when fetching USB strings, first try the full descriptor and
if it works, use it.  if not, fall back to the existing method
that fetches the first 2 bytes and then the whole string, using
the length from the 2 byte reply.

fixes a very strange problem where an axe(4) attaching (either
has ugen(4) or axe(4)) would ask for 2 bytes, usb_mem.c would
allocate a 2 byte fragment, perform the operation, and sometime
shortly afterwards (usually by the time the next allocation
is made for this fragment), would become corrupted (usually
two bytes were written with 0x0304.)

(initial request of 4 bytes also avoids the problem on this
device.  it really seems like a HC problem -- host should not
allow the device to write more than req.wLength!)


Index: usb_subr.c
===================================================================
RCS file: /cvsroot/src/sys/dev/usb/usb_subr.c,v
retrieving revision 1.238
diff -p -u -r1.238 usb_subr.c
--- usb_subr.c	21 Aug 2019 10:48:37 -0000	1.238
+++ usb_subr.c	24 Aug 2019 13:38:50 -0000
@@ -126,7 +126,27 @@ usbd_get_string_desc(struct usbd_device 
 	req.bRequest = UR_GET_DESCRIPTOR;
 	USETW2(req.wValue, UDESC_STRING, sindex);
 	USETW(req.wIndex, langid);
-	USETW(req.wLength, 2);	/* only size byte first */
+	USETW(req.wLength, sizeof *sdesc);
+	err = usbd_do_request_flags(dev, &req, sdesc, USBD_SHORT_XFER_OK,
+		&actlen, USBD_DEFAULT_TIMEOUT);
+
+	if (actlen < 2)
+		return USBD_SHORT_XFER;
+
+	/* Worked on first attempt. */
+	if (err == 0)
+		goto out;
+
+	/*
+	 * Failed. Try again with just the first 2 bytes for length, and
+	 * then fetch the returned length.
+	 */
+
+	req.bmRequestType = UT_READ_DEVICE;
+	req.bRequest = UR_GET_DESCRIPTOR;
+	USETW2(req.wValue, UDESC_STRING, sindex);
+	USETW(req.wIndex, langid);
+	USETW(req.wLength, 2);
 	err = usbd_do_request_flags(dev, &req, sdesc, USBD_SHORT_XFER_OK,
 		&actlen, USBD_DEFAULT_TIMEOUT);
 	if (err)
@@ -143,6 +163,7 @@ usbd_get_string_desc(struct usbd_device 
 	if (err)
 		return err;
 
+out:
 	if (actlen != sdesc->bLength) {
 		DPRINTF("expected %jd, got %jd", sdesc->bLength, actlen, 0, 0);
 	}
@@ -1380,6 +1401,7 @@ usbd_new_device(device_t parent, struct 
 	}
 
 	err = usbd_reload_device_desc(dev);
+
 	if (err) {
 		DPRINTF("addr=%jd, getting full desc failed, err = %jd", addr,
 		    err, 0, 0);