IPC/message queues: introduce msgctl_down

Currently, sys_msgctl is not easy to read.

This patch tries to improve that by introducing the msgctl_down function to
handle all commands requiring the rwmutex to be taken in write mode (ie
IPC_SET and IPC_RMID for now).  It is the equivalent function of semctl_down
for message queues.

This greatly changes the readability of sys_msgctl and also harmonizes the way
these commands are handled among all IPCs.
Signed-off-by: Pierre Peiffer <pierre.peiffer@bull.net>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Nadia Derbey <Nadia.Derbey@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/msg.c

@@ -436,10 +436,95 @@ copy_msqid_from_user(struct msq_setbuf *out, void __user *buf, int version)
 	}
 }
 
-asmlinkage long sys_msgctl(int msqid, int cmd, struct msqid_ds __user *buf)
+/*
+ * 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 msq_setbuf uninitialized_var(setbuf);
+	struct msq_setbuf setbuf;
+	struct msg_queue *msq;
+	int err;
+
+	if (cmd == IPC_SET) {
+		if (copy_msqid_from_user(&setbuf, buf, version))
+			return -EFAULT;
+	}
+
+	down_write(&msg_ids(ns).rw_mutex);
+	msq = msg_lock_check_down(ns, msqid);
+	if (IS_ERR(msq)) {
+		err = PTR_ERR(msq);
+		goto out_up;
+	}
+
+	ipcp = &msq->q_perm;
+
+	err = audit_ipc_obj(ipcp);
+	if (err)
+		goto out_unlock;
+
+	if (cmd == IPC_SET) {
+		err = audit_ipc_set_perm(setbuf.qbytes, setbuf.uid, setbuf.gid,
+					 setbuf.mode);
+		if (err)
+			goto out_unlock;
+	}
+
+	if (current->euid != ipcp->cuid &&
+	    current->euid != ipcp->uid &&
+	    !capable(CAP_SYS_ADMIN)) {
+		/* We _could_ check for CAP_CHOWN above, but we don't */
+		err = -EPERM;
+		goto out_unlock;
+	}
+
+	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 (setbuf.qbytes > ns->msg_ctlmnb &&
+		    !capable(CAP_SYS_RESOURCE)) {
+			err = -EPERM;
+			goto out_unlock;
+		}
+
+		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);
+		break;
+	default:
+		err = -EINVAL;
+	}
+out_unlock:
+	msg_unlock(msq);
+out_up:
+	up_write(&msg_ids(ns).rw_mutex);
+	return err;
+}
+
+asmlinkage long sys_msgctl(int msqid, int cmd, struct msqid_ds __user *buf)
+{
 	struct msg_queue *msq;
 	int err, version;
 	struct ipc_namespace *ns;
@@ -535,82 +620,13 @@ asmlinkage long sys_msgctl(int msqid, int cmd, struct msqid_ds __user *buf)
 		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;
+		err = msgctl_down(ns, msqid, cmd, buf, version);
+		return err;
 	default:
 		return  -EINVAL;
 	}
 
-	down_write(&msg_ids(ns).rw_mutex);
-	msq = msg_lock_check_down(ns, msqid);
-	if (IS_ERR(msq)) {
-		err = PTR_ERR(msq);
-		goto out_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;
-	}
-
-	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 > ns->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(ns, &msq->q_perm);
-		break;
-	}
-	err = 0;
-out_up:
-	up_write(&msg_ids(ns).rw_mutex);
-	return err;
-out_unlock_up:
-	msg_unlock(msq);
-	goto out_up;
 out_unlock:
 	msg_unlock(msq);
 	return err;