On Fri, 18 Sep 2015, Sudip Mukherjee wrote: > On Wed, Sep 16, 2015 at 12:54:03PM -0400, Alan Stern wrote: > > On Wed, 16 Sep 2015, Sudip Mukherjee wrote: > > > > > On error find_tt() returns either a NULL pointer or the error value in > > > ERR_PTR. But we were dereferencing it directly without even checking if > > > find_tt() returned a valid pointer or not. > > > > > > Signed-off-by: Sudip Mukherjee <sudip@xxxxxxxxxxxxxxx> > > > --- > <snip> > > > @@ -1373,6 +1375,8 @@ static void reserve_release_iso_bandwidth(struct ehci_hcd *ehci, > > > } > > > > > > tt = find_tt(stream->ps.udev); > > > + if (IS_ERR_OR_NULL(tt)) > > > + return; > > > if (sign > 0) > > > list_add_tail(&stream->ps.ps_list, &tt->ps_list); > > > else > > > > This patch isn't needed. In both reserve_release_intr_bandwidth() and > > reserve_release_iso_bandwidth() it is known that find_tt() will return > > a valid pointer. > > > > This is because each of those functions is called from only one place. > > For example, reserve_release_intr_bandwidth() is called only at the end > > of qh_schedule(). But near the start of qh_schedule() there is earlier > > call to tt_find(), and there we do test for error pointers. If the > > first call doesn't return an error then the second call won't either. > > > > The same sort of thing happens in reserve_release_iso_bandwidth(). > Yes, I should have looked more before sending. Sorry for the noise. > But in those checkes for find_tt() only IS_ERR is checked, shouldn't we > check for IS_ERR_OR_NULL as find_tt() can return NULL also? I knew someone would ask about that! :-) The NULL case is similar to the ERR_PTR case, but more complicated. Basically, find_tt() returns NULL only when the device doesn't lie below a TT -- all the other invalid returns are ERR_PTRs. In reserve_release_intr_bandwidth(), for instance, the call to find_tt() occurs only if tt_usecs = qh->ps.tt_usecs is nonzero. This value is guaranteed to be 0 if the device doesn't run at low speed or full speed -- see qh_make() in ehci-q.c, where qh->ps.tt_usecs is initialized only when urb->dev->speed != USB_SPEED_HIGH. To see that udev->tt is non-NULL whenever the speed isn't USB_SPEED_HIGH, you have to look through the hub driver code. The relevant routine is hub_port_init() in core/hub.c, the section headed by the comment: /* Set up TT records, if needed */ Similar reasoning applies to reserve_release_iso_bandwidth(); here the condition is that stream->splits is nonzero, which is true only if the device is full speed (see iso_stream_init()). Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
