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From: Kees Cook <keescook@chromium.org>
To: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	linux-kernel@vger.kernel.org
Subject: [PATCH 03/82] overflow: Introduce add_wrap()
Date: Mon, 22 Jan 2024 16:26:38 -0800
Message-Id: <20240123002814.1396804-3-keescook@chromium.org>
In-Reply-To: <20240122235208.work.748-kees@kernel.org>
References: <20240122235208.work.748-kees@kernel.org>
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Provide a helper that will perform wrapping addition without tripping
the arithmetic wrap-around sanitizers.

Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
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 ac088f73e0fd..30779905a77a 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -124,6 +124,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 		check_add_overflow(a, b, &__result);\
 	}))
 
+/**
+ * add_wrap() - Intentionally perform a wrapping addition
+ * @a: first addend
+ * @b: second addend
+ *
+ * Return the potentially wrapped-around addition without
+ * tripping any overflow sanitizers that may be enabled.
+ */
+#define add_wrap(a, b)					\
+	({						\
+		typeof(a) __sum;			\
+		if (check_add_overflow(a, b, &__sum))	\
+			/* do nothing */;		\
+		__sum;					\
+	})
+
 /**
  * check_sub_overflow() - Calculate subtraction with overflow checking
  * @a: minuend; value to subtract from
-- 
2.34.1