linux-kernel - Re: [PATCH v9 1/6] media: uvcvideo: Fix negative modulus calculation

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Message-ID: <20240219104046.GB13043@pendragon.ideasonboard.com>
Date: Mon, 19 Feb 2024 12:40:46 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	linux-kernel@...r.kernel.org, "hn.chen" <hn.chen@...plusit.com>,
	linux-media@...r.kernel.org
Subject: Re: [PATCH v9 1/6] media: uvcvideo: Fix negative modulus calculation

Hi Ricardo,

On Mon, Feb 19, 2024 at 11:28:03AM +0100, Ricardo Ribalda wrote:
> On Mon, 12 Feb 2024 at 23:59, Laurent Pinchart wrote:
> > On Wed, Mar 15, 2023 at 02:30:12PM +0100, Ricardo Ribalda wrote:
> > > If head is 0, last will be addressing the index 0 instead of clock->size
> > > -1. Luckily clock->head is unsiged, otherwise it would be addressing
> > > 0xffffffff.
> >
> > I'm not following you. In the expression
> >
> >         (clock->head - 1) % clock->size
> >
> > clock->head is an unsigned int, and 1 as a signed int, so the result of
> > the subtraction is promoted to an unsigned int. When clock->head is 0, the expression evaluates to
> >
> >         0xffffffff % clock->size
> >
> > clock->size is a power of two (hardcoded to 32 at the moment), so the
> > expression evaluates to 31, as intended.
> >
> > Am I missing something ?
> 
> Take a look to: https://godbolt.org/z/xYeqTx6ba
> 
> The expression only works because the size is a power of two. In this
> set I am allowing sizes that are not powers of two.

Could you then update the commit message to explain that ?

I'll review the rest of the series this week.

> > > Nontheless, this is not the intented behaviour and should be fixed.
> > >
> > > Fixes: 66847ef013cc ("[media] uvcvideo: Add UVC timestamps support")
> > > Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> > > ---
> > >  drivers/media/usb/uvc/uvc_video.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/media/usb/uvc/uvc_video.c b/drivers/media/usb/uvc/uvc_video.c
> > > index d4b023d4de7c..4ff4ab4471fe 100644
> > > --- a/drivers/media/usb/uvc/uvc_video.c
> > > +++ b/drivers/media/usb/uvc/uvc_video.c
> > > @@ -732,7 +732,7 @@ void uvc_video_clock_update(struct uvc_streaming *stream,
> > >               goto done;
> > >
> > >       first = &clock->samples[clock->head];
> > > -     last = &clock->samples[(clock->head - 1) % clock->size];
> > > +     last = &clock->samples[(clock->head - 1 + clock->size) % clock->size];
> > >
> > >       /* First step, PTS to SOF conversion. */
> > >       delta_stc = buf->pts - (1UL << 31);

-- 
Regards,

Laurent Pinchart