ipc,msg: make msgctl_nolock lockless

commit ac0ba20e upstream.

While the INFO cmd doesn't take the ipc lock, the STAT commands do
acquire it unnecessarily.  We can do the permissions and security checks
only holding the rcu lock.

This function now mimics semctl_nolock().
Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

ipc/msg.c

@@ -545,17 +545,25 @@ static int msgctl_nolock(struct ipc_namespace *ns, int msqid,
 		if (!buf)
 			return -EFAULT;
 
+		memset(&tbuf, 0, sizeof(tbuf));
+
+		rcu_read_lock();
 		if (cmd == MSG_STAT) {
-			msq = msg_lock(ns, msqid);
-			if (IS_ERR(msq))
-				return PTR_ERR(msq);
+			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 = msg_lock_check(ns, msqid);
-			if (IS_ERR(msq))
-				return PTR_ERR(msq);
+			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;
@@ -564,8 +572,6 @@ static int msgctl_nolock(struct ipc_namespace *ns, int msqid,
 		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;
@@ -575,7 +581,8 @@ static int msgctl_nolock(struct ipc_namespace *ns, int msqid,
 		tbuf.msg_qbytes = msq->q_qbytes;
 		tbuf.msg_lspid  = msq->q_lspid;
 		tbuf.msg_lrpid  = msq->q_lrpid;
-		msg_unlock(msq);
+		rcu_read_unlock();
+
 		if (copy_msqid_to_user(buf, &tbuf, version))
 			return -EFAULT;
 		return success_return;
@@ -587,7 +594,7 @@ static int msgctl_nolock(struct ipc_namespace *ns, int msqid,
 
 	return err;
 out_unlock:
-	msg_unlock(msq);
+	rcu_read_unlock();
 	return err;
 }