Re: [PATCH 1/2] media: v4l2-core: Shifting signed 32-bit value by 31 bits error

[Shuah Khan]

On 6/6/19 12:33 AM, Hans Verkuil wrote:
> On 6/6/19 5:22 AM, Randy Dunlap wrote:
> > On 6/5/19 2:53 PM, Shuah Khan wrote:
> > > Fix the following cppcheck error:
> > >
> > > Checking drivers/media/v4l2-core/v4l2-ioctl.c ...
> > > [drivers/media/v4l2-core/v4l2-ioctl.c:1370]: (error) Shifting signed 32-bit value by 31 bits is undefined behaviour
> > >
> > > Signed-off-by: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
> > > ---
> > >   drivers/media/v4l2-core/v4l2-ioctl.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
> > > index 6859bdac86fe..333e387bafeb 100644
> > > --- a/drivers/media/v4l2-core/v4l2-ioctl.c
> > > +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
> > > @@ -1364,7 +1364,7 @@ static void v4l_fill_fmtdesc(struct v4l2_fmtdesc *fmt)
> > >   					(char)((fmt->pixelformat >> 8) & 0x7f),
> > >   					(char)((fmt->pixelformat >> 16) & 0x7f),
> > >   					(char)((fmt->pixelformat >> 24) & 0x7f),
> > > -					(fmt->pixelformat & (1 << 31)) ? "-BE" : "");
> > > +					(fmt->pixelformat & BIT(31)) ? "-BE" : "");
> > >   			break;
> > >   		}
> > >   	}
> >
> > If this builds, I guess #define BIT(x) got pulled in indirectly
> > since bits.h nor bitops.h is currently #included in that source file.

It does build. You are right that I should have included bitops.h

> > Documentation/process/submit-checklist.rst rule #1 says:
> > 1) If you use a facility then #include the file that defines/declares
> >     that facility.  Don't depend on other header files pulling in ones
> >     that you use.
> >
> > Please add #include <linux/bits or bitops.h>
>
> I'm not sure about this patch. '1 << 31' is used all over in the kernel,
> including in public headers (e.g. media.h, videodev2.h).
>
> It seems arbitrary to change it only here, but not anywhere else.

Right. We have several places in the kernel that do that.

> In this particular example for the fourcc handling I would prefer to just
> use '1U << 31', both in v4l2-ioctl.c and videodev2.h.

If you would like to take the patch, I can send v2 fixing it using
1U << 31 - This is simpler since it doesn't nee additional includes.

> A separate patch doing the same for MEDIA_ENT_ID_FLAG_NEXT in media.h would
> probably be a good idea either: that way the public API at least will do
> the right thing.

I should have explained it better. I wanted to start with one or two
places first to see if it is worth our time to fix these:

The full kernel cppcheck log for "Shifting signed 32-bit value by 31 
bits is undefined behaviour" can be found at:

https://drive.google.com/file/d/19Xu7UqBGJ7BpzxEp92ZQYb6F8UPrk3z3/view

thanks,
-- Shuah