Re: [usb-host] question about pointer dereference before null check

[Gustavo A. R. Silva]

Hi Alan,

Quoting Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:

> On Mon, 1 May 2017, Gustavo A. R. Silva wrote:
>
> > Hello everybody,
> >
> > While taking a look into Coverity ID 100828 I ran into the following
> > piece of code at drivers/usb/host/ehci-sched.c:1096:
> >
> > u32             addr;
> > int             think_time;
> > int             hs_transfers;
> >
> > addr = dev->ttport << 24;
> > if (!ehci_is_TDI(ehci)
> >                  || (dev->tt->hub !=
> >                          ehci_to_hcd(ehci)->self.root_hub))
> >          addr |= dev->tt->hub->devnum << 16;
> > addr |= epnum << 8;
> > addr |= dev->devnum;
> > stream->ps.usecs = HS_USECS_ISO(maxp);
> > think_time = dev->tt ? dev->tt->think_time : 0;
> >
> > The issue here is that dev->tt is being dereferenced before null check.
> >
> > I was thinking on placing think_time = dev->tt ? dev->tt->think_time :
> > 0; just before the _if_ statement. But this doesn't address the
> > problem of dev->tt actually being NULL.
> >
> > While looking into the callers of the function containing this piece
> > of code (iso_stream_init()) my impression is that dev->tt is not NULL
> > at the time this function is called and, a very simple patch like the
> > following can be applied in order to avoid the Coverity issue:
> >
> > -think_time = dev->tt ? dev->tt->think_time : 0;
> > +think_time = dev->tt->think_time;
> >
> > But I can't tell for sure, so in case dev->tt is NULL, a good strategy
> > to properly update the _addr_ variable would be needed.
> >
> > What do you think?
> >
> > I would really appreciate any comment on this,
> > Thank you!
>
> You are right; udev->tt cannot ever be NULL when this section of code
> runs.  The test should be removed.

Thanks for confirming, I'll send a patch shortly.
--
Gustavo A. R. Silva