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Tue, 21 Jan 2025 06:52:07 -0800 (PST) Received: from server.roeck-us.net ([2600:1700:e321:62f0:da43:aeff:fecc:bfd5]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-72dabac5a46sm9354747b3a.177.2025.01.21.06.52.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 21 Jan 2025 06:52:06 -0800 (PST) Sender: Guenter Roeck From: Guenter Roeck To: Jani Nikula Cc: Rodrigo Vivi , Joonas Lahtinen , Tvrtko Ursulin , David Airlie , Simona Vetter , intel-gfx@lists.freedesktop.org, intel-xe@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Guenter Roeck , Linus Torvalds , David Laight , Andy Shevchenko Subject: [PATCH v2] drm/i915/backlight: Return immediately when scale() finds invalid parameters Date: Tue, 21 Jan 2025 06:52:03 -0800 Message-ID: <20250121145203.2851237-1-linux@roeck-us.net> X-Mailer: git-send-email 2.45.2 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The scale() functions detects invalid parameters, but continues its calculations anyway. This causes bad results if negative values are used for unsigned operations. Worst case, a division by 0 error will be seen if source_min =3D=3D source_max. On top of that, after v6.13, the sequence of WARN_ON() followed by clamp() may result in a build error with gcc 13.x. drivers/gpu/drm/i915/display/intel_backlight.c: In function 'scale': include/linux/compiler_types.h:542:45: error: call to '__compiletime_assert_415' declared with attribute error: clamp() low limit source_min greater than high limit source_max This happens if the compiler decides to rearrange the code as follows. if (source_min > source_max) { WARN(..); /* Do the clamp() knowing that source_min > source_max */ source_val =3D clamp(source_val, source_min, source_max); } else { /* Do the clamp knowing that source_min <=3D source_max */ source_val =3D clamp(source_val, source_min, source_max); } Fix the problem by evaluating the return values from WARN_ON and returning immediately after a warning. While at it, fix divide by zero error seen if source_min =3D=3D source_max. Analyzed-by: Linus Torvalds Suggested-by: Linus Torvalds Suggested-by: David Laight Cc: David Laight Cc: Jani Nikula Cc: Andy Shevchenko Signed-off-by: Guenter Roeck Reviewed-by: Rodrigo Vivi --- v2: Simplify code to always return target_min after a warning, and also warn if source_min =3D=3D source_max. drivers/gpu/drm/i915/display/intel_backlight.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/d= rm/i915/display/intel_backlight.c index 3f81a726cc7d..ca588bed82b9 100644 --- a/drivers/gpu/drm/i915/display/intel_backlight.c +++ b/drivers/gpu/drm/i915/display/intel_backlight.c @@ -40,8 +40,9 @@ static u32 scale(u32 source_val, { u64 target_val; =20 - WARN_ON(source_min > source_max); - WARN_ON(target_min > target_max); + if (WARN_ON(source_min >=3D source_max) || + WARN_ON(target_min > target_max)) + return target_min; =20 /* defensive */ source_val =3D clamp(source_val, source_min, source_max); --=20 2.45.2