overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping_mul()

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:

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

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

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index ad64d810c8aa817466e8b0830db9d746fbc44d17..d3ff8e2bec2972b68249197ffeabb4e7f0bae522 100644 (file)
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -65,6 +65,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
 #define check_add_overflow(a, b, d)    \
        __must_check_overflow(__builtin_add_overflow(a, b, d))
 
+/**
+ * wrapping_add() - 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 wrapping_add(type, a, b)                               \
+       ({                                                      \
+               type __val;                                     \
+               __builtin_add_overflow(a, b, &__val);           \
+               __val;                                          \
+       })
+
 /**
  * check_sub_overflow() - Calculate subtraction with overflow checking
  * @a: minuend; value to subtract from
@@ -79,6 +95,22 @@ 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))
 
+/**
+ * wrapping_sub() - 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 wrapping_sub(type, a, b)                               \
+       ({                                                      \
+               type __val;                                     \
+               __builtin_sub_overflow(a, b, &__val);           \
+               __val;                                          \
+       })
+
 /**
  * check_mul_overflow() - Calculate multiplication with overflow checking
  * @a: first factor
@@ -93,6 +125,22 @@ 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))
 
+/**
+ * wrapping_mul() - 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 wrapping_mul(type, a, b)                               \
+       ({                                                      \
+               type __val;                                     \
+               __builtin_mul_overflow(a, b, &__val);           \
+               __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 c527f6b75789462d7f57606eef241aeb3ef3718f..d3fdb906d3fef7e2022830a4d3d61ee422186fbe 100644 (file)
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -258,20 +258,36 @@ DEFINE_TEST_ARRAY(s64) = {
                                                                        \
        _of = check_ ## op ## _overflow(a, b, &_r);                     \
        KUNIT_EXPECT_EQ_MSG(test, _of, of,                              \
-               "expected "fmt" "sym" "fmt" to%s overflow (type %s)\n", \
+               "expected check "fmt" "sym" "fmt" to%s overflow (type %s)\n",   \
                a, b, of ? "" : " not", #t);                            \
        KUNIT_EXPECT_EQ_MSG(test, _r, r,                                \
-               "expected "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
+               "expected check "fmt" "sym" "fmt" == "fmt", got "fmt" (type %s)\n", \
                a, b, r, _r, #t);                                       \
        /* Check for internal macro side-effects. */                    \
        _of = check_ ## op ## _overflow(_a_orig++, _b_orig++, &_r);     \
-       KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump, "Unexpected " #op " macro side-effect!\n"); \
-       KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, "Unexpected " #op " macro side-effect!\n"); \
+       KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump,                     \
+               "Unexpected check " #op " macro side-effect!\n");       \
+       KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump,                     \
+               "Unexpected check " #op " macro side-effect!\n");       \
+                                                                       \
+       _r = wrapping_ ## op(t, a, b);                                  \
+       KUNIT_EXPECT_TRUE_MSG(test, _r == r,                            \
+               "expected wrap "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 = wrapping_ ## op(t, _a_orig++, _b_orig++);                  \
+       KUNIT_EXPECT_EQ_MSG(test, _a_orig, _a_bump,                     \
+               "Unexpected wrap " #op " macro side-effect!\n");        \
+       KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump,                     \
+               "Unexpected wrap " #op " macro side-effect!\n");        \
 } while (0)
 
 #define DEFINE_TEST_FUNC_TYPED(n, t, fmt)                              \
 static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
 {                                                                      \
+       /* check_{add,sub,mul}_overflow() and wrapping_{add,sub,mul} */ \
        check_one_op(t, fmt, add, "+", p->a, p->b, p->sum, p->s_of);    \
        check_one_op(t, fmt, add, "+", p->b, p->a, p->sum, p->s_of);    \
        check_one_op(t, fmt, sub, "-", p->a, p->b, p->diff, p->d_of);   \