xfrm: prevent ipcomp scratch buffer race condition

[karo-tx-linux.git] / ipc / sem.c

/*
 * 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_semncnt() and
 *   count_semzcnt()
 * - 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 <asm/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 */
        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\n",
                                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 (!ipcp->deleted)
                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;
        struct ipc_ops sem_ops;
        struct ipc_params sem_params;

        ns = current->nsproxy->ipc_ns;

        if (nsems < 0 || nsems > ns->sc_semmsl)
                return -EINVAL;

        sem_ops.getnew = newary;
        sem_ops.associate = sem_security;
        sem_ops.more_checks = sem_more_checks;

        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
 * @sops: array with operations that should be checked
 * @nsems: number of sops
 * @un: undo array
 * @pid: pid that did the change
 *
 * 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 sembuf *sops,
                             int nsops, struct sem_undo *un, int pid)
{
        int result, sem_op;
        struct sembuf *sop;
        struct sem * curr;

        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;
                }
                curr->semval = result;
        }

        sop--;
        while (sop >= sops) {
                sma->sem_base[sop->sem_num].sempid = pid;
                if (sop->sem_flg & SEM_UNDO)
                        un->semadj[sop->sem_num] -= sop->sem_op;
                sop--;
        }
        
        return 0;

out_of_range:
        result = -ERANGE;
        goto undo;

would_block:
        if (sop->sem_flg & IPC_NOWAIT)
                result = -EAGAIN;
        else
                result = 1;

undo:
        sop--;
        while (sop >= sops) {
                sma->sem_base[sop->sem_num].semval -= sop->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(pt) - 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(sma, semnum, pt) - 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->sops, q->nsops,
                                                 q->undo, q->pid);

                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(sma, sops, nsops, pt) - 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.
 *
 * do_smart_wakeup_zero() 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(sma, semnum): 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->sops, q->nsops,
                                         q->undo, q->pid);

                /* 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(sma, sops) - 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(sma, sops, nsops, otime, pt) - 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);
}

/* 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
 * This model assumes that a task waits on exactly one semaphore.
 * Since semaphore operations are to be performed atomically, tasks actually
 * wait on a whole sequence of semaphores simultaneously.
 * The counts we return here are a rough approximation, but still
 * warrant that semncnt+semzcnt>0 if the task is on the pending queue.
 */
static int count_semncnt (struct sem_array * sma, ushort semnum)
{
        int semncnt;
        struct sem_queue * q;

        semncnt = 0;
        list_for_each_entry(q, &sma->sem_base[semnum].pending_alter, list) {
                struct sembuf * sops = q->sops;
                BUG_ON(sops->sem_num != semnum);
                if ((sops->sem_op < 0) && !(sops->sem_flg & IPC_NOWAIT))
                        semncnt++;
        }

        list_for_each_entry(q, &sma->pending_alter, list) {
                struct sembuf * sops = q->sops;
                int nsops = q->nsops;
                int i;
                for (i = 0; i < nsops; i++)
                        if (sops[i].sem_num == semnum
                            && (sops[i].sem_op < 0)
                            && !(sops[i].sem_flg & IPC_NOWAIT))
                                semncnt++;
        }
        return semncnt;
}

static int count_semzcnt (struct sem_array * sma, ushort semnum)
{
        int semzcnt;
        struct sem_queue * q;

        semzcnt = 0;
        list_for_each_entry(q, &sma->sem_base[semnum].pending_const, list) {
                struct sembuf * sops = q->sops;
                BUG_ON(sops->sem_num != semnum);
                if ((sops->sem_op == 0) && !(sops->sem_flg & IPC_NOWAIT))
                        semzcnt++;
        }

        list_for_each_entry(q, &sma->pending_const, list) {
                struct sembuf * sops = q->sops;
                int nsops = q->nsops;
                int i;
                for (i = 0; i < nsops; i++)
                        if (sops[i].sem_num == semnum
                            && (sops[i].sem_op == 0)
                            && !(sops[i].sem_flg & IPC_NOWAIT))
                                semzcnt++;
        }
        return semzcnt;
}

/* 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);

        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(nsems > SEMMSL_FAST) {
                        if (!ipc_rcu_getref(sma)) {
                                sem_unlock(sma, -1);
                                rcu_read_unlock();
                                err = -EIDRM;
                                goto out_free;
                        }
                        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 (sma->sem_perm.deleted) {
                                sem_unlock(sma, -1);
                                rcu_read_unlock();
                                err = -EIDRM;
                                goto out_free;
                        }
                }
                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)) {
                        rcu_read_unlock();
                        return -EIDRM;
                }
                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 (sma->sem_perm.deleted) {
                        sem_unlock(sma, -1);
                        rcu_read_unlock();
                        err = -EIDRM;
                        goto out_free;
                }

                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);
        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_semncnt(sma,semnum);
                goto out_unlock;
        case GETZCNT:
                err = count_semzcnt(sma,semnum);
                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;

        undo_list = current->sysvsem.undo_list;
        if (!undo_list) {
                undo_list = kzalloc(sizeof(*undo_list), GFP_KERNEL);
                if (undo_list == NULL)
                        return -ENOMEM;
                spin_lock_init(&undo_list->lock);
                atomic_set(&undo_list->refcnt, 1);
                INIT_LIST_HEAD(&undo_list->list_proc);

                current->sysvsem.undo_list = undo_list;
        }
        *undo_listp = undo_list;
        return 0;
}

static struct sem_undo *__lookup_undo(struct sem_undo_list *ulp, int semid)
{
        struct sem_undo *un;

        list_for_each_entry_rcu(un, &ulp->list_proc, list_proc) {
                if (un->semid == semid)
                        return un;
        }
        return NULL;
}

static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid)
{
        struct sem_undo *un;

        assert_spin_locked(&ulp->lock);

        un = __lookup_undo(ulp, semid);
        if (un) {
                list_del_rcu(&un->list_proc);
                list_add_rcu(&un->list_proc, &ulp->list_proc);
        }
        return un;
}

/**
 * find_alloc_undo - Lookup (and if not present create) undo array
 * @ns: namespace
 * @semid: semaphore array id
 *
 * The function looks up (and if not present creates) the undo structure.
 * The size of the undo structure depends on the size of the semaphore
 * array, thus the alloc path is not that straightforward.
 * Lifetime-rules: sem_undo is rcu-protected, on success, the function
 * performs a rcu_read_lock().
 */
static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid)
{
        struct sem_array *sma;
        struct sem_undo_list *ulp;
        struct sem_undo *un, *new;
        int nsems, error;

        error = get_undo_list(&ulp);
        if (error)
                return ERR_PTR(error);

        rcu_read_lock();
        spin_lock(&ulp->lock);
        un = lookup_undo(ulp, semid);
        spin_unlock(&ulp->lock);
        if (likely(un!=NULL))
                goto out;

        /* no undo structure around - allocate one. */
        /* step 1: figure out the size of the semaphore array */
        sma = sem_obtain_object_check(ns, semid);
        if (IS_ERR(sma)) {
                rcu_read_unlock();
                return ERR_CAST(sma);
        }

        nsems = sma->sem_nsems;
        if (!ipc_rcu_getref(sma)) {
                rcu_read_unlock();
                un = ERR_PTR(-EIDRM);
                goto out;
        }
        rcu_read_unlock();

        /* step 2: allocate new undo structure */
        new = kzalloc(sizeof(struct sem_undo) + sizeof(short)*nsems, GFP_KERNEL);
        if (!new) {
                ipc_rcu_putref(sma, ipc_rcu_free);
                return ERR_PTR(-ENOMEM);
        }

        /* step 3: Acquire the lock on semaphore array */
        rcu_read_lock();
        sem_lock_and_putref(sma);
        if (sma->sem_perm.deleted) {
                sem_unlock(sma, -1);
                rcu_read_unlock();
                kfree(new);
                un = ERR_PTR(-EIDRM);
                goto out;
        }
        spin_lock(&ulp->lock);

        /*
         * step 4: check for races: did someone else allocate the undo struct?
         */
        un = lookup_undo(ulp, semid);
        if (un) {
                kfree(new);
                goto success;
        }
        /* step 5: initialize & link new undo structure */
        new->semadj = (short *) &new[1];
        new->ulp = ulp;
        new->semid = semid;
        assert_spin_locked(&ulp->lock);
        list_add_rcu(&new->list_proc, &ulp->list_proc);
        ipc_assert_locked_object(&sma->sem_perm);
        list_add(&new->list_id, &sma->list_id);
        un = new;

success:
        spin_unlock(&ulp->lock);
        sem_unlock(sma, -1);
out:
        return un;
}


/**
 * get_queue_result - Retrieve the result code from sem_queue
 * @q: Pointer to queue structure
 *
 * Retrieve the return code from the pending queue. If IN_WAKEUP is found in
 * q->status, then we must loop until the value is replaced with the final
 * value: This may happen if a task is woken up by an unrelated event (e.g.
 * signal) and in parallel the task is woken up by another task because it got
 * the requested semaphores.
 *
 * The function can be called with or without holding the semaphore spinlock.
 */
static int get_queue_result(struct sem_queue *q)
{
        int error;

        error = q->status;
        while (unlikely(error == IN_WAKEUP)) {
                cpu_relax();
                error = q->status;
        }

        return error;
}

SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
                unsigned, nsops, const struct timespec __user *, timeout)
{
        int error = -EINVAL;
        struct sem_array *sma;
        struct sembuf fast_sops[SEMOPM_FAST];
        struct sembuf* sops = fast_sops, *sop;
        struct sem_undo *un;
        int undos = 0, alter = 0, max, locknum;
        struct sem_queue queue;
        unsigned long jiffies_left = 0;
        struct ipc_namespace *ns;
        struct list_head tasks;

        ns = current->nsproxy->ipc_ns;

        if (nsops < 1 || semid < 0)
                return -EINVAL;
        if (nsops > ns->sc_semopm)
                return -E2BIG;
        if(nsops > SEMOPM_FAST) {
                sops = kmalloc(sizeof(*sops)*nsops,GFP_KERNEL);
                if(sops==NULL)
                        return -ENOMEM;
        }
        if (copy_from_user (sops, tsops, nsops * sizeof(*tsops))) {
                error=-EFAULT;
                goto out_free;
        }
        if (timeout) {
                struct timespec _timeout;
                if (copy_from_user(&_timeout, timeout, sizeof(*timeout))) {
                        error = -EFAULT;
                        goto out_free;
                }
                if (_timeout.tv_sec < 0 || _timeout.tv_nsec < 0 ||
                        _timeout.tv_nsec >= 1000000000L) {
                        error = -EINVAL;
                        goto out_free;
                }
                jiffies_left = timespec_to_jiffies(&_timeout);
        }
        max = 0;
        for (sop = sops; sop < sops + nsops; sop++) {
                if (sop->sem_num >= max)
                        max = sop->sem_num;
                if (sop->sem_flg & SEM_UNDO)
                        undos = 1;
                if (sop->sem_op != 0)
                        alter = 1;
        }

        INIT_LIST_HEAD(&tasks);

        if (undos) {
                /* On success, find_alloc_undo takes the rcu_read_lock */
                un = find_alloc_undo(ns, semid);
                if (IS_ERR(un)) {
                        error = PTR_ERR(un);
                        goto out_free;
                }
        } else {
                un = NULL;
                rcu_read_lock();
        }

        sma = sem_obtain_object_check(ns, semid);
        if (IS_ERR(sma)) {
                rcu_read_unlock();
                error = PTR_ERR(sma);
                goto out_free;
        }

        error = -EFBIG;
        if (max >= sma->sem_nsems)
                goto out_rcu_wakeup;

        error = -EACCES;
        if (ipcperms(ns, &sma->sem_perm, alter ? S_IWUGO : S_IRUGO))
                goto out_rcu_wakeup;

        error = security_sem_semop(sma, sops, nsops, alter);
        if (error)
                goto out_rcu_wakeup;

        /*
         * semid identifiers are not unique - find_alloc_undo may have
         * allocated an undo structure, it was invalidated by an RMID
         * and now a new array with received the same id. Check and fail.
         * This case can be detected checking un->semid. The existence of
         * "un" itself is guaranteed by rcu.
         */
        error = -EIDRM;
        locknum = sem_lock(sma, sops, nsops);
        if (un && un->semid == -1)
                goto out_unlock_free;

        error = perform_atomic_semop(sma, sops, nsops, un,
                                        task_tgid_vnr(current));
        if (error == 0) {
                /* If the operation was successful, then do
                 * the required updates.
                 */
                if (alter)
                        do_smart_update(sma, sops, nsops, 1, &tasks);
                else
                        set_semotime(sma, sops);
        }
        if (error <= 0)
                goto out_unlock_free;

        /* We need to sleep on this operation, so we put the current
         * task into the pending queue and go to sleep.
         */
                
        queue.sops = sops;
        queue.nsops = nsops;
        queue.undo = un;
        queue.pid = task_tgid_vnr(current);
        queue.alter = alter;

        if (nsops == 1) {
                struct sem *curr;
                curr = &sma->sem_base[sops->sem_num];

                if (alter) {
                        if (sma->complex_count) {
                                list_add_tail(&queue.list,
                                                &sma->pending_alter);
                        } else {

                                list_add_tail(&queue.list,
                                                &curr->pending_alter);
                        }
                } else {
                        list_add_tail(&queue.list, &curr->pending_const);
                }
        } else {
                if (!sma->complex_count)
                        merge_queues(sma);

                if (alter)
                        list_add_tail(&queue.list, &sma->pending_alter);
                else
                        list_add_tail(&queue.list, &sma->pending_const);

                sma->complex_count++;
        }

        queue.status = -EINTR;
        queue.sleeper = current;

sleep_again:
        current->state = TASK_INTERRUPTIBLE;
        sem_unlock(sma, locknum);
        rcu_read_unlock();

        if (timeout)
                jiffies_left = schedule_timeout(jiffies_left);
        else
                schedule();

        error = get_queue_result(&queue);

        if (error != -EINTR) {
                /* fast path: update_queue already obtained all requested
                 * resources.
                 * Perform a smp_mb(): User space could assume that semop()
                 * is a memory barrier: Without the mb(), the cpu could
                 * speculatively read in user space stale data that was
                 * overwritten by the previous owner of the semaphore.
                 */
                smp_mb();

                goto out_free;
        }

        rcu_read_lock();
        sma = sem_obtain_lock(ns, semid, sops, nsops, &locknum);

        /*
         * Wait until it's guaranteed that no wakeup_sem_queue_do() is ongoing.
         */
        error = get_queue_result(&queue);

        /*
         * Array removed? If yes, leave without sem_unlock().
         */
        if (IS_ERR(sma)) {
                rcu_read_unlock();
                goto out_free;
        }


        /*
         * If queue.status != -EINTR we are woken up by another process.
         * Leave without unlink_queue(), but with sem_unlock().
         */

        if (error != -EINTR) {
                goto out_unlock_free;
        }

        /*
         * If an interrupt occurred we have to clean up the queue
         */
        if (timeout && jiffies_left == 0)
                error = -EAGAIN;

        /*
         * If the wakeup was spurious, just retry
         */
        if (error == -EINTR && !signal_pending(current))
                goto sleep_again;

        unlink_queue(sma, &queue);

out_unlock_free:
        sem_unlock(sma, locknum);
out_rcu_wakeup:
        rcu_read_unlock();
        wake_up_sem_queue_do(&tasks);
out_free:
        if(sops != fast_sops)
                kfree(sops);
        return error;
}

SYSCALL_DEFINE3(semop, int, semid, struct sembuf __user *, tsops,
                unsigned, nsops)
{
        return sys_semtimedop(semid, tsops, nsops, NULL);
}

/* If CLONE_SYSVSEM is set, establish sharing of SEM_UNDO state between
 * parent and child tasks.
 */

int copy_semundo(unsigned long clone_flags, struct task_struct *tsk)
{
        struct sem_undo_list *undo_list;
        int error;

        if (clone_flags & CLONE_SYSVSEM) {
                error = get_undo_list(&undo_list);
                if (error)
                        return error;
                atomic_inc(&undo_list->refcnt);
                tsk->sysvsem.undo_list = undo_list;
        } else 
                tsk->sysvsem.undo_list = NULL;

        return 0;
}

/*
 * add semadj values to semaphores, free undo structures.
 * undo structures are not freed when semaphore arrays are destroyed
 * so some of them may be out of date.
 * IMPLEMENTATION NOTE: There is some confusion over whether the
 * set of adjustments that needs to be done should be done in an atomic
 * manner or not. That is, if we are attempting to decrement the semval
 * should we queue up and wait until we can do so legally?
 * The original implementation attempted to do this (queue and wait).
 * The current implementation does not do so. The POSIX standard
 * and SVID should be consulted to determine what behavior is mandated.
 */
void exit_sem(struct task_struct *tsk)
{
        struct sem_undo_list *ulp;

        ulp = tsk->sysvsem.undo_list;
        if (!ulp)
                return;
        tsk->sysvsem.undo_list = NULL;

        if (!atomic_dec_and_test(&ulp->refcnt))
                return;

        for (;;) {
                struct sem_array *sma;
                struct sem_undo *un;
                struct list_head tasks;
                int semid, i;

                rcu_read_lock();
                un = list_entry_rcu(ulp->list_proc.next,
                                    struct sem_undo, list_proc);
                if (&un->list_proc == &ulp->list_proc)
                        semid = -1;
                 else
                        semid = un->semid;

                if (semid == -1) {
                        rcu_read_unlock();
                        break;
                }

                sma = sem_obtain_object_check(tsk->nsproxy->ipc_ns, un->semid);
                /* exit_sem raced with IPC_RMID, nothing to do */
                if (IS_ERR(sma)) {
                        rcu_read_unlock();
                        continue;
                }

                sem_lock(sma, NULL, -1);
                un = __lookup_undo(ulp, semid);
                if (un == NULL) {
                        /* exit_sem raced with IPC_RMID+semget() that created
                         * exactly the same semid. Nothing to do.
                         */
                        sem_unlock(sma, -1);
                        rcu_read_unlock();
                        continue;
                }

                /* remove un from the linked lists */
                ipc_assert_locked_object(&sma->sem_perm);
                list_del(&un->list_id);

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

                /* perform adjustments registered in un */
                for (i = 0; i < sma->sem_nsems; i++) {
                        struct sem * semaphore = &sma->sem_base[i];
                        if (un->semadj[i]) {
                                semaphore->semval += un->semadj[i];
                                /*
                                 * Range checks of the new semaphore value,
                                 * not defined by sus:
                                 * - Some unices ignore the undo entirely
                                 *   (e.g. HP UX 11i 11.22, Tru64 V5.1)
                                 * - some cap the value (e.g. FreeBSD caps
                                 *   at 0, but doesn't enforce SEMVMX)
                                 *
                                 * Linux caps the semaphore value, both at 0
                                 * and at SEMVMX.
                                 *
                                 *      Manfred <manfred@colorfullife.com>
                                 */
                                if (semaphore->semval < 0)
                                        semaphore->semval = 0;
                                if (semaphore->semval > SEMVMX)
                                        semaphore->semval = SEMVMX;
                                semaphore->sempid = task_tgid_vnr(current);
                        }
                }
                /* maybe some queued-up processes were waiting for this */
                INIT_LIST_HEAD(&tasks);
                do_smart_update(sma, NULL, 0, 1, &tasks);
                sem_unlock(sma, -1);
                rcu_read_unlock();
                wake_up_sem_queue_do(&tasks);

                kfree_rcu(un, rcu);
        }
        kfree(ulp);
}

#ifdef CONFIG_PROC_FS
static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
{
        struct user_namespace *user_ns = seq_user_ns(s);
        struct sem_array *sma = it;
        time_t sem_otime;

        /*
         * The proc interface isn't aware of sem_lock(), it calls
         * ipc_lock_object() directly (in sysvipc_find_ipc).
         * In order to stay compatible with sem_lock(), we must wait until
         * all simple semop() calls have left their critical regions.
         */
        sem_wait_array(sma);

        sem_otime = get_semotime(sma);

        return seq_printf(s,
                          "%10d %10d  %4o %10u %5u %5u %5u %5u %10lu %10lu\n",
                          sma->sem_perm.key,
                          sma->sem_perm.id,
                          sma->sem_perm.mode,
                          sma->sem_nsems,
                          from_kuid_munged(user_ns, sma->sem_perm.uid),
                          from_kgid_munged(user_ns, sma->sem_perm.gid),
                          from_kuid_munged(user_ns, sma->sem_perm.cuid),
                          from_kgid_munged(user_ns, sma->sem_perm.cgid),
                          sem_otime,
                          sma->sem_ctime);
}
#endif