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[34.168.148.209]) by smtp.gmail.com with UTF8SMTPSA id v7-20020a170902b7c700b001d989dd19b0sm1988210plz.140.2024.02.09.15.07.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Feb 2024 15:07:38 -0800 (PST) From: Abhishek Pandit-Subedi To: linux-kernel@vger.kernel.org Cc: David.Laight@ACULAB.COM, pmalani@chromium.org, andriy.shevchenko@linux.intel.com, keescook@chromium.org, torvalds@linux-foundation.org, Abhishek Pandit-Subedi , Andrew Morton , Herve Codina , "Matthew Wilcox (Oracle)" Subject: [PATCH] minmax: Add notes to min_t and max_t Date: Fri, 9 Feb 2024 15:07:02 -0800 Message-ID: <20240209150657.1.I45addf7579e1233fa97c05ba72120cd1c57b4310@changeid> X-Mailer: git-send-email 2.43.0.687.g38aa6559b0-goog 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" Both min_t and max_t are problematic as they can hide issues when comparing differently sized types (and especially differently signed types). Update the comments to nudge users to other options until there is a better fix for these macros. Link: https://lore.kernel.org/all/01e3e09005e9434b8f558a893a47c053@AcuMS.ac= ulab.com/ Link: https://lore.kernel.org/all/CAHk-=3DwhwEAc22wm8h9FESPB5X+P4bLDgv0erBQ= Ma1buTNQW7tA@mail.gmail.com/ Signed-off-by: Abhishek Pandit-Subedi --- Andy Shevchenko made me aware of this particular footgun in https://lore.kernel.org/linux-usb/ZcZ_he1jYx8w57mK@smile.fi.intel.com/. While David + others work on the full fix, I'm hoping to apply a bandaid in the form of comments so the problem doesn't get worse by devs (**cough** me **cough**) inadvertently doing the wrong thing. include/linux/minmax.h | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/include/linux/minmax.h b/include/linux/minmax.h index 2ec559284a9f..96646f840a1f 100644 --- a/include/linux/minmax.h +++ b/include/linux/minmax.h @@ -154,6 +154,18 @@ =20 /** * min_t - return minimum of two values, using the specified type + * + * Note: Downcasting types in this macro can cause incorrect results. Pref= er to + * use min() which does typechecking. + * + * Prefer to use clamp if you are trying to compare to size_t. + * + * Don't: + * min_t(size_t, buf_size, sizeof(foobar)) + * + * Do: + * clamp(buf_size, 0, sizeof(foobar)) + * * @type: data type to use * @x: first value * @y: second value @@ -162,6 +174,10 @@ =20 /** * max_t - return maximum of two values, using the specified type + * + * Note: Downcasting types in this macro can cause incorrect results. Pref= er to + * use max() which does typechecking. + * * @type: data type to use * @x: first value * @y: second value --=20 2.43.0.687.g38aa6559b0-goog