[PATCH 5/5] overflow: Introduce inc_wrap() and dec_wrap()

[Kees Cook]

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@xxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
Cc: linux-hardening@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 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