[PATCH AUTOSEL 6.17] fbdev: core: Fix ubsan warning in pixel_to_pat

[Sasha Levin <sashal@kernel.org>]

From: Zsolt Kajtar <soci@c64.rulez.org>

[ Upstream commit aad1d99beaaf132e2024a52727c24894cdf9474a ]

It could be triggered on 32 bit big endian machines at 32 bpp in the
pattern realignment. In this case just return early as the result is
an identity.

Signed-off-by: Zsolt Kajtar <soci@c64.rulez.org>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

YES
- Returning early in the default branch
  (`drivers/video/fbdev/core/fb_fillrect.h:94`) keeps `pixel_to_pat()`
  from reaching the big-endian realignment block at
  `drivers/video/fbdev/core/fb_fillrect.h:97-99`, eliminating the
  undefined `pattern >> bpp` shift that UBSAN reports when `bpp ==
  BITS_PER_LONG` (e.g., 32 bpp on 32-bit BE). The skipped logic was an
  identity operation in that case, so correctness is preserved.
- The old undefined shift was triggered during ordinary rectangle fills
  (`drivers/video/fbdev/core/fb_fillrect.h:266`) on big-endian
  framebuffers, causing sanitizer aborts and risking miscompilation even
  without UBSAN, so this is a real bug fix with direct user impact.
- The change is tightly scoped, has no dependencies, and leaves little-
  endian paths and sub-word `bpp` handling untouched, keeping regression
  risk very low while restoring defined behavior.

Next steps: 1. If feasible, rebuild a BE configuration with UBSAN to
confirm the warning is gone.

 drivers/video/fbdev/core/fb_fillrect.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/core/fb_fillrect.h b/drivers/video/fbdev/core/fb_fillrect.h
index 66042e534de77..f366670a53af8 100644
--- a/drivers/video/fbdev/core/fb_fillrect.h
+++ b/drivers/video/fbdev/core/fb_fillrect.h
@@ -92,8 +92,7 @@ static unsigned long pixel_to_pat(int bpp, u32 color)
 		pattern = pattern | pattern << bpp;
 		break;
 	default:
-		pattern = color;
-		break;
+		return color;
 	}
 #ifndef __LITTLE_ENDIAN
 	pattern <<= (BITS_PER_LONG % bpp);
-- 
2.51.0