Re: [PATCH] usb: gadget: udc: bdc: Remove unnecessary NULL checks in bdc_req_complete

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Feb 20, 2020 at 8:57 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> I know it has been a while but ping?

Sorry! Too many bugs...barely treading water! Send help!

>
> On Tue, Oct 22, 2019 at 05:20:15PM -0700, Nathan Chancellor wrote:
> > When building with Clang + -Wtautological-pointer-compare:
> >
> > drivers/usb/gadget/udc/bdc/bdc_ep.c:543:28: warning: comparison of
> > address of 'req->queue' equal to a null pointer is always false
> > [-Wtautological-pointer-compare]
> >         if (req == NULL  || &req->queue == NULL || &req->usb_req == NULL)
> >                              ~~~~~^~~~~    ~~~~
> > drivers/usb/gadget/udc/bdc/bdc_ep.c:543:51: warning: comparison of
> > address of 'req->usb_req' equal to a null pointer is always false
> > [-Wtautological-pointer-compare]
> >         if (req == NULL  || &req->queue == NULL || &req->usb_req == NULL)
> >                                                     ~~~~~^~~~~~~    ~~~~
> > 2 warnings generated.
> >
> > As it notes, these statements will always evaluate to false so remove
> > them.

`req` is an instance of a `struct bdc_req` defined in
drivers/usb/gadget/udc/bdc/bdc.h as:
333 struct bdc_req {
334   struct usb_request  usb_req;
335   struct list_head  queue;
336   struct bdc_ep   *ep;
337   /* only one Transfer per request */
338   struct bd_transfer bd_xfr;
339   int epnum;
340 };

So indeed the non-pointer, struct members can never be NULL.

I think the second check that was removed should be
`req->usb_req.complete == NULL`, since otherwise `&req->usb_req` may
be passed to usb_gadget_giveback_request which tries to invoke the
`complete` member as a callback.  There are numerous places in
drivers/usb/gadget/udc/bdc/bdc_ep.c that assign `complete = NULL`.

Can the maintainers clarify?

> >
> > Fixes: efed421a94e6 ("usb: gadget: Add UDC driver for Broadcom USB3.0 device controller IP BDC")
> > Link: https://github.com/ClangBuiltLinux/linux/issues/749
> > Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> > ---
> >
> > Note: I am not sure if these checks were intended to check if the
> > contents of these arrays were NULL or if there should be some other
> > checks in lieu of these; I am not familiar with the USB subsystem to
> > answer this but I will happily respin the patch if this is not correct.
> >
> >  drivers/usb/gadget/udc/bdc/bdc_ep.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/usb/gadget/udc/bdc/bdc_ep.c b/drivers/usb/gadget/udc/bdc/bdc_ep.c
> > index a4d9b5e1e50e..d49c6dc1082d 100644
> > --- a/drivers/usb/gadget/udc/bdc/bdc_ep.c
> > +++ b/drivers/usb/gadget/udc/bdc/bdc_ep.c
> > @@ -540,7 +540,7 @@ static void bdc_req_complete(struct bdc_ep *ep, struct bdc_req *req,
> >  {
> >       struct bdc *bdc = ep->bdc;
> >
> > -     if (req == NULL  || &req->queue == NULL || &req->usb_req == NULL)
> > +     if (req == NULL)
> >               return;
> >
> >       dev_dbg(bdc->dev, "%s ep:%s status:%d\n", __func__, ep->name, status);
> > --
> > 2.23.0
> >
>
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-- 
Thanks,
~Nick Desaulniers