On Tue, 5 Nov 2013, David Cohen wrote:
> Hi Alan,
>
> On 11/05/2013 07:38 AM, Alan Stern wrote:
> > On Tue, 5 Nov 2013, David Cohen wrote:
> >
> >>>> + /*
> >>>> + * Controller requires buffer size to be aligned to
> >>>> + * maxpacketsize of an out endpoint.
> >>>> + */
> >>>> + if (gadget->quirk_ep_out_aligned_size && read) {
> >>>> + /*
> >>>> + * We pass 'orig_len' to usp_ep_align_maxpacketsize()
> >>>> + * due to we're in a loop and 'len' may have been
> >>>> + * changed.
> >>>> + */
> >>>> + len = usb_ep_align_maxpacketsize(ep->ep, orig_len);
> >>>> + if (data && len > data_len) {
> >>>> + kfree(data);
> >>>> + data = NULL;
> >>>> + data_len = 0;
> >>>> + }
> >>>> + }
> >>>
> >>> Since the value of orig_len never changes, there's no point calling
> >>> usb_ep_align_maxpacketsize() inside the loop. You should call it only
> >>> once, before the loop starts. Once you do that, you won't need
> >>> orig_len at all.
> >>
> >> orig_len doesn't change but ep->ep does. If USB specs say max packet
> >> size won't change even if ep does, than we can call it from outside the
> >> loop.
> >
> > I'm not too familiar with this driver. It looks like the only way
> > ep->ep can change is if the endpoint gets enabled while you're sitting
> > inside the wait_event_interruptible() call.
> >
> > In fact, the whole structure of that loop looks peculiar. Why not
> > acquire the mutex first and then do everything else?
>
> I'm not 100% familiar with this driver too. I'd keep this change to
> another patch.
>
> >
> > Does it even make sense for ep to change? Would this change be visible
> > to the host? What if the host changes the alternate setting while this
> > loop is running -- does it make sense for the userspace program to
> > start a read or write under one altsetting but then have the read/write
> > take place under a different altsetting?
>
> It doesn't make sense to do so, but gadget driver allows it. If we just
> ignore, it would be a security or instability issue possible to xploit
> (for DWC3 and any other controller which may depend on this quirk).
Maybe Michal can enlighten us.
Alan Stern
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