linux-kernel - Re: [PATCH v2] media: uvcvideo: Enforce alignment of frame and interval

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Message-ID: <20240610130910.GA12787@pendragon.ideasonboard.com>
Date: Mon, 10 Jun 2024 16:09:10 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Mauro Carvalho Chehab <mchehab@...nel.org>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] media: uvcvideo: Enforce alignment of frame and
 interval

Hi Ricardo,

Thank you for the patch.

On Thu, Apr 04, 2024 at 05:56:18PM +0000, Ricardo Ribalda wrote:
> Struct uvc_frame and interval (u32*) are packaged together on
> streaming->formats on a single contiguous allocation.
> 
> Right now they allocated right after uvc_format, without taking into

s/they/they are/

> consideration their required alignment.
> 
> This is working fine because both structures have a field with a
> pointer, but it will stop working when the sizeof() of any of those
> structs is not a multiple of the sizeof(void*).
> 
> Enforce that alignment during the allocation.
> 
> Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
> ---
> This is better than 3 allocations, and do not have any performance
> penalty.
> 
> I have tried this patch printing the size and the address of the
> pointers in the old and the new mode, and it looks the same.
> 
> [    2.235223] drivers/media/usb/uvc/uvc_driver.c:694 uvc_parse_streaming 432
> [    2.235249] drivers/media/usb/uvc/uvc_driver.c:704 uvc_parse_streaming 432
> [    2.235256] drivers/media/usb/uvc/uvc_driver.c:714 uvc_parse_streaming 00000000d32087cc 00000000d3803788
> [    2.235265] drivers/media/usb/uvc/uvc_driver.c:720 uvc_parse_streaming 00000000d32087cc 00000000d3803788
> ---
> Changes in v2: Thanks Laurent.
> - Enforce alignment during allocation instead of using __aligned()
>   macros.
> - Link to v1: https://lore.kernel.org/r/20230501-uvc-align-v1-1-0f713e4b84c3@chromium.org
> ---
>  drivers/media/usb/uvc/uvc_driver.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index 7aefa76a42b31..7d9844ba3b205 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -663,16 +663,26 @@ static int uvc_parse_streaming(struct uvc_device *dev,
>  		goto error;
>  	}
>  
> -	size = nformats * sizeof(*format) + nframes * sizeof(*frame)
> -	     + nintervals * sizeof(*interval);
> +	/*
> +	 * Allocate memory for the formats, the frames and the intervals,
> +	 * plus any required padding to guarantee that everything has the
> +	 * correct alignment.
> +	 */
> +	size = nformats * sizeof(*format);
> +	size = ALIGN(size, __alignof__(*frame)) + nframes * sizeof(*frame);
> +	size = ALIGN(size, __alignof__(*interval))
> +	       + nintervals * sizeof(*interval);

You have two extra spaces here. I'll fix when applying.

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

> +
>  	format = kzalloc(size, GFP_KERNEL);
> -	if (format == NULL) {
> +	if (!format) {
>  		ret = -ENOMEM;
>  		goto error;
>  	}
>  
> -	frame = (struct uvc_frame *)&format[nformats];
> -	interval = (u32 *)&frame[nframes];
> +	frame = (void *)format + nformats * sizeof(*format);
> +	frame = PTR_ALIGN(frame, __alignof__(*frame));
> +	interval = (void *)frame + nframes * sizeof(*frame);
> +	interval = PTR_ALIGN(interval, __alignof__(*interval));
>  
>  	streaming->format = format;
>  	streaming->nformats = nformats;
> 
> ---
> base-commit: 58390c8ce1bddb6c623f62e7ed36383e7fa5c02f
> change-id: 20230501-uvc-align-6ff202b68dab

-- 
Regards,

Laurent Pinchart