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

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Date: Tue, 13 Feb 2024 00:59:40 +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,

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@...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