[S390] vmur: add "top of queue" sanity check for reader open

If the z/VM reader is already open, it can happen that after opening the
Linux reader device, not the topmost file is processed. According the
semantics of the Linux z/VM unit record device driver, always the topmost
file has to be processed. With this fix an error is returned if that is
not the case.
Signed-off-by: Michael Holzheu <holzheu@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/vmur.c

@@ -565,9 +565,14 @@ static int verify_device(struct urdev *urd)
 			return -ENOMEM;
 		rc = diag_read_file(urd->dev_id.devno, buf);
 		kfree(buf);
-
 		if ((rc != 0) && (rc != -ENODATA)) /* EOF does not hurt */
 			return rc;
+		/* check if the file on top of the queue is open now */
+		rc = diag_read_next_file_info(&fcb, 0);
+		if (rc)
+			return rc;
+		if (!(fcb.file_stat & FLG_IN_USE))
+			return -EMFILE;
 		return 0;
 	default:
 		return -ENOTSUPP;

drivers/s390/char/vmur.h

@@ -53,6 +53,7 @@ struct file_control_block {
 #define FLG_SYSTEM_HOLD	0x04
 #define FLG_CP_DUMP	0x10
 #define FLG_USER_HOLD	0x20
+#define FLG_IN_USE	0x80
 
 /*
  * A struct urdev is created for each ur device that is made available