linux-hardening - [PATCH 5/5] overflow: Introduce inc_wrap() and dec

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Message-Id: <20240129183411.3791340-5-keescook@chromium.org>
Date: Mon, 29 Jan 2024 10:34:09 -0800
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: Kees Cook <keescook@...omium.org>,
	Mark Rutland <mark.rutland@....com>,
	"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>,
	Miguel Ojeda <ojeda@...nel.org>,
	Marco Elver <elver@...gle.com>,
	linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev
Subject: [PATCH 5/5] overflow: Introduce inc_wrap() and dec_wrap()

This allows replacements of the idioms "var += offset" and "var -= offset"
with the inc_wrap() and dec_wrap() helpers respectively. They will avoid
wrap-around sanitizer instrumentation.

Cc: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: Mark Rutland <mark.rutland@....com>
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 | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 4f945e9e7881..080b18b84498 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -138,6 +138,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 		__sum;					\
 	})
 
+/**
+ * add_wrap() - Intentionally perform a wrapping increment
+ * @a: variable to be incremented
+ * @b: amount to add
+ *
+ * Increments @a by @b with wrap-around. Returns the resulting
+ * value of @a. Will not trip any wrap-around sanitizers.
+ */
+#define inc_wrap(var, offset)					\
+	({							\
+		if (check_add_overflow(var, offset, &var)) {	\
+			/* do nothing */			\
+		}						\
+		var;						\
+	})
+
 /**
  * check_sub_overflow() - Calculate subtraction with overflow checking
  * @a: minuend; value to subtract from
@@ -169,6 +185,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 		__val;					\
 	})
 
+/**
+ * dec_wrap() - Intentionally perform a wrapping decrement
+ * @a: variable to be decremented
+ * @b: amount to subtract
+ *
+ * Decrements @a by @b with wrap-around. Returns the resulting
+ * value of @a. Will not trip any wrap-around sanitizers.
+ */
+#define dec_wrap(var, offset)					\
+	({							\
+		if (check_sub_overflow(var, offset, &var)) {	\
+			/* do nothing */			\
+		}						\
+		var;						\
+	})
+
 /**
  * check_mul_overflow() - Calculate multiplication with overflow checking
  * @a: first factor
-- 
2.34.1