root/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 <manfreds@colorfullife.com>
 */

#include <linux/config.h>
#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 <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_read_proc(char *buffer, char **start, off_t offset, int length, int *eof, void *data);
#endif

void __init msg_init (void)
{
        ipc_init_ids(&msg_ids,msg_ctlmni);

#ifdef CONFIG_PROC_FS
        create_proc_read_entry("sysvipc/msg", 0, 0, sysvipc_msg_read_proc, NULL);
#endif
}

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_free(msq, sizeof(*msq));
                return retval;
        }

        id = ipc_addid(&msg_ids, &msq->q_perm, msg_ctlmni);
        if(id == -1) {
                security_msg_queue_free(msq);
                ipc_rcu_free(msq, sizeof(*msq));
                return -ENOSPC;
        }

        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);

        return msg_buildid(id,msq->q_perm.seq);
}

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 = ERR_PTR(res);
                wake_up_process(msr->r_tsk);
        }
}
/* 
 * 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.sem and the spinlock for this message queue is hold
 * before freeque() is called. msg_ids.sem remains locked on exit.
 */
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_free(msq, sizeof(struct msg_queue));
}

asmlinkage long sys_msgget (key_t key, int msgflg)
{
        int id, ret = -EPERM;
        struct msg_queue *msq;
        
        down(&msg_ids.sem);
        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);
                if(msq==NULL)
                        BUG();
                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);
        }
        up(&msg_ids.sem);
        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;
                down(&msg_ids.sem);
                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;
                up(&msg_ids.sem);
                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.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;
        }

        down(&msg_ids.sem);
        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 = -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:
        up(&msg_ids.sem);
        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 = ERR_PTR(-E2BIG);
                                wake_up_process(msr->r_tsk);
                        } else {
                                msr->r_msg = msg;
                                msq->q_lrpid = msr->r_tsk->pid;
                                msq->q_rtime = get_seconds();
                                wake_up_process(msr->r_tsk);
                                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;
retry:
        err= -EIDRM;
        if (msg_checkid(msq,msqid))
                goto out_unlock_free;

        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) {
                struct msg_sender s;

                if(msgflg&IPC_NOWAIT) {
                        err=-EAGAIN;
                        goto out_unlock_free;
                }
                ss_add(msq, &s);
                msg_unlock(msq);
                schedule();
                current->state= TASK_RUNNING;

                msq = msg_lock(msqid);
                err = -EIDRM;
                if(msq==NULL)
                        goto out_free;
                ss_del(&s);
                
                if (signal_pending(current)) {
                        err=-EINTR;
                        goto out_unlock_free;
                }
                goto retry;
        }

        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_receiver msr_d;
        struct list_head* tmp;
        struct msg_msg* msg, *found_msg;
        int err;
        int mode;

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

        msq = msg_lock(msqid);
        if(msq==NULL)
                return -EINVAL;
retry:
        err = -EIDRM;
        if (msg_checkid(msq,msqid))
                goto out_unlock;

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

        tmp = msq->q_messages.next;
        found_msg=NULL;
        while (tmp != &msq->q_messages) {
                msg = list_entry(tmp,struct msg_msg,m_list);
                if(testmsg(msg,msgtyp,mode) &&
                   !security_msg_queue_msgrcv(msq, msg, current, msgtyp, mode)) {
                        found_msg = msg;
                        if(mode == SEARCH_LESSEQUAL && msg->m_type != 1) {
                                found_msg=msg;
                                msgtyp=msg->m_type-1;
                        } else {
                                found_msg=msg;
                                break;
                        }
                }
                tmp = tmp->next;
        }
        if(found_msg) {
                msg=found_msg;
                if ((msgsz < msg->m_ts) && !(msgflg & MSG_NOERROR)) {
                        err=-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);
out_success:
                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;
        } else
        {
                /* no message waiting. Prepare for pipelined
                 * receive.
                 */
                if (msgflg & IPC_NOWAIT) {
                        err=-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();

                /*
                 * The below optimisation is buggy.  A sleeping thread that is
                 * woken up checks if it got a message and if so, copies it to
                 * userspace and just returns without taking any locks.
                 * But this return to user space can be faster than the message
                 * send, and if the receiver immediately exits the
                 * wake_up_process performed by the sender will oops.
                 */
#if 0
                msg = (struct msg_msg*) msr_d.r_msg;
                if(!IS_ERR(msg)) 
                        goto out_success;
#endif

                msq = msg_lock(msqid);
                msg = (struct msg_msg*)msr_d.r_msg;
                if(!IS_ERR(msg)) {
                        /* our message arived while we waited for
                         * the spinlock. Process it.
                         */
                        if(msq)
                                msg_unlock(msq);
                        goto out_success;
                }
                err = PTR_ERR(msg);
                if(err == -EAGAIN) {
                        if(!msq)
                                BUG();
                        list_del(&msr_d.r_list);
                        if (signal_pending(current))
                                err=-EINTR;
                         else
                                goto retry;
                }
        }
out_unlock:
        if(msq)
                msg_unlock(msq);
        return err;
}

#ifdef CONFIG_PROC_FS
static int sysvipc_msg_read_proc(char *buffer, char **start, off_t offset, int length, int *eof, void *data)
{
        off_t pos = 0;
        off_t begin = 0;
        int i, len = 0;

        down(&msg_ids.sem);
        len += sprintf(buffer, "       key      msqid perms      cbytes       qnum lspid lrpid   uid   gid  cuid  cgid      stime      rtime      ctime\n");

        for(i = 0; i <= msg_ids.max_id; i++) {
                struct msg_queue * msq;
                msq = msg_lock(i);
                if(msq != NULL) {
                        len += sprintf(buffer + len, "%10d %10d  %4o  %10lu %10lu %5u %5u %5u %5u %5u %5u %10lu %10lu %10lu\n",
                                msq->q_perm.key,
                                msg_buildid(i,msq->q_perm.seq),
                                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);
                        msg_unlock(msq);

                        pos += len;
                        if(pos < offset) {
                                len = 0;
                                begin = pos;
                        }
                        if(pos > offset + length)
                                goto done;
                }

        }
        *eof = 1;
done:
        up(&msg_ids.sem);
        *start = buffer + (offset - begin);
        len -= (offset - begin);
        if(len > length)
                len = length;
        if(len < 0)
                len = 0;
        return len;
}
#endif