[PATCH v2 4/5] overflow: Introduce add_wrap(), sub_wrap(), and mul_wrap()

[Kees Cook]

Provide helpers that will perform wrapping addition, subtraction, or
multiplication without tripping the arithmetic wrap-around sanitizers. The
first argument is the type under which the wrap-around should happen
with. In other words, these two calls will get very different results:

	mul_wrap(int, 50, 50) == 2500
	mul_wrap(u8,  50, 50) ==  196

Add to the selftests to validate behavior and lack of side-effects.

Cc: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Justin Stitt <justinstitt@xxxxxxxxxx>
Cc: linux-hardening@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
 lib/overflow_kunit.c     | 11 ++++++++
 2 files changed, 65 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 3c46c648d2e8..c9139f88578b 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -120,6 +120,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 		check_add_overflow(var, offset, &__result);	\
 	}))
 
+/**
+ * add_wrap() - Intentionally perform a wrapping addition
+ * @type: type for result of calculation
+ * @a: first addend
+ * @b: second addend
+ *
+ * Return the potentially wrapped-around addition without
+ * tripping any wrap-around sanitizers that may be enabled.
+ */
+#define add_wrap(type, a, b)				\
+	({						\
+		type __val;				\
+		if (check_add_overflow(a, b, &__val)) {	\
+			/* do nothing */		\
+		}					\
+		__val;					\
+	})
+
 /**
  * check_sub_overflow() - Calculate subtraction with overflow checking
  * @a: minuend; value to subtract from
@@ -133,6 +151,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 #define check_sub_overflow(a, b, d)	\
 	__must_check_overflow(__builtin_sub_overflow(a, b, d))
 
+/**
+ * sub_wrap() - Intentionally perform a wrapping subtraction
+ * @type: type for result of calculation
+ * @a: minuend; value to subtract from
+ * @b: subtrahend; value to subtract from @a
+ *
+ * Return the potentially wrapped-around subtraction without
+ * tripping any wrap-around sanitizers that may be enabled.
+ */
+#define sub_wrap(type, a, b)				\
+	({						\
+		type __val;				\
+		if (check_sub_overflow(a, b, &__val)) {	\
+			/* do nothing */		\
+		}					\
+		__val;					\
+	})
+
 /**
  * check_mul_overflow() - Calculate multiplication with overflow checking
  * @a: first factor
@@ -146,6 +182,24 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 #define check_mul_overflow(a, b, d)	\
 	__must_check_overflow(__builtin_mul_overflow(a, b, d))
 
+/**
+ * mul_wrap() - Intentionally perform a wrapping multiplication
+ * @type: type for result of calculation
+ * @a: first factor
+ * @b: second factor
+ *
+ * Return the potentially wrapped-around multiplication without
+ * tripping any wrap-around sanitizers that may be enabled.
+ */
+#define mul_wrap(type, a, b)				\
+	({						\
+		type __val;				\
+		if (check_mul_overflow(a, b, &__val)) {	\
+			/* do nothing */		\
+		}					\
+		__val;					\
+	})
+
 /**
  * check_shl_overflow() - Calculate a left-shifted value and check overflow
  * @a: Value to be shifted
diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index 2d106e880956..319f950872bd 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -273,6 +273,17 @@ DEFINE_TEST_ARRAY(s64) = {
 	/* Check for internal macro side-effects. */			\
 	_of = check_ ## op ## _overflow(_a_orig++, _b_orig++, &_r);	\
 	KUNIT_EXPECT_TRUE_MSG(test, _a_orig == _a_bump, "Unexpected " #op " macro side-effect!\n"); \
+	KUNIT_EXPECT_TRUE_MSG(test, _b_orig == _b_bump, "Unexpected " #op " macro side-effect!\n"); \
+									\
+	_r = op ## _wrap(t, a, b);					\
+	KUNIT_EXPECT_TRUE_MSG(test, _r == r,				\
+		"expected "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
+		a, b, r, _r, #t);					\
+	/* Check for internal macro side-effects. */			\
+	_a_orig = a;							\
+	_b_orig = b;							\
+	_r = op ## _wrap(t, _a_orig++, _b_orig++);			\
+	KUNIT_EXPECT_TRUE_MSG(test, _a_orig == _a_bump, "Unexpected " #op " macro side-effect!\n"); \
 	KUNIT_EXPECT_TRUE_MSG(test, _b_orig == _b_bump, "Unexpected " #op " macro side-effect!\n"); \
 } while (0)
 
-- 
2.34.1