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From: Kees Cook <keescook@chromium.org>
To: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Kees Cook <keescook@chromium.org>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Justin Stitt <justinstitt@google.com>,
	linux-hardening@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Bill Wendling <morbo@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Miguel Ojeda <ojeda@kernel.org>,
	Marco Elver <elver@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Przemek Kitszel <przemyslaw.kitszel@intel.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev
Subject: [PATCH v2 3/5] overflow: Introduce add_would_overflow()
Date: Tue, 30 Jan 2024 14:06:08 -0800
Message-Id: <20240130220614.1154497-3-keescook@chromium.org>
In-Reply-To: <20240130220218.it.154-kees@kernel.org>
References: <20240130220218.it.154-kees@kernel.org>
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For instances where only the overflow needs to be checked (and the sum
isn't used), provide the new helper add_would_overflow(), which is
a wrapper for check_add_overflow().

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Justin Stitt <justinstitt@google.com>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/overflow.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 210e5602e89b..3c46c648d2e8 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -104,6 +104,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 		__builtin_add_overflow(__filter_integral(a), b,		\
 				       __filter_ptrint(d))))
 
+/**
+ * add_would_overflow() - Check if an addition would overflow
+ * @var: variable to add to that is checked for overflow
+ * @offset: value to add
+ *
+ * Returns true if the sum would overflow.
+ *
+ * To keep a copy of the sum when the addition doesn't overflow, use
+ * check_add_overflow() instead.
+ */
+#define add_would_overflow(var, offset)				\
+	__must_check_overflow(({				\
+		typeof(var) __result;				\
+		check_add_overflow(var, offset, &__result);	\
+	}))
+
 /**
  * check_sub_overflow() - Calculate subtraction with overflow checking
  * @a: minuend; value to subtract from
-- 
2.34.1