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Message-ID: <1a98bb80-9990-dc65-5f14-066315a7e7d8@suse.de>
Date: Thu, 31 Aug 2023 09:40:19 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
Javier Martinez Canillas <javierm@...hat.com>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
David Airlie <airlied@...il.com>,
Daniel Vetter <daniel@...ll.ch>
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/8] drm/dumb-buffers: Fix drm_mode_create_dumb() for
bpp < 8
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@...ux-m68k.org>
> Reviewed-by: Javier Martinez Canillas <javierm@...hat.com>
> Tested-by: Javier Martinez Canillas <javierm@...hat.com>
> ---
> 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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