Re: [PATCH v2 1/8] drm/dumb-buffers: Fix drm_mode_create_dumb() for bpp < 8

[Thomas Zimmermann]

Hi

Am 24.08.23 um 17:08 schrieb Geert Uytterhoeven:
> drm_mode_create_dumb() calculates the number of characters per pixel
> from the number of bits per pixel by rounding up, which is not correct
> as the actual value of cpp may be non-integer.  While we do not need to
> care here about complex formats like YUV, bpp < 8 is a valid use case.
>
>    - The overflow check for the buffer width is not correct if bpp < 8.
>      However, it doesn't hurt, as widths larger than U32_MAX / 8 should
>      not happen for real anyway.  Add a comment to clarify.
>    - Calculating the stride from the number of characters per pixel is
>      not correct.  Fix this by calculating it from the number of bits per
>      pixel instead.
>
> Signed-off-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Reviewed-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
> Tested-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
> ---
> v2:
>    - Add Reviewed-by, Tested-by.
> ---
>   drivers/gpu/drm/drm_dumb_buffers.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_dumb_buffers.c b/drivers/gpu/drm/drm_dumb_buffers.c
> index 70032bba1c97e787..21a04c32a5e3d785 100644
> --- a/drivers/gpu/drm/drm_dumb_buffers.c
> +++ b/drivers/gpu/drm/drm_dumb_buffers.c
> @@ -71,10 +71,11 @@ int drm_mode_create_dumb(struct drm_device *dev,
>   	/* overflow checks for 32bit size calculations */
>   	if (args->bpp > U32_MAX - 8)
>   		return -EINVAL;
> +	/* Incorrect (especially if bpp < 8), but doesn't hurt much */
>   	cpp = DIV_ROUND_UP(args->bpp, 8);
>   	if (cpp > U32_MAX / args->width)
>   		return -EINVAL;

At first, I was confused by this. So I'd really prefer to outright 
delete this code. As you say, it's incorrect and doesn't add anything. 
The concept of cpp is somewhat wrong anyway.

> -	stride = cpp * args->width;
> +	stride = DIV_ROUND_UP(args->bpp * args->width, 8);

Do we need an overflow check for (bpp * width < U32_MAX) ?

Best regards
Thomas

>   	if (args->height > U32_MAX / stride)
>   		return -EINVAL;
>   

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)

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