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

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Date:   Thu, 13 Jul 2023 15:17:10 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Javier Martinez Canillas <javierm@...hat.com>,
        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: [PATCH 2/8] drm/dumb-buffers: Fix drm_mode_create_dumb() for bpp < 8

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 >
U32_MAX / stride" rejects all sizes not fitting in __u32 anyway.
---
 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;
 
-- 
2.34.1