staging: comedi: COMEDI_BUFINFO: force bytes_read or bytes_written to 0

The `COMEDI_BUFINFO` ioctl is used to advance the current position in
the buffer by a specified amount (which can be 0) and get the new
position.  On input, the `bytes_read` member of `struct comedi_bufinfo`
specifies the amount to advance the "read" position for an asynchronous
command in the "read" direction, and the `bytes_written` member
specifies the amount to advance the "write" position for a command in
the "write" direction.  The handler `do_bufinfo_ioctl()` may adjust
these by the amount the position is actually advanced before copying
them back to the user.  Currently, it ignores the specified `bytes_read`
value for a command in the "write" direction, and ignores the specified
`bytes_written` for a command in the "read" direction, so the values
copied back to the user are unchanged.  Change it to force the ignored
value to 0 before copying the values back to the user.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_fops.c

@@ -1141,19 +1141,25 @@ static int do_bufinfo_ioctl(struct comedi_device *dev,
 	if (s->busy != file)
 		return -EACCES;
 
-	if (bi.bytes_read && !(async->cmd.flags & CMDF_WRITE)) {
-		comedi_buf_read_alloc(s, bi.bytes_read);
-		bi.bytes_read = comedi_buf_read_free(s, bi.bytes_read);
-
-		if (comedi_is_subdevice_idle(s) &&
-		    comedi_buf_read_n_available(s) == 0) {
-			do_become_nonbusy(dev, s);
+	if (!(async->cmd.flags & CMDF_WRITE)) {
+		/* command was set up in "read" direction */
+		if (bi.bytes_read) {
+			comedi_buf_read_alloc(s, bi.bytes_read);
+			bi.bytes_read = comedi_buf_read_free(s, bi.bytes_read);
+
+			if (comedi_is_subdevice_idle(s) &&
+			    comedi_buf_read_n_available(s) == 0)
+				do_become_nonbusy(dev, s);
 		}
-	}
-
-	if (bi.bytes_written && (async->cmd.flags & CMDF_WRITE)) {
-		comedi_buf_write_alloc(s, bi.bytes_written);
-		bi.bytes_written = comedi_buf_write_free(s, bi.bytes_written);
+		bi.bytes_written = 0;
+	} else {
+		/* command was set up in "write" direction */
+		if (bi.bytes_written) {
+			comedi_buf_write_alloc(s, bi.bytes_written);
+			bi.bytes_written =
+			    comedi_buf_write_free(s, bi.bytes_written);
+		}
+		bi.bytes_read = 0;
 	}
 
 copyback_position: