Re: [PATCH v8 4/5] ipc: message queue copy feature introduced

This patch is required for checkpoint/restore in userspace.
IOW, c/r requires some way to get all pending IPC messages without deleting
them from the queue (checkpoint can fail and in this case tasks will be resumed,
so queue have to be valid).
To achive this, new operation flag MSG_COPY for sys_msgrcv() system call was
introduced. If this flag was specified, then mtype is interpreted as number of
the message to copy.
If MSG_COPY is set, then kernel will allocate dummy message with passed size,
and then use new copy_msg() helper function to copy desired message (instead of
unlinking it from the queue).

...

@@ -777,19 +777,48 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
 	struct msg_msg *msg;
 	int mode;
 	struct ipc_namespace *ns;
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	struct msg_msg *copy = NULL;
+	unsigned long copy_number = 0;
+#endif
 
 	if (msqid < 0 || (long) bufsz < 0)
 		return -EINVAL;
+	if (msgflg & MSG_COPY) {
+#ifdef CONFIG_CHECKPOINT_RESTORE
+
+		if (msgflg & MSG_COPY) {

+			copy_number = msgtyp;
+			msgtyp = 0;
+		}
+
+		/*
+		 * Create dummy message to copy real message to.
+		 */
+		copy = load_msg(buf, bufsz);
+		if (IS_ERR(copy))
+			return PTR_ERR(copy);
+		copy->m_ts = bufsz;
+#else
+		return -ENOSYS;
+#endif
+	}
 	mode = convert_mode(&msgtyp, msgflg);
 	ns = current->nsproxy->ipc_ns;
 
 	msq = msg_lock_check(ns, msqid);
-	if (IS_ERR(msq))
+	if (IS_ERR(msq)) {
+#ifdef CONFIG_CHECKPOINT_RESTORE
+		if (msgflg & MSG_COPY)
+			free_msg(copy);
+#endif
 		return PTR_ERR(msq);
+	}
 
 	for (;;) {
 		struct msg_receiver msr_d;
 		struct list_head *tmp;
+		long msg_counter = 0;
 
 		msg = ERR_PTR(-EACCES);
 		if (ipcperms(ns, &msq->q_perm, S_IRUGO))

@@ -809,8 +838,16 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
 				if (mode == SEARCH_LESSEQUAL &&
 						walk_msg->m_type != 1) {
 					msgtyp = walk_msg->m_type - 1;
+#ifdef CONFIG_CHECKPOINT_RESTORE
+				} else if (msgflg & MSG_COPY) {
+					if (copy_number == msg_counter) {
+						msg = copy_msg(walk_msg, copy);
+						break;
+					}
+#endif
 				} else
 					break;
+				msg_counter++;
 			}
 			tmp = tmp->next;
 		}

@@ -823,6 +860,10 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
 				msg = ERR_PTR(-E2BIG);
 				goto out_unlock;
 			}
+#ifdef CONFIG_CHECKPOINT_RESTORE
+			if (msgflg & MSG_COPY)
+				goto out_unlock;
+#endif
 			list_del(&msg->m_list);
 			msq->q_qnum--;
 			msq->q_rtime = get_seconds();

@@ -906,8 +947,13 @@ out_unlock:
 			break;
 		}
 	}
-	if (IS_ERR(msg))
+	if (IS_ERR(msg)) {
+#ifdef CONFIG_CHECKPOINT_RESTORE
+		if (msgflg & MSG_COPY)
+			free_msg(copy);
+#endif
 		return PTR_ERR(msg);
+	}
 
 	bufsz = msg_handler(buf, msg, bufsz);
 	free_msg(msg);