linux-kernel - Re: [PATCH 2/8] drm/dumb-buffers: Fix drm_mode_create

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Message-ID: <87o7keu8j0.fsf@minerva.mail-host-address-is-not-set>
Date:   Fri, 14 Jul 2023 11:50:11 +0200
From:   Javier Martinez Canillas <javierm@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH 2/8] drm/dumb-buffers: Fix drm_mode_create_dumb() for
 bpp < 8

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

> 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>
> ---
> Why is drm_mode_create_dumb.size __u64?  The test for "args->height >

I don't think can be changed since is a DRM_IOCTL_MODE_CREATE_DUMB uAPI ?

> U32_MAX / stride" rejects all sizes not fitting in __u32 anyway.

Hmm, wonder if should be U64_MAX instead ?

> ---
>  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;
> -	stride = cpp * args->width;
> +	stride = DIV_ROUND_UP(args->bpp * args->width, 8);
>  	if (args->height > U32_MAX / stride)
>  		return -EINVAL;
>  

Good catch.

Reviewed-by: Javier Martinez Canillas <javierm@...hat.com>

-- 
Best regards,

Javier Martinez Canillas
Core Platforms
Red Hat