Re: [PATCH 4/6] USB: hub: fix non-SS hub-descriptor handling

[Johan Hovold <johan@kernel.org>]

On Wed, May 10, 2017 at 04:31:19PM +0200, Johan Hovold wrote:
> On Wed, May 10, 2017 at 10:12:56AM -0400, Alan Stern wrote:
> > On Wed, 10 May 2017, Johan Hovold wrote:

> > >  /* USB 2.0 spec Section 11.24.4.5 */
> > > -static int get_hub_descriptor(struct usb_device *hdev, void *data)
> > > +static int get_hub_descriptor(struct usb_device *hdev,
> > > +		struct usb_hub_descriptor *desc)
> > >  {
> > >  	int i, ret, size;
> > >  	unsigned dtype;
> > > @@ -378,12 +379,16 @@ static int get_hub_descriptor(struct usb_device *hdev, void *data)
> > >  	for (i = 0; i < 3; i++) {
> > >  		ret = usb_control_msg(hdev, usb_rcvctrlpipe(hdev, 0),
> > >  			USB_REQ_GET_DESCRIPTOR, USB_DIR_IN | USB_RT_HUB,
> > > -			dtype << 8, 0, data, size,
> > > +			dtype << 8, 0, desc, size,
> > >  			USB_CTRL_GET_TIMEOUT);
> > >  		if (hub_is_superspeed(hdev)) {
> > >  			if (ret == size)
> > >  				return ret;
> > > -		} else if (ret >= (USB_DT_HUB_NONVAR_SIZE + 2)) {
> > > +		} else if (ret >= USB_DT_HUB_NONVAR_SIZE + 2) {
> > > +			/* Make sure we have the DeviceRemovable field. */
> > > +			size = USB_DT_HUB_NONVAR_SIZE + desc->bNbrPorts / 8 + 1;
> > > +			if (ret < size)
> > > +				return -EMSGSIZE;
> > 
> > The logic could be simplified a little.  Since we don't really care 
> > about the return code when an error occurs, you could just do:
> > 
> > 		} else if (ret >= USB_DT_HUB_NONVAR_SIZE +
> > 				desc->bNbrPorts / 8 + 1) {
> > 			/* We have the entire DeviceRemovable field. */
> >  			return ret;
> >  		}
> 
> Sure, that would work, but I it doesn't feel right to access bNbrPorts
> without first verifying we got the non-variable fields.
> 
> I considered dropping the +2 bit, but decided to keep it in the unlikely
> even that there are quirky devices out there that rely on it (e.g. first
> read always return 7 bytes). Spelling it out makes it sound overly
> conservative though. How about I drop that instead?

Then again, a non-SS hub descriptor is always at least
(USB_DT_HUB_NONVAR_SIZE + 2) bytes long so keeping it kind of makes
sense anyway.

Johan