overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

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

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

Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-hardening@vger.kernel.org

include/linux/overflow.h

@@ -81,6 +81,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 		__val;						\
 	})
 
+/**
+ * wrapping_assign_add() - Intentionally perform a wrapping increment assignment
+ * @var: variable to be incremented
+ * @offset: amount to add
+ *
+ * Increments @var by @offset with wrap-around. Returns the resulting
+ * value of @var. Will not trip any wrap-around sanitizers.
+ *
+ * Returns the new value of @var.
+ */
+#define wrapping_assign_add(var, offset)				\
+	({								\
+		typeof(var) *__ptr = &(var);				\
+		*__ptr = wrapping_add(typeof(var), *__ptr, offset);	\
+	})
+
 /**
  * check_sub_overflow() - Calculate subtraction with overflow checking
  * @a: minuend; value to subtract from
@@ -111,6 +127,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 		__val;						\
 	})
 
+/**
+ * wrapping_assign_sub() - Intentionally perform a wrapping decrement assign
+ * @var: variable to be decremented
+ * @offset: amount to subtract
+ *
+ * Decrements @var by @offset with wrap-around. Returns the resulting
+ * value of @var. Will not trip any wrap-around sanitizers.
+ *
+ * Returns the new value of @var.
+ */
+#define wrapping_assign_sub(var, offset)				\
+	({								\
+		typeof(var) *__ptr = &(var);				\
+		*__ptr = wrapping_sub(typeof(var), *__ptr, offset);	\
+	})
+
 /**
  * check_mul_overflow() - Calculate multiplication with overflow checking
  * @a: first factor

lib/overflow_kunit.c

@@ -284,6 +284,45 @@ DEFINE_TEST_ARRAY(s64) = {
 		"Unexpected wrap " #op " macro side-effect!\n");	\
 } while (0)
 
+static int global_counter;
+static void bump_counter(void)
+{
+	global_counter++;
+}
+
+static int get_index(void)
+{
+	volatile int index = 0;
+	bump_counter();
+	return index;
+}
+
+#define check_self_op(fmt, op, sym, a, b) do {				\
+	typeof(a + 0) _a = a;						\
+	typeof(b + 0) _b = b;						\
+	typeof(a + 0) _a_sym = a;					\
+	typeof(a + 0) _a_orig[1] = { a };				\
+	typeof(b + 0) _b_orig = b;					\
+	typeof(b + 0) _b_bump = b + 1;					\
+	typeof(a + 0) _r;						\
+									\
+	_a_sym sym _b;							\
+	_r = wrapping_ ## op(_a, _b);					\
+	KUNIT_EXPECT_TRUE_MSG(test, _r == _a_sym,			\
+		"expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n",	\
+		a, b, _a_sym, _r);					\
+	KUNIT_EXPECT_TRUE_MSG(test, _a == _a_sym,			\
+		"expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n",	\
+		a, b, _a_sym, _a);					\
+	/* Check for internal macro side-effects. */			\
+	global_counter = 0;						\
+	wrapping_ ## op(_a_orig[get_index()], _b_orig++);		\
+	KUNIT_EXPECT_EQ_MSG(test, global_counter, 1,			\
+		"Unexpected wrapping_" #op " macro side-effect on arg1!\n"); \
+	KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump,			\
+		"Unexpected wrapping_" #op " macro side-effect on arg2!\n"); \
+} while (0)
+
 #define DEFINE_TEST_FUNC_TYPED(n, t, fmt)				\
 static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
 {									\
@@ -293,6 +332,10 @@ static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
 	check_one_op(t, fmt, sub, "-", p->a, p->b, p->diff, p->d_of);	\
 	check_one_op(t, fmt, mul, "*", p->a, p->b, p->prod, p->p_of);	\
 	check_one_op(t, fmt, mul, "*", p->b, p->a, p->prod, p->p_of);	\
+	/* wrapping_assign_{add,sub}() */				\
+	check_self_op(fmt, assign_add, +=, p->a, p->b);			\
+	check_self_op(fmt, assign_add, +=, p->b, p->a);			\
+	check_self_op(fmt, assign_sub, -=, p->a, p->b);			\
 }									\
 									\
 static void n ## _overflow_test(struct kunit *test) {			\