usb: cdc-wdm: avoid setting WDM_READ for ZLP-s

Don't set WDM_READ flag in wdm_in_callback() for ZLP-s, otherwise when
userspace tries to poll for available data, it might - incorrectly -
believe there is something available, and when it tries to non-blocking
read it, it might get stuck in the read loop.

For example this is what glib does for non-blocking read (briefly):

1. poll()
2. if poll returns with non-zero, starts a read data loop:
a. loop on poll() (EINTR disabled)
b. if revents was set, reads data
I. if read returns with EINTR or EAGAIN, goto 2.a.
II. otherwise return with data

So if ZLP sets WDM_READ (#1), we expect data, and try to read it (#2).
But as that was a ZLP, and we are doing non-blocking read, wdm_read()
returns with EAGAIN (#2.b.I), so loop again, and try to read again
(#2.a.).

With glib, we might stuck in this loop forever, as EINTR is disabled
(#2.a).

Signed-off-by: Robert Hodaszi <[e-mail]>
Acked-by: Oliver Neukum <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/class/cdc-wdm.c

···
 	u16			wMaxCommand;
 	u16			wMaxPacketSize;
 	__le16			inum;
-	int			reslength;
 	int			length;
 	int			read;
 	int			count;
···
 	if (desc->rerr == 0 && status != -EPIPE)
 		desc->rerr = status;
 
+	if (length == 0) {
+		dev_dbg(&desc->intf->dev, "received ZLP\n");
+		goto skip_zlp;
+	}
+
 	if (length + desc->length > desc->wMaxCommand) {
 		/* The buffer would overflow */
 		set_bit(WDM_OVERFLOW, &desc->flags);
···
 		if (!test_bit(WDM_OVERFLOW, &desc->flags)) {
 			memmove(desc->ubuf + desc->length, desc->inbuf, length);
 			desc->length += length;
-			desc->reslength = length;
 		}
 	}
 skip_error:
 
 	if (desc->rerr) {
 		/*
-		 * Since there was an error, userspace may decide to not read
-		 * any data after poll'ing.
+		 * If there was a ZLP or an error, userspace may decide to not
+		 * read any data after poll'ing.
 		 * We should respond to further attempts from the device to send
 		 * data, so that we can get unstuck.
 		 */
+skip_zlp:
 		schedule_work(&desc->service_outs_intr);
 	} else {
 		set_bit(WDM_READ, &desc->flags);
···
 			goto retry;
 		}
 
-		if (!desc->reslength) { /* zero length read */
-			dev_dbg(&desc->intf->dev, "zero length - clearing WDM_READ\n");
-			clear_bit(WDM_READ, &desc->flags);
-			rv = service_outstanding_interrupt(desc);
-			spin_unlock_irq(&desc->iuspin);
-			if (rv < 0)
-				goto err;
-			goto retry;
-		}
 		cntr = desc->length;
 		spin_unlock_irq(&desc->iuspin);
 	}
···
 
 	spin_lock_irq(&desc->iuspin);
 	service_outstanding_interrupt(desc);
-	if (!desc->resp_count) {
+	if (!desc->resp_count && (desc->length || desc->rerr)) {
 		set_bit(WDM_READ, &desc->flags);
 		wake_up(&desc->wait);
 	}