   * - FIFO ordering for semop() operations (just FIFO, not starvation
   *   protection)
   * - multiple semaphore operations that alter the same semaphore in
   *   one semop() are handled.
   * - sem_ctime (time of last semctl()) is updated in the IPC_SET, SETVAL and
   *   SETALL calls.
   * - two Linux specific semctl() commands: SEM_STAT, SEM_INFO.
   * - undo adjustments at process exit are limited to 0..SEMVMX.
   * - namespace are supported.
   * - SEMMSL, SEMMNS, SEMOPM and SEMMNI can be configured at runtine by writing
   *   to /proc/sys/kernel/sem.
   * - statistics about the usage are reported in /proc/sysvipc/sem.
   *
   * Internals:
   * - scalability:
   *   - all global variables are read-mostly.
   *   - semop() calls and semctl(RMID) are synchronized by RCU.
   *   - most operations do write operations (actually: spin_lock calls) to
   *     the per-semaphore array structure.
   *   Thus: Perfect SMP scaling between independent semaphore arrays.
   *         If multiple semaphores in one array are used, then cache line
   *         trashing on the semaphore array spinlock will limit the scaling.

   * - semncnt and semzcnt are calculated on demand in count_semcnt()

   * - the task that performs a successful semop() scans the list of all
   *   sleeping tasks and completes any pending operations that can be fulfilled.
   *   Semaphores are actively given to waiting tasks (necessary for FIFO).
   *   (see update_queue())
   * - To improve the scalability, the actual wake-up calls are performed after
   *   dropping all locks. (see wake_up_sem_queue_prepare(),
   *   wake_up_sem_queue_do())
   * - All work is done by the waker, the woken up task does not have to do
   *   anything - not even acquiring a lock or dropping a refcount.
   * - A woken up task may not even touch the semaphore array anymore, it may
   *   have been destroyed already by a semctl(RMID).
   * - The synchronizations between wake-ups due to a timeout/signal and a
   *   wake-up due to a completed semaphore operation is achieved by using an
   *   intermediate state (IN_WAKEUP).
   * - UNDO values are stored in an array (one per process and per
   *   semaphore array, lazily allocated). For backwards compatibility, multiple
   *   modes for the UNDO variables are supported (per process, per thread)
   *   (see copy_semundo, CLONE_SYSVSEM)
   * - There are two lists of the pending operations: a per-array list
   *   and per-semaphore list (stored in the array). This allows to achieve FIFO
   *   ordering without always scanning all pending operations.
   *   The worst-case behavior is nevertheless O(N^2) for N wakeups.

   */

  #include <linux/slab.h>
  #include <linux/spinlock.h>
  #include <linux/init.h>
  #include <linux/proc_fs.h>
  #include <linux/time.h>

  #include <linux/security.h>
  #include <linux/syscalls.h>
  #include <linux/audit.h>

  #include <linux/capability.h>

  #include <linux/seq_file.h>

  #include <linux/rwsem.h>

  #include <linux/nsproxy.h>

  #include <linux/ipc_namespace.h>

  #include <linux/uaccess.h>

  #include "util.h"

  /* One semaphore structure for each semaphore in the system. */
  struct sem {
  	int	semval;		/* current value */
  	int	sempid;		/* pid of last operation */

  	spinlock_t	lock;	/* spinlock for fine-grained semtimedop */

  	struct list_head pending_alter; /* pending single-sop operations */
  					/* that alter the semaphore */
  	struct list_head pending_const; /* pending single-sop operations */
  					/* that do not alter the semaphore*/

  	time_t	sem_otime;	/* candidate for sem_otime */

  } ____cacheline_aligned_in_smp;

  
  /* One queue for each sleeping process in the system. */
  struct sem_queue {

  	struct list_head	list;	 /* queue of pending operations */
  	struct task_struct	*sleeper; /* this process */
  	struct sem_undo		*undo;	 /* undo structure */
  	int			pid;	 /* process id of requesting process */
  	int			status;	 /* completion status of operation */
  	struct sembuf		*sops;	 /* array of pending operations */

  	struct sembuf		*blocking; /* the operation that blocked */

  	int			nsops;	 /* number of operations */
  	int			alter;	 /* does *sops alter the array? */
  };
  
  /* Each task has a list of undo requests. They are executed automatically
   * when the process exits.
   */
  struct sem_undo {
  	struct list_head	list_proc;	/* per-process list: *
  						 * all undos from one process
  						 * rcu protected */
  	struct rcu_head		rcu;		/* rcu struct for sem_undo */
  	struct sem_undo_list	*ulp;		/* back ptr to sem_undo_list */
  	struct list_head	list_id;	/* per semaphore array list:
  						 * all undos for one array */
  	int			semid;		/* semaphore set identifier */
  	short			*semadj;	/* array of adjustments */
  						/* one per semaphore */
  };
  
  /* sem_undo_list controls shared access to the list of sem_undo structures
   * that may be shared among all a CLONE_SYSVSEM task group.
   */
  struct sem_undo_list {
  	atomic_t		refcnt;
  	spinlock_t		lock;
  	struct list_head	list_proc;
  };

  #define sem_ids(ns)	((ns)->ids[IPC_SEM_IDS])

  #define sem_checkid(sma, semid)	ipc_checkid(&sma->sem_perm, semid)

  static int newary(struct ipc_namespace *, struct ipc_params *);

  static void freeary(struct ipc_namespace *, struct kern_ipc_perm *);

  #ifdef CONFIG_PROC_FS

  static int sysvipc_sem_proc_show(struct seq_file *s, void *it);

  #endif
  
  #define SEMMSL_FAST	256 /* 512 bytes on stack */
  #define SEMOPM_FAST	64  /* ~ 372 bytes on stack */
  
  /*

   * Locking:

   *	sem_undo.id_next,

   *	sem_array.complex_count,

   *	sem_array.pending{_alter,_cont},

   *	sem_array.sem_undo: global sem_lock() for read/write

   *	sem_undo.proc_next: only "current" is allowed to read/write that field.

   *

   *	sem_array.sem_base[i].pending_{const,alter}:
   *		global or semaphore sem_lock() for read/write

   */

  #define sc_semmsl	sem_ctls[0]
  #define sc_semmns	sem_ctls[1]
  #define sc_semopm	sem_ctls[2]
  #define sc_semmni	sem_ctls[3]

  void sem_init_ns(struct ipc_namespace *ns)

  {

  	ns->sc_semmsl = SEMMSL;
  	ns->sc_semmns = SEMMNS;
  	ns->sc_semopm = SEMOPM;
  	ns->sc_semmni = SEMMNI;
  	ns->used_sems = 0;

  	ipc_init_ids(&ns->ids[IPC_SEM_IDS]);

  }

  #ifdef CONFIG_IPC_NS

  void sem_exit_ns(struct ipc_namespace *ns)
  {

  	free_ipcs(ns, &sem_ids(ns), freeary);

  	idr_destroy(&ns->ids[IPC_SEM_IDS].ipcs_idr);

  }

  #endif

  void __init sem_init(void)

  {

  	sem_init_ns(&init_ipc_ns);

  	ipc_init_proc_interface("sysvipc/sem",
  				"       key      semid perms      nsems   uid   gid  cuid  cgid      otime      ctime
  ",

  				IPC_SEM_IDS, sysvipc_sem_proc_show);

  }

  /**
   * unmerge_queues - unmerge queues, if possible.
   * @sma: semaphore array
   *
   * The function unmerges the wait queues if complex_count is 0.
   * It must be called prior to dropping the global semaphore array lock.
   */
  static void unmerge_queues(struct sem_array *sma)
  {
  	struct sem_queue *q, *tq;
  
  	/* complex operations still around? */
  	if (sma->complex_count)
  		return;
  	/*
  	 * We will switch back to simple mode.
  	 * Move all pending operation back into the per-semaphore
  	 * queues.
  	 */
  	list_for_each_entry_safe(q, tq, &sma->pending_alter, list) {
  		struct sem *curr;
  		curr = &sma->sem_base[q->sops[0].sem_num];
  
  		list_add_tail(&q->list, &curr->pending_alter);
  	}
  	INIT_LIST_HEAD(&sma->pending_alter);
  }
  
  /**

   * merge_queues - merge single semop queues into global queue

   * @sma: semaphore array
   *
   * This function merges all per-semaphore queues into the global queue.
   * It is necessary to achieve FIFO ordering for the pending single-sop
   * operations when a multi-semop operation must sleep.
   * Only the alter operations must be moved, the const operations can stay.
   */
  static void merge_queues(struct sem_array *sma)
  {
  	int i;
  	for (i = 0; i < sma->sem_nsems; i++) {
  		struct sem *sem = sma->sem_base + i;
  
  		list_splice_init(&sem->pending_alter, &sma->pending_alter);
  	}
  }

  static void sem_rcu_free(struct rcu_head *head)
  {
  	struct ipc_rcu *p = container_of(head, struct ipc_rcu, rcu);
  	struct sem_array *sma = ipc_rcu_to_struct(p);
  
  	security_sem_free(sma);
  	ipc_rcu_free(head);
  }

  /*

   * Wait until all currently ongoing simple ops have completed.
   * Caller must own sem_perm.lock.
   * New simple ops cannot start, because simple ops first check
   * that sem_perm.lock is free.

   * that a) sem_perm.lock is free and b) complex_count is 0.

   */
  static void sem_wait_array(struct sem_array *sma)
  {
  	int i;
  	struct sem *sem;

  	if (sma->complex_count)  {
  		/* The thread that increased sma->complex_count waited on
  		 * all sem->lock locks. Thus we don't need to wait again.
  		 */
  		return;
  	}

  	for (i = 0; i < sma->sem_nsems; i++) {
  		sem = sma->sem_base + i;
  		spin_unlock_wait(&sem->lock);
  	}
  }
  
  /*

   * If the request contains only one semaphore operation, and there are
   * no complex transactions pending, lock only the semaphore involved.
   * Otherwise, lock the entire semaphore array, since we either have
   * multiple semaphores in our own semops, or we need to look at
   * semaphores from other pending complex operations.

   */
  static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
  			      int nsops)
  {

  	struct sem *sem;

  	if (nsops != 1) {
  		/* Complex operation - acquire a full lock */
  		ipc_lock_object(&sma->sem_perm);

  		/* And wait until all simple ops that are processed
  		 * right now have dropped their locks.

  		 */

  		sem_wait_array(sma);
  		return -1;
  	}
  
  	/*
  	 * Only one semaphore affected - try to optimize locking.
  	 * The rules are:
  	 * - optimized locking is possible if no complex operation
  	 *   is either enqueued or processed right now.
  	 * - The test for enqueued complex ops is simple:
  	 *      sma->complex_count != 0
  	 * - Testing for complex ops that are processed right now is
  	 *   a bit more difficult. Complex ops acquire the full lock
  	 *   and first wait that the running simple ops have completed.
  	 *   (see above)
  	 *   Thus: If we own a simple lock and the global lock is free
  	 *	and complex_count is now 0, then it will stay 0 and
  	 *	thus just locking sem->lock is sufficient.
  	 */
  	sem = sma->sem_base + sops->sem_num;

  	if (sma->complex_count == 0) {

  		/*

  		 * It appears that no complex operation is around.
  		 * Acquire the per-semaphore lock.

  		 */

  		spin_lock(&sem->lock);
  
  		/* Then check that the global lock is free */
  		if (!spin_is_locked(&sma->sem_perm.lock)) {
  			/* spin_is_locked() is not a memory barrier */
  			smp_mb();
  
  			/* Now repeat the test of complex_count:
  			 * It can't change anymore until we drop sem->lock.
  			 * Thus: if is now 0, then it will stay 0.
  			 */
  			if (sma->complex_count == 0) {
  				/* fast path successful! */
  				return sops->sem_num;
  			}

  		}

  		spin_unlock(&sem->lock);
  	}
  
  	/* slow path: acquire the full lock */
  	ipc_lock_object(&sma->sem_perm);

  	if (sma->complex_count == 0) {
  		/* False alarm:
  		 * There is no complex operation, thus we can switch
  		 * back to the fast path.
  		 */
  		spin_lock(&sem->lock);
  		ipc_unlock_object(&sma->sem_perm);
  		return sops->sem_num;

  	} else {

  		/* Not a false alarm, thus complete the sequence for a
  		 * full lock.

  		 */

  		sem_wait_array(sma);
  		return -1;

  	}

  }
  
  static inline void sem_unlock(struct sem_array *sma, int locknum)
  {
  	if (locknum == -1) {

  		unmerge_queues(sma);

  		ipc_unlock_object(&sma->sem_perm);

  	} else {
  		struct sem *sem = sma->sem_base + locknum;
  		spin_unlock(&sem->lock);
  	}

  }
  
  /*

   * sem_lock_(check_) routines are called in the paths where the rwsem

   * is not held.

   *
   * The caller holds the RCU read lock.

   */

  static inline struct sem_array *sem_obtain_lock(struct ipc_namespace *ns,
  			int id, struct sembuf *sops, int nsops, int *locknum)

  {

  	struct kern_ipc_perm *ipcp;
  	struct sem_array *sma;

  	ipcp = ipc_obtain_object(&sem_ids(ns), id);

  	if (IS_ERR(ipcp))
  		return ERR_CAST(ipcp);

  	sma = container_of(ipcp, struct sem_array, sem_perm);
  	*locknum = sem_lock(sma, sops, nsops);

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

  	if (ipc_valid_object(ipcp))

  		return container_of(ipcp, struct sem_array, sem_perm);

  	sem_unlock(sma, *locknum);

  	return ERR_PTR(-EINVAL);

  }

  static inline struct sem_array *sem_obtain_object(struct ipc_namespace *ns, int id)
  {
  	struct kern_ipc_perm *ipcp = ipc_obtain_object(&sem_ids(ns), id);
  
  	if (IS_ERR(ipcp))
  		return ERR_CAST(ipcp);
  
  	return container_of(ipcp, struct sem_array, sem_perm);
  }

  static inline struct sem_array *sem_obtain_object_check(struct ipc_namespace *ns,
  							int id)
  {
  	struct kern_ipc_perm *ipcp = ipc_obtain_object_check(&sem_ids(ns), id);
  
  	if (IS_ERR(ipcp))
  		return ERR_CAST(ipcp);

  	return container_of(ipcp, struct sem_array, sem_perm);

  }

  static inline void sem_lock_and_putref(struct sem_array *sma)
  {

  	sem_lock(sma, NULL, -1);

  	ipc_rcu_putref(sma, ipc_rcu_free);

  }

  static inline void sem_rmid(struct ipc_namespace *ns, struct sem_array *s)
  {
  	ipc_rmid(&sem_ids(ns), &s->sem_perm);
  }

  /*
   * Lockless wakeup algorithm:
   * Without the check/retry algorithm a lockless wakeup is possible:
   * - queue.status is initialized to -EINTR before blocking.
   * - wakeup is performed by

   *	* unlinking the queue entry from the pending list

   *	* setting queue.status to IN_WAKEUP
   *	  This is the notification for the blocked thread that a
   *	  result value is imminent.
   *	* call wake_up_process
   *	* set queue.status to the final value.
   * - the previously blocked thread checks queue.status:

   *	* if it's IN_WAKEUP, then it must wait until the value changes
   *	* if it's not -EINTR, then the operation was completed by
   *	  update_queue. semtimedop can return queue.status without
   *	  performing any operation on the sem array.
   *	* otherwise it must acquire the spinlock and check what's up.

   *
   * The two-stage algorithm is necessary to protect against the following
   * races:
   * - if queue.status is set after wake_up_process, then the woken up idle
   *   thread could race forward and try (and fail) to acquire sma->lock
   *   before update_queue had a chance to set queue.status
   * - if queue.status is written before wake_up_process and if the
   *   blocked process is woken up by a signal between writing
   *   queue.status and the wake_up_process, then the woken up
   *   process could return from semtimedop and die by calling
   *   sys_exit before wake_up_process is called. Then wake_up_process
   *   will oops, because the task structure is already invalid.
   *   (yes, this happened on s390 with sysv msg).
   *
   */
  #define IN_WAKEUP	1

  /**
   * newary - Create a new semaphore set
   * @ns: namespace
   * @params: ptr to the structure that contains key, semflg and nsems
   *

   * Called with sem_ids.rwsem held (as a writer)

   */

  static int newary(struct ipc_namespace *ns, struct ipc_params *params)

  {
  	int id;
  	int retval;
  	struct sem_array *sma;
  	int size;

  	key_t key = params->key;
  	int nsems = params->u.nsems;
  	int semflg = params->flg;

  	int i;

  
  	if (!nsems)
  		return -EINVAL;

  	if (ns->used_sems + nsems > ns->sc_semmns)

  		return -ENOSPC;

  	size = sizeof(*sma) + nsems * sizeof(struct sem);

  	sma = ipc_rcu_alloc(size);

  	if (!sma)

  		return -ENOMEM;

  	memset(sma, 0, size);

  
  	sma->sem_perm.mode = (semflg & S_IRWXUGO);
  	sma->sem_perm.key = key;
  
  	sma->sem_perm.security = NULL;
  	retval = security_sem_alloc(sma);
  	if (retval) {

  		ipc_rcu_putref(sma, ipc_rcu_free);

  		return retval;
  	}

  	id = ipc_addid(&sem_ids(ns), &sma->sem_perm, ns->sc_semmni);

  	if (id < 0) {

  		ipc_rcu_putref(sma, sem_rcu_free);

  		return id;

  	}

  	ns->used_sems += nsems;

  
  	sma->sem_base = (struct sem *) &sma[1];

  	for (i = 0; i < nsems; i++) {

  		INIT_LIST_HEAD(&sma->sem_base[i].pending_alter);
  		INIT_LIST_HEAD(&sma->sem_base[i].pending_const);

  		spin_lock_init(&sma->sem_base[i].lock);
  	}

  
  	sma->complex_count = 0;

  	INIT_LIST_HEAD(&sma->pending_alter);
  	INIT_LIST_HEAD(&sma->pending_const);

  	INIT_LIST_HEAD(&sma->list_id);

  	sma->sem_nsems = nsems;
  	sma->sem_ctime = get_seconds();

  	sem_unlock(sma, -1);

  	rcu_read_unlock();

  	return sma->sem_perm.id;

  }

  /*

   * Called with sem_ids.rwsem and ipcp locked.

   */

  static inline int sem_security(struct kern_ipc_perm *ipcp, int semflg)

  {

  	struct sem_array *sma;
  
  	sma = container_of(ipcp, struct sem_array, sem_perm);
  	return security_sem_associate(sma, semflg);

  }

  /*

   * Called with sem_ids.rwsem and ipcp locked.

   */

  static inline int sem_more_checks(struct kern_ipc_perm *ipcp,
  				struct ipc_params *params)

  {

  	struct sem_array *sma;
  
  	sma = container_of(ipcp, struct sem_array, sem_perm);
  	if (params->u.nsems > sma->sem_nsems)

  		return -EINVAL;
  
  	return 0;
  }

  SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg)

  {

  	struct ipc_namespace *ns;

  	static const struct ipc_ops sem_ops = {
  		.getnew = newary,
  		.associate = sem_security,
  		.more_checks = sem_more_checks,
  	};

  	struct ipc_params sem_params;

  
  	ns = current->nsproxy->ipc_ns;

  	if (nsems < 0 || nsems > ns->sc_semmsl)

  		return -EINVAL;

  	sem_params.key = key;
  	sem_params.flg = semflg;
  	sem_params.u.nsems = nsems;

  	return ipcget(ns, &sem_ids(ns), &sem_ops, &sem_params);

  }

  /**
   * perform_atomic_semop - Perform (if possible) a semaphore operation

   * @sma: semaphore array

   * @q: struct sem_queue that describes the operation

   *
   * Returns 0 if the operation was possible.
   * Returns 1 if the operation is impossible, the caller must sleep.
   * Negative values are error codes.

   */

  static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)

  {

  	int result, sem_op, nsops, pid;

  	struct sembuf *sop;

  	struct sem *curr;

  	struct sembuf *sops;
  	struct sem_undo *un;
  
  	sops = q->sops;
  	nsops = q->nsops;
  	un = q->undo;

  
  	for (sop = sops; sop < sops + nsops; sop++) {
  		curr = sma->sem_base + sop->sem_num;
  		sem_op = sop->sem_op;
  		result = curr->semval;

  		if (!sem_op && result)
  			goto would_block;
  
  		result += sem_op;
  		if (result < 0)
  			goto would_block;
  		if (result > SEMVMX)
  			goto out_of_range;

  		if (sop->sem_flg & SEM_UNDO) {
  			int undo = un->semadj[sop->sem_num] - sem_op;

  			/* Exceeding the undo range is an error. */

  			if (undo < (-SEMAEM - 1) || undo > SEMAEM)
  				goto out_of_range;

  			un->semadj[sop->sem_num] = undo;

  		}

  		curr->semval = result;
  	}
  
  	sop--;

  	pid = q->pid;

  	while (sop >= sops) {
  		sma->sem_base[sop->sem_num].sempid = pid;

  		sop--;
  	}

  	return 0;
  
  out_of_range:
  	result = -ERANGE;
  	goto undo;
  
  would_block:

  	q->blocking = sop;

  	if (sop->sem_flg & IPC_NOWAIT)
  		result = -EAGAIN;
  	else
  		result = 1;
  
  undo:
  	sop--;
  	while (sop >= sops) {

  		sem_op = sop->sem_op;
  		sma->sem_base[sop->sem_num].semval -= sem_op;
  		if (sop->sem_flg & SEM_UNDO)
  			un->semadj[sop->sem_num] += sem_op;

  		sop--;
  	}
  
  	return result;
  }

  /** wake_up_sem_queue_prepare(q, error): Prepare wake-up
   * @q: queue entry that must be signaled
   * @error: Error value for the signal
   *
   * Prepare the wake-up of the queue entry q.

   */

  static void wake_up_sem_queue_prepare(struct list_head *pt,
  				struct sem_queue *q, int error)

  {

  	if (list_empty(pt)) {
  		/*
  		 * Hold preempt off so that we don't get preempted and have the
  		 * wakee busy-wait until we're scheduled back on.
  		 */
  		preempt_disable();
  	}

  	q->status = IN_WAKEUP;

  	q->pid = error;

  	list_add_tail(&q->list, pt);

  }
  
  /**

   * wake_up_sem_queue_do - do the actual wake-up

   * @pt: list of tasks to be woken up
   *
   * Do the actual wake-up.
   * The function is called without any locks held, thus the semaphore array
   * could be destroyed already and the tasks can disappear as soon as the
   * status is set to the actual return code.
   */
  static void wake_up_sem_queue_do(struct list_head *pt)
  {
  	struct sem_queue *q, *t;
  	int did_something;
  
  	did_something = !list_empty(pt);

  	list_for_each_entry_safe(q, t, pt, list) {

  		wake_up_process(q->sleeper);
  		/* q can disappear immediately after writing q->status. */
  		smp_wmb();
  		q->status = q->pid;
  	}
  	if (did_something)
  		preempt_enable();

  }

  static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
  {
  	list_del(&q->list);

  	if (q->nsops > 1)

  		sma->complex_count--;
  }

  /** check_restart(sma, q)
   * @sma: semaphore array
   * @q: the operation that just completed
   *
   * update_queue is O(N^2) when it restarts scanning the whole queue of
   * waiting operations. Therefore this function checks if the restart is
   * really necessary. It is called after a previously waiting operation

   * modified the array.
   * Note that wait-for-zero operations are handled without restart.

   */
  static int check_restart(struct sem_array *sma, struct sem_queue *q)
  {

  	/* pending complex alter operations are too difficult to analyse */
  	if (!list_empty(&sma->pending_alter))

  		return 1;
  
  	/* we were a sleeping complex operation. Too difficult */
  	if (q->nsops > 1)
  		return 1;

  	/* It is impossible that someone waits for the new value:
  	 * - complex operations always restart.
  	 * - wait-for-zero are handled seperately.
  	 * - q is a previously sleeping simple operation that
  	 *   altered the array. It must be a decrement, because
  	 *   simple increments never sleep.
  	 * - If there are older (higher priority) decrements
  	 *   in the queue, then they have observed the original
  	 *   semval value and couldn't proceed. The operation
  	 *   decremented to value - thus they won't proceed either.
  	 */
  	return 0;
  }

  /**

   * wake_const_ops - wake up non-alter tasks

   * @sma: semaphore array.
   * @semnum: semaphore that was modified.
   * @pt: list head for the tasks that must be woken up.
   *
   * wake_const_ops must be called after a semaphore in a semaphore array
   * was set to 0. If complex const operations are pending, wake_const_ops must
   * be called with semnum = -1, as well as with the number of each modified
   * semaphore.
   * The tasks that must be woken up are added to @pt. The return code
   * is stored in q->pid.
   * The function returns 1 if at least one operation was completed successfully.
   */
  static int wake_const_ops(struct sem_array *sma, int semnum,
  				struct list_head *pt)
  {
  	struct sem_queue *q;
  	struct list_head *walk;
  	struct list_head *pending_list;
  	int semop_completed = 0;
  
  	if (semnum == -1)
  		pending_list = &sma->pending_const;
  	else
  		pending_list = &sma->sem_base[semnum].pending_const;

  	walk = pending_list->next;
  	while (walk != pending_list) {
  		int error;
  
  		q = container_of(walk, struct sem_queue, list);
  		walk = walk->next;

  		error = perform_atomic_semop(sma, q);

  
  		if (error <= 0) {
  			/* operation completed, remove from queue & wakeup */
  
  			unlink_queue(sma, q);
  
  			wake_up_sem_queue_prepare(pt, q, error);
  			if (error == 0)
  				semop_completed = 1;
  		}
  	}
  	return semop_completed;
  }
  
  /**

   * do_smart_wakeup_zero - wakeup all wait for zero tasks

   * @sma: semaphore array
   * @sops: operations that were performed
   * @nsops: number of operations
   * @pt: list head of the tasks that must be woken up.
   *

   * Checks all required queue for wait-for-zero operations, based
   * on the actual changes that were performed on the semaphore array.

   * The function returns 1 if at least one operation was completed successfully.
   */
  static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
  					int nsops, struct list_head *pt)
  {
  	int i;
  	int semop_completed = 0;
  	int got_zero = 0;
  
  	/* first: the per-semaphore queues, if known */
  	if (sops) {
  		for (i = 0; i < nsops; i++) {
  			int num = sops[i].sem_num;
  
  			if (sma->sem_base[num].semval == 0) {
  				got_zero = 1;
  				semop_completed |= wake_const_ops(sma, num, pt);
  			}
  		}
  	} else {
  		/*
  		 * No sops means modified semaphores not known.
  		 * Assume all were changed.

  		 */

  		for (i = 0; i < sma->sem_nsems; i++) {
  			if (sma->sem_base[i].semval == 0) {
  				got_zero = 1;
  				semop_completed |= wake_const_ops(sma, i, pt);
  			}
  		}

  	}
  	/*

  	 * If one of the modified semaphores got 0,
  	 * then check the global queue, too.

  	 */

  	if (got_zero)
  		semop_completed |= wake_const_ops(sma, -1, pt);

  	return semop_completed;

  }

  
  /**

   * update_queue - look for tasks that can be completed.

   * @sma: semaphore array.
   * @semnum: semaphore that was modified.

   * @pt: list head for the tasks that must be woken up.

   *
   * update_queue must be called after a semaphore in a semaphore array

   * was modified. If multiple semaphores were modified, update_queue must
   * be called with semnum = -1, as well as with the number of each modified
   * semaphore.

   * The tasks that must be woken up are added to @pt. The return code
   * is stored in q->pid.

   * The function internally checks if const operations can now succeed.
   *

   * The function return 1 if at least one semop was completed successfully.

   */

  static int update_queue(struct sem_array *sma, int semnum, struct list_head *pt)

  {

  	struct sem_queue *q;
  	struct list_head *walk;
  	struct list_head *pending_list;

  	int semop_completed = 0;

  	if (semnum == -1)

  		pending_list = &sma->pending_alter;

  	else

  		pending_list = &sma->sem_base[semnum].pending_alter;

  
  again:

  	walk = pending_list->next;
  	while (walk != pending_list) {

  		int error, restart;

  		q = container_of(walk, struct sem_queue, list);

  		walk = walk->next;

  		/* If we are scanning the single sop, per-semaphore list of
  		 * one semaphore and that semaphore is 0, then it is not

  		 * necessary to scan further: simple increments

  		 * that affect only one entry succeed immediately and cannot
  		 * be in the  per semaphore pending queue, and decrements
  		 * cannot be successful if the value is already 0.
  		 */

  		if (semnum != -1 && sma->sem_base[semnum].semval == 0)

  			break;

  		error = perform_atomic_semop(sma, q);

  
  		/* Does q->sleeper still need to sleep? */

  		if (error > 0)
  			continue;

  		unlink_queue(sma, q);

  		if (error) {

  			restart = 0;

  		} else {
  			semop_completed = 1;

  			do_smart_wakeup_zero(sma, q->sops, q->nsops, pt);

  			restart = check_restart(sma, q);

  		}

  		wake_up_sem_queue_prepare(pt, q, error);

  		if (restart)

  			goto again;

  	}

  	return semop_completed;

  }

  /**

   * set_semotime - set sem_otime

   * @sma: semaphore array
   * @sops: operations that modified the array, may be NULL
   *
   * sem_otime is replicated to avoid cache line trashing.
   * This function sets one instance to the current time.
   */
  static void set_semotime(struct sem_array *sma, struct sembuf *sops)
  {
  	if (sops == NULL) {
  		sma->sem_base[0].sem_otime = get_seconds();
  	} else {
  		sma->sem_base[sops[0].sem_num].sem_otime =
  							get_seconds();
  	}
  }
  
  /**

   * do_smart_update - optimized update_queue

   * @sma: semaphore array
   * @sops: operations that were performed
   * @nsops: number of operations

   * @otime: force setting otime
   * @pt: list head of the tasks that must be woken up.

   *

   * do_smart_update() does the required calls to update_queue and wakeup_zero,
   * based on the actual changes that were performed on the semaphore array.

   * Note that the function does not do the actual wake-up: the caller is
   * responsible for calling wake_up_sem_queue_do(@pt).
   * It is safe to perform this call after dropping all locks.

   */

  static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsops,
  			int otime, struct list_head *pt)

  {
  	int i;

  	otime |= do_smart_wakeup_zero(sma, sops, nsops, pt);

  	if (!list_empty(&sma->pending_alter)) {
  		/* semaphore array uses the global queue - just process it. */
  		otime |= update_queue(sma, -1, pt);
  	} else {
  		if (!sops) {
  			/*
  			 * No sops, thus the modified semaphores are not
  			 * known. Check all.
  			 */
  			for (i = 0; i < sma->sem_nsems; i++)
  				otime |= update_queue(sma, i, pt);
  		} else {
  			/*
  			 * Check the semaphores that were increased:
  			 * - No complex ops, thus all sleeping ops are
  			 *   decrease.
  			 * - if we decreased the value, then any sleeping
  			 *   semaphore ops wont be able to run: If the
  			 *   previous value was too small, then the new
  			 *   value will be too small, too.
  			 */
  			for (i = 0; i < nsops; i++) {
  				if (sops[i].sem_op > 0) {
  					otime |= update_queue(sma,
  							sops[i].sem_num, pt);
  				}

  			}

  		}

  	}

  	if (otime)
  		set_semotime(sma, sops);

  }

  /*

   * check_qop: Test if a queued operation sleeps on the semaphore semnum

   */
  static int check_qop(struct sem_array *sma, int semnum, struct sem_queue *q,
  			bool count_zero)
  {

  	struct sembuf *sop = q->blocking;

  	/*
  	 * Linux always (since 0.99.10) reported a task as sleeping on all
  	 * semaphores. This violates SUS, therefore it was changed to the
  	 * standard compliant behavior.
  	 * Give the administrators a chance to notice that an application
  	 * might misbehave because it relies on the Linux behavior.
  	 */
  	pr_info_once("semctl(GETNCNT/GETZCNT) is since 3.16 Single Unix Specification compliant.
  "
  			"The task %s (%d) triggered the difference, watch for misbehavior.
  ",
  			current->comm, task_pid_nr(current));

  	if (sop->sem_num != semnum)
  		return 0;

  	if (count_zero && sop->sem_op == 0)
  		return 1;
  	if (!count_zero && sop->sem_op < 0)
  		return 1;
  
  	return 0;

  }

  /* The following counts are associated to each semaphore:
   *   semncnt        number of tasks waiting on semval being nonzero
   *   semzcnt        number of tasks waiting on semval being zero

   *
   * Per definition, a task waits only on the semaphore of the first semop
   * that cannot proceed, even if additional operation would block, too.

   */

  static int count_semcnt(struct sem_array *sma, ushort semnum,
  			bool count_zero)

  {

  	struct list_head *l;

  	struct sem_queue *q;

  	int semcnt;

  	semcnt = 0;
  	/* First: check the simple operations. They are easy to evaluate */
  	if (count_zero)
  		l = &sma->sem_base[semnum].pending_const;
  	else
  		l = &sma->sem_base[semnum].pending_alter;

  	list_for_each_entry(q, l, list) {
  		/* all task on a per-semaphore list sleep on exactly
  		 * that semaphore
  		 */
  		semcnt++;

  	}

  	/* Then: check the complex operations. */

  	list_for_each_entry(q, &sma->pending_alter, list) {

  		semcnt += check_qop(sma, semnum, q, count_zero);
  	}
  	if (count_zero) {
  		list_for_each_entry(q, &sma->pending_const, list) {
  			semcnt += check_qop(sma, semnum, q, count_zero);
  		}

  	}

  	return semcnt;

  }

  /* Free a semaphore set. freeary() is called with sem_ids.rwsem locked
   * as a writer and the spinlock for this semaphore set hold. sem_ids.rwsem

   * remains locked on exit.

   */

  static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)

  {

  	struct sem_undo *un, *tu;
  	struct sem_queue *q, *tq;

  	struct sem_array *sma = container_of(ipcp, struct sem_array, sem_perm);

  	struct list_head tasks;

  	int i;

  	/* Free the existing undo structures for this semaphore set.  */

  	ipc_assert_locked_object(&sma->sem_perm);

  	list_for_each_entry_safe(un, tu, &sma->list_id, list_id) {
  		list_del(&un->list_id);
  		spin_lock(&un->ulp->lock);

  		un->semid = -1;

  		list_del_rcu(&un->list_proc);
  		spin_unlock(&un->ulp->lock);

  		kfree_rcu(un, rcu);

  	}

  
  	/* Wake up all pending processes and let them fail with EIDRM. */

  	INIT_LIST_HEAD(&tasks);

  	list_for_each_entry_safe(q, tq, &sma->pending_const, list) {
  		unlink_queue(sma, q);
  		wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
  	}
  
  	list_for_each_entry_safe(q, tq, &sma->pending_alter, list) {

  		unlink_queue(sma, q);

  		wake_up_sem_queue_prepare(&tasks, q, -EIDRM);

  	}

  	for (i = 0; i < sma->sem_nsems; i++) {
  		struct sem *sem = sma->sem_base + i;

  		list_for_each_entry_safe(q, tq, &sem->pending_const, list) {
  			unlink_queue(sma, q);
  			wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
  		}
  		list_for_each_entry_safe(q, tq, &sem->pending_alter, list) {

  			unlink_queue(sma, q);
  			wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
  		}
  	}

  	/* Remove the semaphore set from the IDR */
  	sem_rmid(ns, sma);

  	sem_unlock(sma, -1);

  	rcu_read_unlock();

  	wake_up_sem_queue_do(&tasks);

  	ns->used_sems -= sma->sem_nsems;

  	ipc_rcu_putref(sma, sem_rcu_free);

  }
  
  static unsigned long copy_semid_to_user(void __user *buf, struct semid64_ds *in, int version)
  {

  	switch (version) {

  	case IPC_64:
  		return copy_to_user(buf, in, sizeof(*in));
  	case IPC_OLD:
  	    {
  		struct semid_ds out;

  		memset(&out, 0, sizeof(out));

  		ipc64_perm_to_ipc_perm(&in->sem_perm, &out.sem_perm);
  
  		out.sem_otime	= in->sem_otime;
  		out.sem_ctime	= in->sem_ctime;
  		out.sem_nsems	= in->sem_nsems;
  
  		return copy_to_user(buf, &out, sizeof(out));
  	    }
  	default:
  		return -EINVAL;
  	}
  }

  static time_t get_semotime(struct sem_array *sma)
  {
  	int i;
  	time_t res;
  
  	res = sma->sem_base[0].sem_otime;
  	for (i = 1; i < sma->sem_nsems; i++) {
  		time_t to = sma->sem_base[i].sem_otime;
  
  		if (to > res)
  			res = to;
  	}
  	return res;
  }

  static int semctl_nolock(struct ipc_namespace *ns, int semid,

  			 int cmd, int version, void __user *p)

  {

  	int err;

  	struct sem_array *sma;

  	switch (cmd) {

  	case IPC_INFO:
  	case SEM_INFO:
  	{
  		struct seminfo seminfo;
  		int max_id;
  
  		err = security_sem_semctl(NULL, cmd);
  		if (err)
  			return err;

  		memset(&seminfo, 0, sizeof(seminfo));

  		seminfo.semmni = ns->sc_semmni;
  		seminfo.semmns = ns->sc_semmns;
  		seminfo.semmsl = ns->sc_semmsl;
  		seminfo.semopm = ns->sc_semopm;

  		seminfo.semvmx = SEMVMX;
  		seminfo.semmnu = SEMMNU;
  		seminfo.semmap = SEMMAP;
  		seminfo.semume = SEMUME;

  		down_read(&sem_ids(ns).rwsem);

  		if (cmd == SEM_INFO) {

  			seminfo.semusz = sem_ids(ns).in_use;
  			seminfo.semaem = ns->used_sems;

  		} else {
  			seminfo.semusz = SEMUSZ;
  			seminfo.semaem = SEMAEM;
  		}

  		max_id = ipc_get_maxid(&sem_ids(ns));

  		up_read(&sem_ids(ns).rwsem);

  		if (copy_to_user(p, &seminfo, sizeof(struct seminfo)))

  			return -EFAULT;

  		return (max_id < 0) ? 0 : max_id;

  	}

  	case IPC_STAT:

  	case SEM_STAT:
  	{
  		struct semid64_ds tbuf;

  		int id = 0;
  
  		memset(&tbuf, 0, sizeof(tbuf));

  		rcu_read_lock();

  		if (cmd == SEM_STAT) {

  			sma = sem_obtain_object(ns, semid);
  			if (IS_ERR(sma)) {
  				err = PTR_ERR(sma);
  				goto out_unlock;
  			}

  			id = sma->sem_perm.id;
  		} else {

  			sma = sem_obtain_object_check(ns, semid);
  			if (IS_ERR(sma)) {
  				err = PTR_ERR(sma);
  				goto out_unlock;
  			}

  		}

  
  		err = -EACCES;

  		if (ipcperms(ns, &sma->sem_perm, S_IRUGO))

  			goto out_unlock;
  
  		err = security_sem_semctl(sma, cmd);
  		if (err)
  			goto out_unlock;

  		kernel_to_ipc64_perm(&sma->sem_perm, &tbuf.sem_perm);

  		tbuf.sem_otime = get_semotime(sma);
  		tbuf.sem_ctime = sma->sem_ctime;
  		tbuf.sem_nsems = sma->sem_nsems;

  		rcu_read_unlock();

  		if (copy_semid_to_user(p, &tbuf, version))

  			return -EFAULT;
  		return id;
  	}
  	default:
  		return -EINVAL;
  	}

  out_unlock:

  	rcu_read_unlock();

  	return err;
  }

  static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
  		unsigned long arg)
  {
  	struct sem_undo *un;
  	struct sem_array *sma;

  	struct sem *curr;

  	int err;

  	struct list_head tasks;
  	int val;
  #if defined(CONFIG_64BIT) && defined(__BIG_ENDIAN)
  	/* big-endian 64bit */
  	val = arg >> 32;
  #else
  	/* 32bit or little-endian 64bit */
  	val = arg;
  #endif

  	if (val > SEMVMX || val < 0)
  		return -ERANGE;

  
  	INIT_LIST_HEAD(&tasks);

  	rcu_read_lock();
  	sma = sem_obtain_object_check(ns, semid);
  	if (IS_ERR(sma)) {
  		rcu_read_unlock();
  		return PTR_ERR(sma);
  	}
  
  	if (semnum < 0 || semnum >= sma->sem_nsems) {
  		rcu_read_unlock();
  		return -EINVAL;
  	}
  
  
  	if (ipcperms(ns, &sma->sem_perm, S_IWUGO)) {
  		rcu_read_unlock();
  		return -EACCES;
  	}

  
  	err = security_sem_semctl(sma, SETVAL);

  	if (err) {
  		rcu_read_unlock();
  		return -EACCES;
  	}

  	sem_lock(sma, NULL, -1);

  	if (!ipc_valid_object(&sma->sem_perm)) {

  		sem_unlock(sma, -1);
  		rcu_read_unlock();
  		return -EIDRM;
  	}

  	curr = &sma->sem_base[semnum];

  	ipc_assert_locked_object(&sma->sem_perm);

  	list_for_each_entry(un, &sma->list_id, list_id)
  		un->semadj[semnum] = 0;
  
  	curr->semval = val;
  	curr->sempid = task_tgid_vnr(current);
  	sma->sem_ctime = get_seconds();
  	/* maybe some queued-up processes were waiting for this */
  	do_smart_update(sma, NULL, 0, 0, &tasks);

  	sem_unlock(sma, -1);

  	rcu_read_unlock();

  	wake_up_sem_queue_do(&tasks);

  	return 0;

  }

  static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,

  		int cmd, void __user *p)

  {
  	struct sem_array *sma;

  	struct sem *curr;

  	int err, nsems;

  	ushort fast_sem_io[SEMMSL_FAST];

  	ushort *sem_io = fast_sem_io;

  	struct list_head tasks;

  	INIT_LIST_HEAD(&tasks);
  
  	rcu_read_lock();
  	sma = sem_obtain_object_check(ns, semid);
  	if (IS_ERR(sma)) {
  		rcu_read_unlock();

  		return PTR_ERR(sma);

  	}

  
  	nsems = sma->sem_nsems;

  	err = -EACCES;

  	if (ipcperms(ns, &sma->sem_perm, cmd == SETALL ? S_IWUGO : S_IRUGO))
  		goto out_rcu_wakeup;

  
  	err = security_sem_semctl(sma, cmd);

  	if (err)
  		goto out_rcu_wakeup;

  
  	err = -EACCES;
  	switch (cmd) {
  	case GETALL:
  	{

  		ushort __user *array = p;

  		int i;

  		sem_lock(sma, NULL, -1);

  		if (!ipc_valid_object(&sma->sem_perm)) {

  			err = -EIDRM;
  			goto out_unlock;
  		}

  		if (nsems > SEMMSL_FAST) {

  			if (!ipc_rcu_getref(sma)) {

  				err = -EIDRM;

  				goto out_unlock;

  			}
  			sem_unlock(sma, -1);

  			rcu_read_unlock();

  			sem_io = ipc_alloc(sizeof(ushort)*nsems);

  			if (sem_io == NULL) {

  				ipc_rcu_putref(sma, ipc_rcu_free);

  				return -ENOMEM;
  			}

  			rcu_read_lock();

  			sem_lock_and_putref(sma);

  			if (!ipc_valid_object(&sma->sem_perm)) {

  				err = -EIDRM;

  				goto out_unlock;

  			}

  		}

  		for (i = 0; i < sma->sem_nsems; i++)
  			sem_io[i] = sma->sem_base[i].semval;

  		sem_unlock(sma, -1);

  		rcu_read_unlock();

  		err = 0;

  		if (copy_to_user(array, sem_io, nsems*sizeof(ushort)))

  			err = -EFAULT;
  		goto out_free;
  	}
  	case SETALL:
  	{
  		int i;
  		struct sem_undo *un;

  		if (!ipc_rcu_getref(sma)) {

  			err = -EIDRM;
  			goto out_rcu_wakeup;

  		}

  		rcu_read_unlock();

  		if (nsems > SEMMSL_FAST) {

  			sem_io = ipc_alloc(sizeof(ushort)*nsems);

  			if (sem_io == NULL) {

  				ipc_rcu_putref(sma, ipc_rcu_free);

  				return -ENOMEM;
  			}
  		}

  		if (copy_from_user(sem_io, p, nsems*sizeof(ushort))) {

  			ipc_rcu_putref(sma, ipc_rcu_free);

  			err = -EFAULT;
  			goto out_free;
  		}
  
  		for (i = 0; i < nsems; i++) {
  			if (sem_io[i] > SEMVMX) {

  				ipc_rcu_putref(sma, ipc_rcu_free);

  				err = -ERANGE;
  				goto out_free;
  			}
  		}

  		rcu_read_lock();

  		sem_lock_and_putref(sma);

  		if (!ipc_valid_object(&sma->sem_perm)) {

  			err = -EIDRM;

  			goto out_unlock;

  		}
  
  		for (i = 0; i < nsems; i++)
  			sma->sem_base[i].semval = sem_io[i];

  		ipc_assert_locked_object(&sma->sem_perm);

  		list_for_each_entry(un, &sma->list_id, list_id) {

  			for (i = 0; i < nsems; i++)
  				un->semadj[i] = 0;

  		}

  		sma->sem_ctime = get_seconds();
  		/* maybe some queued-up processes were waiting for this */

  		do_smart_update(sma, NULL, 0, 0, &tasks);

  		err = 0;
  		goto out_unlock;
  	}

  	/* GETVAL, GETPID, GETNCTN, GETZCNT: fall-through */

  	}
  	err = -EINVAL;

  	if (semnum < 0 || semnum >= nsems)
  		goto out_rcu_wakeup;

  	sem_lock(sma, NULL, -1);

  	if (!ipc_valid_object(&sma->sem_perm)) {

  		err = -EIDRM;
  		goto out_unlock;
  	}

  	curr = &sma->sem_base[semnum];
  
  	switch (cmd) {
  	case GETVAL:
  		err = curr->semval;
  		goto out_unlock;
  	case GETPID:
  		err = curr->sempid;
  		goto out_unlock;
  	case GETNCNT:

  		err = count_semcnt(sma, semnum, 0);

  		goto out_unlock;
  	case GETZCNT:

  		err = count_semcnt(sma, semnum, 1);

  		goto out_unlock;

  	}

  out_unlock:

  	sem_unlock(sma, -1);

  out_rcu_wakeup:

  	rcu_read_unlock();

  	wake_up_sem_queue_do(&tasks);

  out_free:

  	if (sem_io != fast_sem_io)

  		ipc_free(sem_io, sizeof(ushort)*nsems);
  	return err;
  }

  static inline unsigned long
  copy_semid_from_user(struct semid64_ds *out, void __user *buf, int version)

  {

  	switch (version) {

  	case IPC_64:

  		if (copy_from_user(out, buf, sizeof(*out)))

  			return -EFAULT;

  		return 0;

  	case IPC_OLD:
  	    {
  		struct semid_ds tbuf_old;

  		if (copy_from_user(&tbuf_old, buf, sizeof(tbuf_old)))

  			return -EFAULT;

  		out->sem_perm.uid	= tbuf_old.sem_perm.uid;
  		out->sem_perm.gid	= tbuf_old.sem_perm.gid;
  		out->sem_perm.mode	= tbuf_old.sem_perm.mode;

  
  		return 0;
  	    }
  	default:
  		return -EINVAL;
  	}
  }

  /*

   * This function handles some semctl commands which require the rwsem

   * to be held in write mode.

   * NOTE: no locks must be held, the rwsem is taken inside this function.

   */

  static int semctl_down(struct ipc_namespace *ns, int semid,

  		       int cmd, int version, void __user *p)

  {
  	struct sem_array *sma;
  	int err;

  	struct semid64_ds semid64;

  	struct kern_ipc_perm *ipcp;

  	if (cmd == IPC_SET) {

  		if (copy_semid_from_user(&semid64, p, version))

  			return -EFAULT;

  	}

  	down_write(&sem_ids(ns).rwsem);

  	rcu_read_lock();

  	ipcp = ipcctl_pre_down_nolock(ns, &sem_ids(ns), semid, cmd,
  				      &semid64.sem_perm, 0);

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

  		goto out_unlock1;
  	}

  	sma = container_of(ipcp, struct sem_array, sem_perm);

  
  	err = security_sem_semctl(sma, cmd);

  	if (err)
  		goto out_unlock1;

  	switch (cmd) {

  	case IPC_RMID:

  		sem_lock(sma, NULL, -1);

  		/* freeary unlocks the ipc object and rcu */

  		freeary(ns, ipcp);

  		goto out_up;

  	case IPC_SET:

  		sem_lock(sma, NULL, -1);

  		err = ipc_update_perm(&semid64.sem_perm, ipcp);
  		if (err)

  			goto out_unlock0;

  		sma->sem_ctime = get_seconds();

  		break;
  	default:

  		err = -EINVAL;

  		goto out_unlock1;

  	}

  out_unlock0:

  	sem_unlock(sma, -1);

  out_unlock1:

  	rcu_read_unlock();

  out_up:

  	up_write(&sem_ids(ns).rwsem);

  	return err;
  }

  SYSCALL_DEFINE4(semctl, int, semid, int, semnum, int, cmd, unsigned long, arg)

  {

  	int version;

  	struct ipc_namespace *ns;

  	void __user *p = (void __user *)arg;

  
  	if (semid < 0)
  		return -EINVAL;
  
  	version = ipc_parse_version(&cmd);

  	ns = current->nsproxy->ipc_ns;

  	switch (cmd) {

  	case IPC_INFO:
  	case SEM_INFO:

  	case IPC_STAT:

  	case SEM_STAT:

  		return semctl_nolock(ns, semid, cmd, version, p);

  	case GETALL:
  	case GETVAL:
  	case GETPID:
  	case GETNCNT:
  	case GETZCNT:

  	case SETALL:

  		return semctl_main(ns, semid, semnum, cmd, p);
  	case SETVAL:
  		return semctl_setval(ns, semid, semnum, arg);

  	case IPC_RMID:
  	case IPC_SET:

  		return semctl_down(ns, semid, cmd, version, p);

  	default:
  		return -EINVAL;
  	}
  }

  /* If the task doesn't already have a undo_list, then allocate one
   * here.  We guarantee there is only one thread using this undo list,
   * and current is THE ONE
   *
   * If this allocation and assignment succeeds, but later
   * portions of this code fail, there is no need to free the sem_undo_list.
   * Just let it stay associated with the task, and it'll be freed later
   * at exit time.
   *
   * This can block, so callers must hold no locks.
   */
  static inline int get_undo_list(struct sem_undo_list **undo_listp)
  {
  	struct sem_undo_list *undo_list;