Am Montag, den 19.08.2019, 10:17 -0400 schrieb Alan Stern:
> On Mon, 19 Aug 2019, Oliver Neukum wrote:
>
> > Am Freitag, den 16.08.2019, 13:10 -0400 schrieb Alan Stern:
> > > Oliver and Jiri:
> > >
> > > Why is there duplicated code in
> > > drivers/hid/usbhid/hiddev.c:hiddev_open()?
> > >
> > > Line 267:
> > > /*
> > > * no need for locking because the USB major number
> > > * is shared which usbcore guards against disconnect
> > > */
> > > if (list->hiddev->exist) {
> > > if (!list->hiddev->open++) {
> > > res = hid_hw_open(hiddev->hid);
> > > if (res < 0)
> > > goto bail;
> > > }
> > > } else {
> > > res = -ENODEV;
> > > goto bail;
> > > }
> > >
> > > Line 286:
> > > mutex_lock(&hiddev->existancelock);
> > > if (!list->hiddev->open++)
> > > if (list->hiddev->exist) {
> > > struct hid_device *hid = hiddev->hid;
> > > res = hid_hw_power(hid, PM_HINT_FULLON);
> > > if (res < 0)
> > > goto bail_unlock;
> > > res = hid_hw_open(hid);
> > > if (res < 0)
> > > goto bail_normal_power;
> > > }
> > > mutex_unlock(&hiddev->existancelock);
> > >
> > > The second part can never execute, because the first part ensures that
> > > list->hiddev->open > 0 by the time the second part runs.
> > >
> > > Even more disturbing, why is one of these code sections protected by a
> > > mutex and the other not?
> >
> > I suppose the comment I made back then:
> >
> > 079034073faf9 drivers/hid/usbhid/hiddev.c (Oliver Neukum
> > 2008-12-16 10:55:15 +0100 268) * no need for locking because the USB
> > major number
> > 079034073faf9 drivers/hid/usbhid/hiddev.c (Oliver Neukum
> > 2008-12-16 10:55:15 +0100 269) * is shared which usbcore guards against
> > disconnect
> >
> > has ceased to be true, but the section was not removed, as the check
> > for existance was duplicated.
> >
> > > Note: The second section was added in commit 0361a28d3f9a ("HID:
> > > autosuspend support for USB HID") over ten years ago!
> >
> > Yes and I remember how frustrating keyboards were in testing, but
> > no further details.
>
> Indeed. But more importantly for now, how should this be fixed? This
> may be the culprit in some of the syzbot bug reports (those involving
> hiddev).
I doubt it. This looks like it would cause a resource leak, not the
other way round. But I'd say all operations need to be done under lock.
Regards
oliver
