usbmon: correct length for isochronous

Usually the usbmon returns the amount of data specified in
urb->transfer_buffer_length for output submissions and urb->actual_length
for input callbacks. However, for Isochronous input transfers, this is
not enough, since the returned data buffer may contain "holes".

One easy way to fix this is to use urb->transfer_buffer_length,
but this often transfers a whole lot of unused data, so we find
how much was actually used instead.

Original patch by Márton Németh. See also kernel bug 22182.

Signed-off-by: Pete Zaitcev <[e-mail]>
Signed-off-by: Márton Németh <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/mon/mon_bin.c

···
 	return length;
 }
 
+/*
+ * This is the look-ahead pass in case of 'C Zi', when actual_length cannot
+ * be used to determine the length of the whole contiguous buffer.
+ */
+static unsigned int mon_bin_collate_isodesc(const struct mon_reader_bin *rp,
+    struct urb *urb, unsigned int ndesc)
+{
+	struct usb_iso_packet_descriptor *fp;
+	unsigned int length;
+
+	length = 0;
+	fp = urb->iso_frame_desc;
+	while (ndesc-- != 0) {
+		if (fp->actual_length != 0) {
+			if (fp->offset + fp->actual_length > length)
+				length = fp->offset + fp->actual_length;
+		}
+		fp++;
+	}
+	return length;
+}
+
 static void mon_bin_get_isodesc(const struct mon_reader_bin *rp,
     unsigned int offset, struct urb *urb, char ev_type, unsigned int ndesc)
 {
···
 	/*
 	 * Find the maximum allowable length, then allocate space.
 	 */
+	urb_length = (ev_type == 'S') ?
+	    urb->transfer_buffer_length : urb->actual_length;
+	length = urb_length;
+
 	if (usb_endpoint_xfer_isoc(epd)) {
 		if (urb->number_of_packets < 0) {
 			ndesc = 0;
···
 		} else {
 			ndesc = urb->number_of_packets;
 		}
+		if (ev_type == 'C' && usb_urb_dir_in(urb))
+			length = mon_bin_collate_isodesc(rp, urb, ndesc);
 	} else {
 		ndesc = 0;
 	}
 	lendesc = ndesc*sizeof(struct mon_bin_isodesc);
 
-	urb_length = (ev_type == 'S') ?
-	    urb->transfer_buffer_length : urb->actual_length;
-	length = urb_length;
+	/* not an issue unless there's a subtle bug in a HCD somewhere */
+	if (length >= urb->transfer_buffer_length)
+		length = urb->transfer_buffer_length;
 
 	if (length >= rp->b_size/5)
 		length = rp->b_size/5;