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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 <dragos@iname.com>
     *
     * mostly rewritten, threaded and wake-one semantics added
     * MSGMAX limit removed, sysctl's added
    
     * (c) 1999 Manfred Spraul <manfred@colorfullife.com>
    
     *
     * support for audit of ipc object properties and permission changes
     * Dustin Kirkland <dustin.kirkland@us.ibm.com>
    
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     */
    
    
    #include <linux/capability.h>
    
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    #include <linux/slab.h>
    #include <linux/msg.h>
    #include <linux/spinlock.h>
    #include <linux/init.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/mutex.h>
    
    
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    #include <asm/current.h>
    #include <asm/uaccess.h>
    #include "util.h"
    
    /* sysctl: */
    int msg_ctlmax = MSGMAX;
    int msg_ctlmnb = MSGMNB;
    int msg_ctlmni = MSGMNI;
    
    /* 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
    
    static atomic_t msg_bytes = ATOMIC_INIT(0);
    static atomic_t msg_hdrs = ATOMIC_INIT(0);
    
    static struct ipc_ids msg_ids;
    
    #define msg_lock(id)	((struct msg_queue*)ipc_lock(&msg_ids,id))
    #define msg_unlock(msq)	ipc_unlock(&(msq)->q_perm)
    #define msg_rmid(id)	((struct msg_queue*)ipc_rmid(&msg_ids,id))
    #define msg_checkid(msq, msgid)	\
    	ipc_checkid(&msg_ids,&msq->q_perm,msgid)
    #define msg_buildid(id, seq) \
    	ipc_buildid(&msg_ids, id, seq)
    
    static void freeque (struct msg_queue *msq, int id);
    static int newque (key_t key, int msgflg);
    #ifdef CONFIG_PROC_FS
    
    static int sysvipc_msg_proc_show(struct seq_file *s, void *it);
    
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    #endif
    
    void __init msg_init (void)
    {
    	ipc_init_ids(&msg_ids,msg_ctlmni);
    
    	ipc_init_proc_interface("sysvipc/msg",
    				"       key      msqid perms      cbytes       qnum lspid lrpid   uid   gid  cuid  cgid      stime      rtime      ctime\n",
    				&msg_ids,
    				sysvipc_msg_proc_show);
    
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    }
    
    static int newque (key_t key, int msgflg)
    {
    	int id;
    	int retval;
    	struct msg_queue *msq;
    
    	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;
    	}
    
    	id = ipc_addid(&msg_ids, &msq->q_perm, msg_ctlmni);
    	if(id == -1) {
    		security_msg_queue_free(msq);
    		ipc_rcu_putref(msq);
    		return -ENOSPC;
    	}
    
    
    	msq->q_id = msg_buildid(id,msq->q_perm.seq);
    
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    	msq->q_stime = msq->q_rtime = 0;
    	msq->q_ctime = get_seconds();
    	msq->q_cbytes = msq->q_qnum = 0;
    	msq->q_qbytes = 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);
    
    
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    }
    
    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 
     * array, and cleans up all the messages associated with this queue.
     *
    
     * msg_ids.mutex and the spinlock for this message queue is hold
     * before freeque() is called. msg_ids.mutex remains locked on exit.
    
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     */
    static void freeque (struct msg_queue *msq, int id)
    {
    	struct list_head *tmp;
    
    	expunge_all(msq,-EIDRM);
    	ss_wakeup(&msq->q_senders,1);
    	msq = msg_rmid(id);
    	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(&msg_hdrs);
    		free_msg(msg);
    	}
    	atomic_sub(msq->q_cbytes, &msg_bytes);
    	security_msg_queue_free(msq);
    	ipc_rcu_putref(msq);
    }
    
    asmlinkage long sys_msgget (key_t key, int msgflg)
    {
    	int id, ret = -EPERM;
    	struct msg_queue *msq;
    	
    
    	mutex_lock(&msg_ids.mutex);
    
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    	if (key == IPC_PRIVATE) 
    		ret = newque(key, msgflg);
    	else if ((id = ipc_findkey(&msg_ids, key)) == -1) { /* key not used */
    		if (!(msgflg & IPC_CREAT))
    			ret = -ENOENT;
    		else
    			ret = newque(key, msgflg);
    	} else if (msgflg & IPC_CREAT && msgflg & IPC_EXCL) {
    		ret = -EEXIST;
    	} else {
    		msq = msg_lock(id);
    
    		BUG_ON(msq==NULL);
    
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    		if (ipcperms(&msq->q_perm, msgflg))
    			ret = -EACCES;
    		else {
    			int qid = msg_buildid(id, msq->q_perm.seq);
    		    	ret = security_msg_queue_associate(msq, msgflg);
    			if (!ret)
    				ret = qid;
    		}
    		msg_unlock(msq);
    	}
    
    	mutex_unlock(&msg_ids.mutex);
    
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    	return ret;
    }
    
    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;
    	}
    }
    
    struct msq_setbuf {
    	unsigned long	qbytes;
    	uid_t		uid;
    	gid_t		gid;
    	mode_t		mode;
    };
    
    static inline unsigned long copy_msqid_from_user(struct msq_setbuf *out, void __user *buf, int version)
    {
    	switch(version) {
    	case IPC_64:
    	    {
    		struct msqid64_ds tbuf;
    
    		if (copy_from_user (&tbuf, buf, sizeof (tbuf)))
    			return -EFAULT;
    
    		out->qbytes		= tbuf.msg_qbytes;
    		out->uid		= tbuf.msg_perm.uid;
    		out->gid		= tbuf.msg_perm.gid;
    		out->mode		= tbuf.msg_perm.mode;
    
    		return 0;
    	    }
    	case IPC_OLD:
    	    {
    		struct msqid_ds tbuf_old;
    
    		if (copy_from_user (&tbuf_old, buf, sizeof (tbuf_old)))
    			return -EFAULT;
    
    		out->uid		= tbuf_old.msg_perm.uid;
    		out->gid		= tbuf_old.msg_perm.gid;
    		out->mode		= tbuf_old.msg_perm.mode;
    
    		if(tbuf_old.msg_qbytes == 0)
    			out->qbytes	= tbuf_old.msg_lqbytes;
    		else
    			out->qbytes	= tbuf_old.msg_qbytes;
    
    		return 0;
    	    }
    	default:
    		return -EINVAL;
    	}
    }
    
    asmlinkage long sys_msgctl (int msqid, int cmd, struct msqid_ds __user *buf)
    {
    	int err, version;
    	struct msg_queue *msq;
    	struct msq_setbuf setbuf;
    	struct kern_ipc_perm *ipcp;
    	
    	if (msqid < 0 || cmd < 0)
    		return -EINVAL;
    
    	version = ipc_parse_version(&cmd);
    
    	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 = msg_ctlmni;
    		msginfo.msgmax = msg_ctlmax;
    		msginfo.msgmnb = msg_ctlmnb;
    		msginfo.msgssz = MSGSSZ;
    		msginfo.msgseg = MSGSEG;
    
    		mutex_lock(&msg_ids.mutex);
    
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    		if (cmd == MSG_INFO) {
    			msginfo.msgpool = msg_ids.in_use;
    			msginfo.msgmap = atomic_read(&msg_hdrs);
    			msginfo.msgtql = atomic_read(&msg_bytes);
    		} else {
    			msginfo.msgmap = MSGMAP;
    			msginfo.msgpool = MSGPOOL;
    			msginfo.msgtql = MSGTQL;
    		}
    		max_id = msg_ids.max_id;
    
    		mutex_unlock(&msg_ids.mutex);
    
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    		if (copy_to_user (buf, &msginfo, sizeof(struct msginfo)))
    			return -EFAULT;
    		return (max_id < 0) ? 0: max_id;
    	}
    	case MSG_STAT:
    	case IPC_STAT:
    	{
    		struct msqid64_ds tbuf;
    		int success_return;
    		if (!buf)
    			return -EFAULT;
    		if(cmd == MSG_STAT && msqid >= msg_ids.entries->size)
    			return -EINVAL;
    
    		memset(&tbuf,0,sizeof(tbuf));
    
    		msq = msg_lock(msqid);
    		if (msq == NULL)
    			return -EINVAL;
    
    		if(cmd == MSG_STAT) {
    			success_return = msg_buildid(msqid, msq->q_perm.seq);
    		} else {
    			err = -EIDRM;
    			if (msg_checkid(msq,msqid))
    				goto out_unlock;
    			success_return = 0;
    		}
    		err = -EACCES;
    		if (ipcperms (&msq->q_perm, S_IRUGO))
    			goto out_unlock;
    
    		err = security_msg_queue_msgctl(msq, cmd);
    		if (err)
    			goto out_unlock;
    
    		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:
    		if (!buf)
    			return -EFAULT;
    		if (copy_msqid_from_user (&setbuf, buf, version))
    			return -EFAULT;
    		break;
    	case IPC_RMID:
    		break;
    	default:
    		return  -EINVAL;
    	}
    
    
    	mutex_lock(&msg_ids.mutex);
    
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    	msq = msg_lock(msqid);
    	err=-EINVAL;
    	if (msq == NULL)
    		goto out_up;
    
    	err = -EIDRM;
    	if (msg_checkid(msq,msqid))
    		goto out_unlock_up;
    	ipcp = &msq->q_perm;
    
    
    	err = audit_ipc_obj(ipcp);
    	if (err)
    		goto out_unlock_up;
    
    	if (cmd==IPC_SET) {
    		err = audit_ipc_set_perm(setbuf.qbytes, setbuf.uid, setbuf.gid, setbuf.mode);
    		if (err)
    			goto out_unlock_up;
    	}
    
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    	err = -EPERM;
    	if (current->euid != ipcp->cuid && 
    	    current->euid != ipcp->uid && !capable(CAP_SYS_ADMIN))
    	    /* We _could_ check for CAP_CHOWN above, but we don't */
    		goto out_unlock_up;
    
    	err = security_msg_queue_msgctl(msq, cmd);
    	if (err)
    		goto out_unlock_up;
    
    	switch (cmd) {
    	case IPC_SET:
    	{
    		err = -EPERM;
    		if (setbuf.qbytes > msg_ctlmnb && !capable(CAP_SYS_RESOURCE))
    			goto out_unlock_up;
    
    		msq->q_qbytes = setbuf.qbytes;
    
    		ipcp->uid = setbuf.uid;
    		ipcp->gid = setbuf.gid;
    		ipcp->mode = (ipcp->mode & ~S_IRWXUGO) | 
    			(S_IRWXUGO & setbuf.mode);
    		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);
    		msg_unlock(msq);
    		break;
    	}
    	case IPC_RMID:
    		freeque (msq, msqid); 
    		break;
    	}
    	err = 0;
    out_up:
    
    	mutex_unlock(&msg_ids.mutex);
    
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    	return err;
    out_unlock_up:
    	msg_unlock(msq);
    	goto out_up;
    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 = msr->r_tsk->pid;
    				msq->q_rtime = get_seconds();
    				wake_up_process(msr->r_tsk);
    				smp_mb();
    				msr->r_msg = msg;
    				return 1;
    			}
    		}
    	}
    	return 0;
    }
    
    asmlinkage long sys_msgsnd (int msqid, struct msgbuf __user *msgp, size_t msgsz, int msgflg)
    {
    	struct msg_queue *msq;
    	struct msg_msg *msg;
    	long mtype;
    	int err;
    	
    	if (msgsz > msg_ctlmax || (long) msgsz < 0 || msqid < 0)
    		return -EINVAL;
    	if (get_user(mtype, &msgp->mtype))
    		return -EFAULT; 
    	if (mtype < 1)
    		return -EINVAL;
    
    	msg = load_msg(msgp->mtext, msgsz);
    	if(IS_ERR(msg))
    		return PTR_ERR(msg);
    
    	msg->m_type = mtype;
    	msg->m_ts = msgsz;
    
    	msq = msg_lock(msqid);
    	err=-EINVAL;
    	if(msq==NULL)
    		goto out_free;
    
    	err= -EIDRM;
    	if (msg_checkid(msq,msqid))
    		goto out_unlock_free;
    
    	for (;;) {
    		struct msg_sender s;
    
    		err=-EACCES;
    		if (ipcperms(&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 = current->tgid;
    	msq->q_stime = get_seconds();
    
    	if(!pipelined_send(msq,msg)) {
    		/* noone 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,&msg_bytes);
    		atomic_inc(&msg_hdrs);
    	}
    	
    	err = 0;
    	msg = NULL;
    
    out_unlock_free:
    	msg_unlock(msq);
    out_free:
    	if(msg!=NULL)
    		free_msg(msg);
    	return err;
    }
    
    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;
    }
    
    asmlinkage long sys_msgrcv (int msqid, struct msgbuf __user *msgp, size_t msgsz,
    			    long msgtyp, int msgflg)
    {
    	struct msg_queue *msq;
    	struct msg_msg *msg;
    	int mode;
    
    	if (msqid < 0 || (long) msgsz < 0)
    		return -EINVAL;
    	mode = convert_mode(&msgtyp,msgflg);
    
    	msq = msg_lock(msqid);
    	if(msq==NULL)
    		return -EINVAL;
    
    	msg = ERR_PTR(-EIDRM);
    	if (msg_checkid(msq,msqid))
    		goto out_unlock;
    
    	for (;;) {
    		struct msg_receiver msr_d;
    		struct list_head* tmp;
    
    		msg = ERR_PTR(-EACCES);
    		if (ipcperms (&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 = current->tgid;
    			msq->q_cbytes -= msg->m_ts;
    			atomic_sub(msg->m_ts,&msg_bytes);
    			atomic_dec(&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
    		 * existance 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;
    	if (put_user (msg->m_type, &msgp->mtype) ||
    	    store_msg(msgp->mtext, msg, msgsz)) {
    		    msgsz = -EFAULT;
    	}
    	free_msg(msg);
    	return msgsz;
    }
    
    #ifdef CONFIG_PROC_FS
    
    static int sysvipc_msg_proc_show(struct seq_file *s, void *it)
    
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    {
    
    	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_id,
    			  msq->q_perm.mode,
    			  msq->q_cbytes,
    			  msq->q_qnum,
    			  msq->q_lspid,
    			  msq->q_lrpid,
    			  msq->q_perm.uid,
    			  msq->q_perm.gid,
    			  msq->q_perm.cuid,
    			  msq->q_perm.cgid,
    			  msq->q_stime,
    			  msq->q_rtime,
    			  msq->q_ctime);
    
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    }
    #endif