linux-hardening - [PATCH 3/5] overflow: Introduce add_would

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Date: Mon, 29 Jan 2024 10:34:07 -0800
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: Kees Cook <keescook@...omium.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-hardening@...r.kernel.org,
	Nathan Chancellor <nathan@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Mark Rutland <mark.rutland@....com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Marco Elver <elver@...gle.com>,
	linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: [PATCH 3/5] overflow: Introduce add_would_overflow()

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@...vas.dk>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-hardening@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.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