[v9,1/6] media: uvcvideo: Fix negative modulus calculation

  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.

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@chromium.org>
---
 drivers/media/usb/uvc/uvc_video.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
  

On (23/03/15 14:30), 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.
> 
> 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@chromium.org>

Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
  

Hi Ricardo,

Thank you for the patch.

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 ?

> 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@chromium.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);
  

Hi Laurent

On Mon, 12 Feb 2024 at 23:59, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> Hi Ricardo,
>
> Thank you for the patch.
>
> 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.

Regards!




>
> > 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@chromium.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



--
Ricardo Ribalda
  

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@chromium.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);
  

Hi Laurent

On Mon, 19 Feb 2024 at 11:40, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
>
> 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.
Thanks

Will update with the following text after the review:

The tail of the list lives at the position before the head. This is
mathematically noted as
```
(head-1)  mod size.
```

Unfortunately C, does not have a modulus operator, but a remainder
operator (%).
The reminder operation has a different result than the modulus if
(head -1) is a negative number and size is not a power of two.

Adding size to (head-1) allows the code to run with any value of size.

>
> > > > 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@chromium.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