msg.c

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/*
 * linux/ipc/msg.c
 * Copyright (C) 1992 Krishna Balasubramanian
 *
 * Removed all the remaining kerneld mess
 * Catch the -EFAULT stuff properly
 * Use GFP_KERNEL for messages as in 1.2
 * Fixed up the unchecked user space derefs
 * Copyright (C) 1998 Alan Cox & Andi Kleen
 *
 * /proc/sysvipc/msg support (c) 1999 Dragos Acostachioaie <[email protected]>
 *
 * mostly rewritten, threaded and wake-one semantics added
 * MSGMAX limit removed, sysctl's added
 * (c) 1999 Manfred Spraul <[email protected]>
 *
 * support for audit of ipc object properties and permission changes
 * Dustin Kirkland <[email protected]>
 *
 * namespaces support
 * OpenVZ, SWsoft Inc.
 * Pavel Emelianov <[email protected]>
 */

#include <linux/capability.h>
#include <linux/msg.h>
#include <linux/spinlock.h>
#include <linux/init.h>
#include <linux/mm.h>
#include <linux/proc_fs.h>
#include <linux/list.h>
#include <linux/security.h>
#include <linux/sched.h>
#include <linux/syscalls.h>
#include <linux/audit.h>
#include <linux/seq_file.h>
#include <linux/rwsem.h>
#include <linux/nsproxy.h>
#include <linux/ipc_namespace.h>

#include <asm/current.h>
#include <asm/uaccess.h>
#include "util.h"

/*
 * one msg_receiver structure for each sleeping receiver:
 */
struct msg_receiver {
    struct list_head    r_list;
    struct task_struct  *r_tsk;

    int         r_mode;
    long            r_msgtype;
    long            r_maxsize;

    struct msg_msg      *volatile r_msg;
};

/* one msg_sender for each sleeping sender */
struct msg_sender {
    struct list_head    list;
    struct task_struct  *tsk;
};

#define SEARCH_ANY      1
#define SEARCH_EQUAL        2
#define SEARCH_NOTEQUAL     3
#define SEARCH_LESSEQUAL    4

#define msg_ids(ns) ((ns)->ids[IPC_MSG_IDS])

#define msg_unlock(msq)     ipc_unlock(&(msq)->q_perm)

static void freeque(struct ipc_namespace *, struct kern_ipc_perm *);
static int newque(struct ipc_namespace *, struct ipc_params *);
#ifdef CONFIG_PROC_FS
static int sysvipc_msg_proc_show(struct seq_file *s, void *it);
#endif

/*
 * Scale msgmni with the available lowmem size: the memory dedicated to msg
 * queues should occupy at most 1/MSG_MEM_SCALE of lowmem.
 * Also take into account the number of nsproxies created so far.
 * This should be done staying within the (MSGMNI , IPCMNI/nr_ipc_ns) range.
 */
void recompute_msgmni(struct ipc_namespace *ns)
{
    struct sysinfo i;
    unsigned long allowed;
    int nb_ns;

    si_meminfo(&i);
    allowed = (((i.totalram - i.totalhigh) / MSG_MEM_SCALE) * i.mem_unit)
        / MSGMNB;
    nb_ns = atomic_read(&nr_ipc_ns);
    allowed /= nb_ns;

    if (allowed < MSGMNI) {
        ns->msg_ctlmni = MSGMNI;
        return;
    }

    if (allowed > IPCMNI / nb_ns) {
        ns->msg_ctlmni = IPCMNI / nb_ns;
        return;
    }

    ns->msg_ctlmni = allowed;
}

void msg_init_ns(struct ipc_namespace *ns)
{
    ns->msg_ctlmax = MSGMAX;
    ns->msg_ctlmnb = MSGMNB;

    recompute_msgmni(ns);

    atomic_set(&ns->msg_bytes, 0);
    atomic_set(&ns->msg_hdrs, 0);
    ipc_init_ids(&ns->ids[IPC_MSG_IDS]);
}

#ifdef CONFIG_IPC_NS
void msg_exit_ns(struct ipc_namespace *ns)
{
    free_ipcs(ns, &msg_ids(ns), freeque);
    idr_destroy(&ns->ids[IPC_MSG_IDS].ipcs_idr);
}
#endif

void __init msg_init(void)
{
    msg_init_ns(&init_ipc_ns);

    printk(KERN_INFO "msgmni has been set to %d\n",
        init_ipc_ns.msg_ctlmni);

    ipc_init_proc_interface("sysvipc/msg",
                "       key      msqid perms      cbytes       qnum lspid lrpid   uid   gid  cuid  cgid      stime      rtime      ctime\n",
                IPC_MSG_IDS, sysvipc_msg_proc_show);
}

/*
 * msg_lock_(check_) routines are called in the paths where the rw_mutex
 * is not held.
 */
static inline struct msg_queue *msg_lock(struct ipc_namespace *ns, int id)
{
    struct kern_ipc_perm *ipcp = ipc_lock(&msg_ids(ns), id);

    if (IS_ERR(ipcp))
        return (struct msg_queue *)ipcp;

    return container_of(ipcp, struct msg_queue, q_perm);
}

static inline struct msg_queue *msg_lock_check(struct ipc_namespace *ns,
                        int id)
{
    struct kern_ipc_perm *ipcp = ipc_lock_check(&msg_ids(ns), id);

    if (IS_ERR(ipcp))
        return (struct msg_queue *)ipcp;

    return container_of(ipcp, struct msg_queue, q_perm);
}

static inline void msg_rmid(struct ipc_namespace *ns, struct msg_queue *s)
{
    ipc_rmid(&msg_ids(ns), &s->q_perm);
}

static int newque(struct ipc_namespace *ns, struct ipc_params *params)
{
    struct msg_queue *msq;
    int id, retval;
    key_t key = params->key;
    int msgflg = params->flg;

    msq = ipc_rcu_alloc(sizeof(*msq));
    if (!msq)
        return -ENOMEM;

    msq->q_perm.mode = msgflg & S_IRWXUGO;
    msq->q_perm.key = key;

    msq->q_perm.security = NULL;
    retval = security_msg_queue_alloc(msq);
    if (retval) {
        ipc_rcu_putref(msq);
        return retval;
    }

    /*
     * ipc_addid() locks msq
     */
    id = ipc_addid(&msg_ids(ns), &msq->q_perm, ns->msg_ctlmni);
    if (id < 0) {
        security_msg_queue_free(msq);
        ipc_rcu_putref(msq);
        return id;
    }

    msq->q_stime = msq->q_rtime = 0;
    msq->q_ctime = get_seconds();
    msq->q_cbytes = msq->q_qnum = 0;
    msq->q_qbytes = ns->msg_ctlmnb;
    msq->q_lspid = msq->q_lrpid = 0;
    INIT_LIST_HEAD(&msq->q_messages);
    INIT_LIST_HEAD(&msq->q_receivers);
    INIT_LIST_HEAD(&msq->q_senders);

    msg_unlock(msq);

    return msq->q_perm.id;
}

static inline void ss_add(struct msg_queue *msq, struct msg_sender *mss)
{
    mss->tsk = current;
    current->state = TASK_INTERRUPTIBLE;
    list_add_tail(&mss->list, &msq->q_senders);
}

static inline void ss_del(struct msg_sender *mss)
{
    if (mss->list.next != NULL)
        list_del(&mss->list);
}

static void ss_wakeup(struct list_head *h, int kill)
{
    struct list_head *tmp;

    tmp = h->next;
    while (tmp != h) {
        struct msg_sender *mss;

        mss = list_entry(tmp, struct msg_sender, list);
        tmp = tmp->next;
        if (kill)
            mss->list.next = NULL;
        wake_up_process(mss->tsk);
    }
}

static void expunge_all(struct msg_queue *msq, int res)
{
    struct list_head *tmp;

    tmp = msq->q_receivers.next;
    while (tmp != &msq->q_receivers) {
        struct msg_receiver *msr;

        msr = list_entry(tmp, struct msg_receiver, r_list);
        tmp = tmp->next;
        msr->r_msg = NULL;
        wake_up_process(msr->r_tsk);
        smp_mb();
        msr->r_msg = ERR_PTR(res);
    }
}

/*
 * freeque() wakes up waiters on the sender and receiver waiting queue,
 * removes the message queue from message queue ID IDR, and cleans up all the
 * messages associated with this queue.
 *
 * msg_ids.rw_mutex (writer) and the spinlock for this message queue are held
 * before freeque() is called. msg_ids.rw_mutex remains locked on exit.
 */
static void freeque(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
{
    struct list_head *tmp;
    struct msg_queue *msq = container_of(ipcp, struct msg_queue, q_perm);

    expunge_all(msq, -EIDRM);
    ss_wakeup(&msq->q_senders, 1);
    msg_rmid(ns, msq);
    msg_unlock(msq);

    tmp = msq->q_messages.next;
    while (tmp != &msq->q_messages) {
        struct msg_msg *msg = list_entry(tmp, struct msg_msg, m_list);

        tmp = tmp->next;
        atomic_dec(&ns->msg_hdrs);
        free_msg(msg);
    }
    atomic_sub(msq->q_cbytes, &ns->msg_bytes);
    security_msg_queue_free(msq);
    ipc_rcu_putref(msq);
}

/*
 * Called with msg_ids.rw_mutex and ipcp locked.
 */
static inline int msg_security(struct kern_ipc_perm *ipcp, int msgflg)
{
    struct msg_queue *msq = container_of(ipcp, struct msg_queue, q_perm);

    return security_msg_queue_associate(msq, msgflg);
}

SYSCALL_DEFINE2(msgget, key_t, key, int, msgflg)
{
    struct ipc_namespace *ns;
    struct ipc_ops msg_ops;
    struct ipc_params msg_params;

    ns = current->nsproxy->ipc_ns;

    msg_ops.getnew = newque;
    msg_ops.associate = msg_security;
    msg_ops.more_checks = NULL;

    msg_params.key = key;
    msg_params.flg = msgflg;

    return ipcget(ns, &msg_ids(ns), &msg_ops, &msg_params);
}

static inline unsigned long
copy_msqid_to_user(void __user *buf, struct msqid64_ds *in, int version)
{
    switch(version) {
    case IPC_64:
        return copy_to_user(buf, in, sizeof(*in));
    case IPC_OLD:
    {
        struct msqid_ds out;

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

        ipc64_perm_to_ipc_perm(&in->msg_perm, &out.msg_perm);

        out.msg_stime       = in->msg_stime;
        out.msg_rtime       = in->msg_rtime;
        out.msg_ctime       = in->msg_ctime;

        if (in->msg_cbytes > USHRT_MAX)
            out.msg_cbytes  = USHRT_MAX;
        else
            out.msg_cbytes  = in->msg_cbytes;
        out.msg_lcbytes     = in->msg_cbytes;

        if (in->msg_qnum > USHRT_MAX)
            out.msg_qnum    = USHRT_MAX;
        else
            out.msg_qnum    = in->msg_qnum;

        if (in->msg_qbytes > USHRT_MAX)
            out.msg_qbytes  = USHRT_MAX;
        else
            out.msg_qbytes  = in->msg_qbytes;
        out.msg_lqbytes     = in->msg_qbytes;

        out.msg_lspid       = in->msg_lspid;
        out.msg_lrpid       = in->msg_lrpid;

        return copy_to_user(buf, &out, sizeof(out));
    }
    default:
        return -EINVAL;
    }
}

static inline unsigned long
copy_msqid_from_user(struct msqid64_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 msqid_ds tbuf_old;

        if (copy_from_user(&tbuf_old, buf, sizeof(tbuf_old)))
            return -EFAULT;

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

        if (tbuf_old.msg_qbytes == 0)
            out->msg_qbytes = tbuf_old.msg_lqbytes;
        else
            out->msg_qbytes = tbuf_old.msg_qbytes;

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

/*
 * This function handles some msgctl commands which require the rw_mutex
 * to be held in write mode.
 * NOTE: no locks must be held, the rw_mutex is taken inside this function.
 */
static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
               struct msqid_ds __user *buf, int version)
{
    struct kern_ipc_perm *ipcp;
    struct msqid64_ds uninitialized_var(msqid64);
    struct msg_queue *msq;
    int err;

    if (cmd == IPC_SET) {
        if (copy_msqid_from_user(&msqid64, buf, version))
            return -EFAULT;
    }

    ipcp = ipcctl_pre_down(ns, &msg_ids(ns), msqid, cmd,
                   &msqid64.msg_perm, msqid64.msg_qbytes);
    if (IS_ERR(ipcp))
        return PTR_ERR(ipcp);

    msq = container_of(ipcp, struct msg_queue, q_perm);

    err = security_msg_queue_msgctl(msq, cmd);
    if (err)
        goto out_unlock;

    switch (cmd) {
    case IPC_RMID:
        freeque(ns, ipcp);
        goto out_up;
    case IPC_SET:
        if (msqid64.msg_qbytes > ns->msg_ctlmnb &&
            !capable(CAP_SYS_RESOURCE)) {
            err = -EPERM;
            goto out_unlock;
        }

        err = ipc_update_perm(&msqid64.msg_perm, ipcp);
        if (err)
            goto out_unlock;

        msq->q_qbytes = msqid64.msg_qbytes;

        msq->q_ctime = get_seconds();
        /* sleeping receivers might be excluded by
         * stricter permissions.
         */
        expunge_all(msq, -EAGAIN);
        /* sleeping senders might be able to send
         * due to a larger queue size.
         */
        ss_wakeup(&msq->q_senders, 0);
        break;
    default:
        err = -EINVAL;
    }
out_unlock:
    msg_unlock(msq);
out_up:
    up_write(&msg_ids(ns).rw_mutex);
    return err;
}

SYSCALL_DEFINE3(msgctl, int, msqid, int, cmd, struct msqid_ds __user *, buf)
{
    struct msg_queue *msq;
    int err, version;
    struct ipc_namespace *ns;

    if (msqid < 0 || cmd < 0)
        return -EINVAL;

    version = ipc_parse_version(&cmd);
    ns = current->nsproxy->ipc_ns;

    switch (cmd) {
    case IPC_INFO:
    case MSG_INFO:
    {
        struct msginfo msginfo;
        int max_id;

        if (!buf)
            return -EFAULT;
        /*
         * We must not return kernel stack data.
         * due to padding, it's not enough
         * to set all member fields.
         */
        err = security_msg_queue_msgctl(NULL, cmd);
        if (err)
            return err;

        memset(&msginfo, 0, sizeof(msginfo));
        msginfo.msgmni = ns->msg_ctlmni;
        msginfo.msgmax = ns->msg_ctlmax;
        msginfo.msgmnb = ns->msg_ctlmnb;
        msginfo.msgssz = MSGSSZ;
        msginfo.msgseg = MSGSEG;
        down_read(&msg_ids(ns).rw_mutex);
        if (cmd == MSG_INFO) {
            msginfo.msgpool = msg_ids(ns).in_use;
            msginfo.msgmap = atomic_read(&ns->msg_hdrs);
            msginfo.msgtql = atomic_read(&ns->msg_bytes);
        } else {
            msginfo.msgmap = MSGMAP;
            msginfo.msgpool = MSGPOOL;
            msginfo.msgtql = MSGTQL;
        }
        max_id = ipc_get_maxid(&msg_ids(ns));
        up_read(&msg_ids(ns).rw_mutex);
        if (copy_to_user(buf, &msginfo, sizeof(struct msginfo)))
            return -EFAULT;
        return (max_id < 0) ? 0 : max_id;
    }
    case MSG_STAT:  /* msqid is an index rather than a msg queue id */
    case IPC_STAT:
    {
        struct msqid64_ds tbuf;
        int success_return;

        if (!buf)
            return -EFAULT;

        if (cmd == MSG_STAT) {
            msq = msg_lock(ns, msqid);
            if (IS_ERR(msq))
                return PTR_ERR(msq);
            success_return = msq->q_perm.id;
        } else {
            msq = msg_lock_check(ns, msqid);
            if (IS_ERR(msq))
                return PTR_ERR(msq);
            success_return = 0;
        }
        err = -EACCES;
        if (ipcperms(ns, &msq->q_perm, S_IRUGO))
            goto out_unlock;

        err = security_msg_queue_msgctl(msq, cmd);
        if (err)
            goto out_unlock;

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

        kernel_to_ipc64_perm(&msq->q_perm, &tbuf.msg_perm);
        tbuf.msg_stime  = msq->q_stime;
        tbuf.msg_rtime  = msq->q_rtime;
        tbuf.msg_ctime  = msq->q_ctime;
        tbuf.msg_cbytes = msq->q_cbytes;
        tbuf.msg_qnum   = msq->q_qnum;
        tbuf.msg_qbytes = msq->q_qbytes;
        tbuf.msg_lspid  = msq->q_lspid;
        tbuf.msg_lrpid  = msq->q_lrpid;
        msg_unlock(msq);
        if (copy_msqid_to_user(buf, &tbuf, version))
            return -EFAULT;
        return success_return;
    }
    case IPC_SET:
    case IPC_RMID:
        err = msgctl_down(ns, msqid, cmd, buf, version);
        return err;
    default:
        return  -EINVAL;
    }

out_unlock:
    msg_unlock(msq);
    return err;
}

static int testmsg(struct msg_msg *msg, long type, int mode)
{
    switch(mode)
    {
        case SEARCH_ANY:
            return 1;
        case SEARCH_LESSEQUAL:
            if (msg->m_type <=type)
                return 1;
            break;
        case SEARCH_EQUAL:
            if (msg->m_type == type)
                return 1;
            break;
        case SEARCH_NOTEQUAL:
            if (msg->m_type != type)
                return 1;
            break;
    }
    return 0;
}

static inline int pipelined_send(struct msg_queue *msq, struct msg_msg *msg)
{
    struct list_head *tmp;

    tmp = msq->q_receivers.next;
    while (tmp != &msq->q_receivers) {
        struct msg_receiver *msr;

        msr = list_entry(tmp, struct msg_receiver, r_list);
        tmp = tmp->next;
        if (testmsg(msg, msr->r_msgtype, msr->r_mode) &&
            !security_msg_queue_msgrcv(msq, msg, msr->r_tsk,
                           msr->r_msgtype, msr->r_mode)) {

            list_del(&msr->r_list);
            if (msr->r_maxsize < msg->m_ts) {
                msr->r_msg = NULL;
                wake_up_process(msr->r_tsk);
                smp_mb();
                msr->r_msg = ERR_PTR(-E2BIG);
            } else {
                msr->r_msg = NULL;
                msq->q_lrpid = task_pid_vnr(msr->r_tsk);
                msq->q_rtime = get_seconds();
                wake_up_process(msr->r_tsk);
                smp_mb();
                msr->r_msg = msg;

                return 1;
            }
        }
    }
    return 0;
}

long do_msgsnd(int msqid, long mtype, void __user *mtext,
        size_t msgsz, int msgflg)
{
    struct msg_queue *msq;
    struct msg_msg *msg;
    int err;
    struct ipc_namespace *ns;

    ns = current->nsproxy->ipc_ns;

    if (msgsz > ns->msg_ctlmax || (long) msgsz < 0 || msqid < 0)
        return -EINVAL;
    if (mtype < 1)
        return -EINVAL;

    msg = load_msg(mtext, msgsz);
    if (IS_ERR(msg))
        return PTR_ERR(msg);

    msg->m_type = mtype;
    msg->m_ts = msgsz;

    msq = msg_lock_check(ns, msqid);
    if (IS_ERR(msq)) {
        err = PTR_ERR(msq);
        goto out_free;
    }

    for (;;) {
        struct msg_sender s;

        err = -EACCES;
        if (ipcperms(ns, &msq->q_perm, S_IWUGO))
            goto out_unlock_free;

        err = security_msg_queue_msgsnd(msq, msg, msgflg);
        if (err)
            goto out_unlock_free;

        if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
                1 + msq->q_qnum <= msq->q_qbytes) {
            break;
        }

        /* queue full, wait: */
        if (msgflg & IPC_NOWAIT) {
            err = -EAGAIN;
            goto out_unlock_free;
        }
        ss_add(msq, &s);
        ipc_rcu_getref(msq);
        msg_unlock(msq);
        schedule();

        ipc_lock_by_ptr(&msq->q_perm);
        ipc_rcu_putref(msq);
        if (msq->q_perm.deleted) {
            err = -EIDRM;
            goto out_unlock_free;
        }
        ss_del(&s);

        if (signal_pending(current)) {
            err = -ERESTARTNOHAND;
            goto out_unlock_free;
        }
    }

    msq->q_lspid = task_tgid_vnr(current);
    msq->q_stime = get_seconds();

    if (!pipelined_send(msq, msg)) {
        /* no one is waiting for this message, enqueue it */
        list_add_tail(&msg->m_list, &msq->q_messages);
        msq->q_cbytes += msgsz;
        msq->q_qnum++;
        atomic_add(msgsz, &ns->msg_bytes);
        atomic_inc(&ns->msg_hdrs);
    }

    err = 0;
    msg = NULL;

out_unlock_free:
    msg_unlock(msq);
out_free:
    if (msg != NULL)
        free_msg(msg);
    return err;
}

SYSCALL_DEFINE4(msgsnd, int, msqid, struct msgbuf __user *, msgp, size_t, msgsz,
        int, msgflg)
{
    long mtype;

    if (get_user(mtype, &msgp->mtype))
        return -EFAULT;
    return do_msgsnd(msqid, mtype, msgp->mtext, msgsz, msgflg);
}

static inline int convert_mode(long *msgtyp, int msgflg)
{
    /*
     *  find message of correct type.
     *  msgtyp = 0 => get first.
     *  msgtyp > 0 => get first message of matching type.
     *  msgtyp < 0 => get message with least type must be < abs(msgtype).
     */
    if (*msgtyp == 0)
        return SEARCH_ANY;
    if (*msgtyp < 0) {
        *msgtyp = -*msgtyp;
        return SEARCH_LESSEQUAL;
    }
    if (msgflg & MSG_EXCEPT)
        return SEARCH_NOTEQUAL;
    return SEARCH_EQUAL;
}

long do_msgrcv(int msqid, long *pmtype, void __user *mtext,
        size_t msgsz, long msgtyp, int msgflg)
{
    struct msg_queue *msq;
    struct msg_msg *msg;
    int mode;
    struct ipc_namespace *ns;

    if (msqid < 0 || (long) msgsz < 0)
        return -EINVAL;
    mode = convert_mode(&msgtyp, msgflg);
    ns = current->nsproxy->ipc_ns;

    msq = msg_lock_check(ns, msqid);
    if (IS_ERR(msq))
        return PTR_ERR(msq);

    for (;;) {
        struct msg_receiver msr_d;
        struct list_head *tmp;

        msg = ERR_PTR(-EACCES);
        if (ipcperms(ns, &msq->q_perm, S_IRUGO))
            goto out_unlock;

        msg = ERR_PTR(-EAGAIN);
        tmp = msq->q_messages.next;
        while (tmp != &msq->q_messages) {
            struct msg_msg *walk_msg;

            walk_msg = list_entry(tmp, struct msg_msg, m_list);
            if (testmsg(walk_msg, msgtyp, mode) &&
                !security_msg_queue_msgrcv(msq, walk_msg, current,
                               msgtyp, mode)) {

                msg = walk_msg;
                if (mode == SEARCH_LESSEQUAL &&
                        walk_msg->m_type != 1) {
                    msg = walk_msg;
                    msgtyp = walk_msg->m_type - 1;
                } else {
                    msg = walk_msg;
                    break;
                }
            }
            tmp = tmp->next;
        }
        if (!IS_ERR(msg)) {
            /*
             * Found a suitable message.
             * Unlink it from the queue.
             */
            if ((msgsz < msg->m_ts) && !(msgflg & MSG_NOERROR)) {
                msg = ERR_PTR(-E2BIG);
                goto out_unlock;
            }
            list_del(&msg->m_list);
            msq->q_qnum--;
            msq->q_rtime = get_seconds();
            msq->q_lrpid = task_tgid_vnr(current);
            msq->q_cbytes -= msg->m_ts;
            atomic_sub(msg->m_ts, &ns->msg_bytes);
            atomic_dec(&ns->msg_hdrs);
            ss_wakeup(&msq->q_senders, 0);
            msg_unlock(msq);
            break;
        }
        /* No message waiting. Wait for a message */
        if (msgflg & IPC_NOWAIT) {
            msg = ERR_PTR(-ENOMSG);
            goto out_unlock;
        }
        list_add_tail(&msr_d.r_list, &msq->q_receivers);
        msr_d.r_tsk = current;
        msr_d.r_msgtype = msgtyp;
        msr_d.r_mode = mode;
        if (msgflg & MSG_NOERROR)
            msr_d.r_maxsize = INT_MAX;
        else
            msr_d.r_maxsize = msgsz;
        msr_d.r_msg = ERR_PTR(-EAGAIN);
        current->state = TASK_INTERRUPTIBLE;
        msg_unlock(msq);

        schedule();

        /* Lockless receive, part 1:
         * Disable preemption.  We don't hold a reference to the queue
         * and getting a reference would defeat the idea of a lockless
         * operation, thus the code relies on rcu to guarantee the
         * existence of msq:
         * Prior to destruction, expunge_all(-EIRDM) changes r_msg.
         * Thus if r_msg is -EAGAIN, then the queue not yet destroyed.
         * rcu_read_lock() prevents preemption between reading r_msg
         * and the spin_lock() inside ipc_lock_by_ptr().
         */
        rcu_read_lock();

        /* Lockless receive, part 2:
         * Wait until pipelined_send or expunge_all are outside of
         * wake_up_process(). There is a race with exit(), see
         * ipc/mqueue.c for the details.
         */
        msg = (struct msg_msg*)msr_d.r_msg;
        while (msg == NULL) {
            cpu_relax();
            msg = (struct msg_msg *)msr_d.r_msg;
        }

        /* Lockless receive, part 3:
         * If there is a message or an error then accept it without
         * locking.
         */
        if (msg != ERR_PTR(-EAGAIN)) {
            rcu_read_unlock();
            break;
        }

        /* Lockless receive, part 3:
         * Acquire the queue spinlock.
         */
        ipc_lock_by_ptr(&msq->q_perm);
        rcu_read_unlock();

        /* Lockless receive, part 4:
         * Repeat test after acquiring the spinlock.
         */
        msg = (struct msg_msg*)msr_d.r_msg;
        if (msg != ERR_PTR(-EAGAIN))
            goto out_unlock;

        list_del(&msr_d.r_list);
        if (signal_pending(current)) {
            msg = ERR_PTR(-ERESTARTNOHAND);
out_unlock:
            msg_unlock(msq);
            break;
        }
    }
    if (IS_ERR(msg))
        return PTR_ERR(msg);

    msgsz = (msgsz > msg->m_ts) ? msg->m_ts : msgsz;
    *pmtype = msg->m_type;
    if (store_msg(mtext, msg, msgsz))
        msgsz = -EFAULT;

    free_msg(msg);

    return msgsz;
}

SYSCALL_DEFINE5(msgrcv, int, msqid, struct msgbuf __user *, msgp, size_t, msgsz,
        long, msgtyp, int, msgflg)
{
    long err, mtype;

    err =  do_msgrcv(msqid, &mtype, msgp->mtext, msgsz, msgtyp, msgflg);
    if (err < 0)
        goto out;

    if (put_user(mtype, &msgp->mtype))
        err = -EFAULT;
out:
    return err;
}

#ifdef CONFIG_PROC_FS
static int sysvipc_msg_proc_show(struct seq_file *s, void *it)
{
    struct user_namespace *user_ns = seq_user_ns(s);
    struct msg_queue *msq = it;

    return seq_printf(s,
            "%10d %10d  %4o  %10lu %10lu %5u %5u %5u %5u %5u %5u %10lu %10lu %10lu\n",
            msq->q_perm.key,
            msq->q_perm.id,
            msq->q_perm.mode,
            msq->q_cbytes,
            msq->q_qnum,
            msq->q_lspid,
            msq->q_lrpid,
            from_kuid_munged(user_ns, msq->q_perm.uid),
            from_kgid_munged(user_ns, msq->q_perm.gid),
            from_kuid_munged(user_ns, msq->q_perm.cuid),
            from_kgid_munged(user_ns, msq->q_perm.cgid),
            msq->q_stime,
            msq->q_rtime,
            msq->q_ctime);
}
#endif