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

Kees Cook
2024-01-29T18:34:14Z

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:

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

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 3c46c648d2e8..4f945e9e7881 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 to check overflow against
+ * @a: first addend
+ * @b: second addend
+ *
+ * Return the potentially wrapped-around addition without
+ * tripping any overflow sanitizers that may be enabled.
+ */
+#define add_wrap(type, a, b) \
+ ({ \
+ type __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
@@ -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 to check underflow against
+ * @a: minuend; value to subtract from
+ * @b: subtrahend; value to subtract from @a
+ *
+ * Return the potentially wrapped-around subtraction without
+ * tripping any overflow 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 to check underflow against
+ * @a: first factor
+ * @b: second factor
+ *
+ * Return the potentially wrapped-around multiplication without
+ * tripping any overflow 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
--
2.34.1