ipc/msg.c

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

#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 SEARCH_NUMBER		5

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

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);
}

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

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

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

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

	if (IS_ERR(ipcp))
		return ERR_CAST(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 void msg_rcu_free(struct rcu_head *head)
{
	struct ipc_rcu *p = container_of(head, struct ipc_rcu, rcu);
	struct msg_queue *msq = ipc_rcu_to_struct(p);

	security_msg_queue_free(msq);
	ipc_rcu_free(head);
}

/**
 * newque - Create a new msg queue
 * @ns: namespace
 * @params: ptr to the structure that contains the key and msgflg
 *
 * Called with msg_ids.rwsem held (writer)
 */
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, ipc_rcu_free);
		return retval;
	}

	/* ipc_addid() locks msq upon success. */
	id = ipc_addid(&msg_ids(ns), &msq->q_perm, ns->msg_ctlmni);
	if (id < 0) {
		ipc_rcu_putref(msq, msg_rcu_free);
		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);

	ipc_unlock_object(&msq->q_perm);
	rcu_read_unlock();

	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 msg_sender *mss, *t;

	list_for_each_entry_safe(mss, t, h, list) {
		if (kill)
			mss->list.next = NULL;
		wake_up_process(mss->tsk);
	}
}

static void expunge_all(struct msg_queue *msq, int res)
{
	struct msg_receiver *msr, *t;

	list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
		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.rwsem (writer) and the spinlock for this message queue are held
 * before freeque() is called. msg_ids.rwsem remains locked on exit.
 */
static void freeque(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
{
	struct msg_msg *msg, *t;
	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);
	ipc_unlock_object(&msq->q_perm);
	rcu_read_unlock();

	list_for_each_entry_safe(msg, t, &msq->q_messages, m_list) {
		atomic_dec(&ns->msg_hdrs);
		free_msg(msg);
	}
	atomic_sub(msq->q_cbytes, &ns->msg_bytes);
	ipc_rcu_putref(msq, msg_rcu_free);
}

/*
 * Called with msg_ids.rwsem 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 rwsem
 * to be held in write mode.
 * NOTE: no locks must be held, the rwsem 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;
	}

	down_write(&msg_ids(ns).rwsem);
	rcu_read_lock();

	ipcp = ipcctl_pre_down_nolock(ns, &msg_ids(ns), msqid, cmd,
				      &msqid64.msg_perm, msqid64.msg_qbytes);
	if (IS_ERR(ipcp)) {
		err = PTR_ERR(ipcp);
		goto out_unlock1;
	}

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

	err = security_msg_queue_msgctl(msq, cmd);
	if (err)
		goto out_unlock1;

	switch (cmd) {
	case IPC_RMID:
		ipc_lock_object(&msq->q_perm);
		/* freeque unlocks the ipc object and rcu */
		freeque(ns, ipcp);
		goto out_up;
	case IPC_SET:
		if (msqid64.msg_qbytes > ns->msg_ctlmnb &&
		    !capable(CAP_SYS_RESOURCE)) {
			err = -EPERM;
			goto out_unlock1;
		}

		ipc_lock_object(&msq->q_perm);
		err = ipc_update_perm(&msqid64.msg_perm, ipcp);
		if (err)
			goto out_unlock0;

		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;
		goto out_unlock1;
	}

out_unlock0:
	ipc_unlock_object(&msq->q_perm);
out_unlock1:
	rcu_read_unlock();
out_up:
	up_write(&msg_ids(ns).rwsem);
	return err;
}

static int msgctl_nolock(struct ipc_namespace *ns, int msqid,
			 int cmd, int version, void __user *buf)
{
	int err;
	struct msg_queue *msq;

	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).rwsem);
		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).rwsem);
		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;

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

		rcu_read_lock();
		if (cmd == MSG_STAT) {
			msq = msq_obtain_object(ns, msqid);
			if (IS_ERR(msq)) {
				err = PTR_ERR(msq);
				goto out_unlock;
			}
			success_return = msq->q_perm.id;
		} else {
			msq = msq_obtain_object_check(ns, msqid);
			if (IS_ERR(msq)) {
				err = PTR_ERR(msq);
				goto out_unlock;
			}
			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;

		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;
		rcu_read_unlock();

		if (copy_msqid_to_user(buf, &tbuf, version))
			return -EFAULT;
		return success_return;
	}

	default:
		return -EINVAL;
	}

	return err;
out_unlock:
	rcu_read_unlock();
	return err;
}

SYSCALL_DEFINE3(msgctl, int, msqid, int, cmd, struct msqid_ds __user *, buf)
{
	int 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:
	case MSG_STAT:	/* msqid is an index rather than a msg queue id */
	case IPC_STAT:
		return msgctl_nolock(ns, msqid, cmd, version, buf);
	case IPC_SET:
	case IPC_RMID:
		return msgctl_down(ns, msqid, cmd, buf, version);
	default:
		return  -EINVAL;
	}
}

static int testmsg(struct msg_msg *msg, long type, int mode)
{
	switch (mode)
	{
		case SEARCH_ANY:
		case SEARCH_NUMBER:
			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 msg_receiver *msr, *t;

	list_for_each_entry_safe(msr, t, &msq->q_receivers, r_list) {
		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;

	rcu_read_lock();
	msq = msq_obtain_object_check(ns, msqid);
	if (IS_ERR(msq)) {
		err = PTR_ERR(msq);
		goto out_unlock1;
	}

	ipc_lock_object(&msq->q_perm);

	for (;;) {
		struct msg_sender s;

		err = -EACCES;
		if (ipcperms(ns, &msq->q_perm, S_IWUGO))
			goto out_unlock0;

		/* raced with RMID? */
		if (!ipc_valid_object(&msq->q_perm)) {
			err = -EIDRM;
			goto out_unlock0;
		}

		err = security_msg_queue_msgsnd(msq, msg, msgflg);
		if (err)
			goto out_unlock0;

		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_unlock0;
		}

		ss_add(msq, &s);

		if (!ipc_rcu_getref(msq)) {
			err = -EIDRM;
			goto out_unlock0;
		}

		ipc_unlock_object(&msq->q_perm);
		rcu_read_unlock();
		schedule();

		rcu_read_lock();
		ipc_lock_object(&msq->q_perm);

		ipc_rcu_putref(msq, ipc_rcu_free);
		/* raced with RMID? */
		if (!ipc_valid_object(&msq->q_perm)) {
			err = -EIDRM;
			goto out_unlock0;
		}

		ss_del(&s);

		if (signal_pending(current)) {
			err = -ERESTARTNOHAND;
			goto out_unlock0;
		}

	}
	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_unlock0:
	ipc_unlock_object(&msq->q_perm);
out_unlock1:
	rcu_read_unlock();
	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)
{
	if (msgflg & MSG_COPY)
		return SEARCH_NUMBER;
	/*
	 *  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;
}

static long do_msg_fill(void __user *dest, struct msg_msg *msg, size_t bufsz)
{
	struct msgbuf __user *msgp = dest;
	size_t msgsz;

	if (put_user(msg->m_type, &msgp->mtype))
		return -EFAULT;

	msgsz = (bufsz > msg->m_ts) ? msg->m_ts : bufsz;
	if (store_msg(msgp->mtext, msg, msgsz))
		return -EFAULT;
	return msgsz;
}

#ifdef CONFIG_CHECKPOINT_RESTORE
/*
 * This function creates new kernel message structure, large enough to store
 * bufsz message bytes.
 */
static inline struct msg_msg *prepare_copy(void __user *buf, size_t bufsz)
{
	struct msg_msg *copy;

	/*
	 * Create dummy message to copy real message to.
	 */
	copy = load_msg(buf, bufsz);
	if (!IS_ERR(copy))
		copy->m_ts = bufsz;
	return copy;
}

static inline void free_copy(struct msg_msg *copy)
{
	if (copy)
		free_msg(copy);
}
#else
static inline struct msg_msg *prepare_copy(void __user *buf, size_t bufsz)
{
	return ERR_PTR(-ENOSYS);
}

static inline void free_copy(struct msg_msg *copy)
{
}
#endif

static struct msg_msg *find_msg(struct msg_queue *msq, long *msgtyp, int mode)
{
	struct msg_msg *msg, *found = NULL;
	long count = 0;

	list_for_each_entry(msg, &msq->q_messages, m_list) {
		if (testmsg(msg, *msgtyp, mode) &&
		    !security_msg_queue_msgrcv(msq, msg, current,
					       *msgtyp, mode)) {
			if (mode == SEARCH_LESSEQUAL && msg->m_type != 1) {
				*msgtyp = msg->m_type - 1;
				found = msg;
			} else if (mode == SEARCH_NUMBER) {
				if (*msgtyp == count)
					return msg;
			} else
				return msg;
			count++;
		}
	}

	return found ?: ERR_PTR(-EAGAIN);
}

long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp, int msgflg,
	       long (*msg_handler)(void __user *, struct msg_msg *, size_t))
{
	int mode;
	struct msg_queue *msq;
	struct ipc_namespace *ns;
	struct msg_msg *msg, *copy = NULL;

	ns = current->nsproxy->ipc_ns;

	if (msqid < 0 || (long) bufsz < 0)
		return -EINVAL;

	if (msgflg & MSG_COPY) {
		copy = prepare_copy(buf, min_t(size_t, bufsz, ns->msg_ctlmax));
		if (IS_ERR(copy))
			return PTR_ERR(copy);
	}
	mode = convert_mode(&msgtyp, msgflg);

	rcu_read_lock();
	msq = msq_obtain_object_check(ns, msqid);
	if (IS_ERR(msq)) {
		rcu_read_unlock();
		free_copy(copy);
		return PTR_ERR(msq);
	}

	for (;;) {
		struct msg_receiver msr_d;

		msg = ERR_PTR(-EACCES);
		if (ipcperms(ns, &msq->q_perm, S_IRUGO))
			goto out_unlock1;

		ipc_lock_object(&msq->q_perm);

		/* raced with RMID? */
		if (!ipc_valid_object(&msq->q_perm)) {
			msg = ERR_PTR(-EIDRM);
			goto out_unlock0;
		}

		msg = find_msg(msq, &msgtyp, mode);
		if (!IS_ERR(msg)) {
			/*
			 * Found a suitable message.
			 * Unlink it from the queue.
			 */
			if ((bufsz < msg->m_ts) && !(msgflg & MSG_NOERROR)) {
				msg = ERR_PTR(-E2BIG);
				goto out_unlock0;
			}
			/*
			 * If we are copying, then do not unlink message and do
			 * not update queue parameters.
			 */
			if (msgflg & MSG_COPY) {
				msg = copy_msg(msg, copy);
				goto out_unlock0;
			}

			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);

			goto out_unlock0;
		}

		/* No message waiting. Wait for a message */
		if (msgflg & IPC_NOWAIT) {
			msg = ERR_PTR(-ENOMSG);
			goto out_unlock0;
		}

		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 = bufsz;
		msr_d.r_msg = ERR_PTR(-EAGAIN);
		current->state = TASK_INTERRUPTIBLE;

		ipc_unlock_object(&msq->q_perm);
		rcu_read_unlock();
		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 acquiring the q_perm.lock in ipc_lock_object().
		 */
		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))
			goto out_unlock1;

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

		/* 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_unlock0;

		list_del(&msr_d.r_list);
		if (signal_pending(current)) {
			msg = ERR_PTR(-ERESTARTNOHAND);
			goto out_unlock0;
		}

		ipc_unlock_object(&msq->q_perm);
	}

out_unlock0:
	ipc_unlock_object(&msq->q_perm);
out_unlock1:
	rcu_read_unlock();
	if (IS_ERR(msg)) {
		free_copy(copy);
		return PTR_ERR(msg);
	}

	bufsz = msg_handler(buf, msg, bufsz);
	free_msg(msg);

	return bufsz;
}

SYSCALL_DEFINE5(msgrcv, int, msqid, struct msgbuf __user *, msgp, size_t, msgsz,
		long, msgtyp, int, msgflg)
{
	return do_msgrcv(msqid, msgp, msgsz, msgtyp, msgflg, do_msg_fill);
}

#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