diff mbox series

[v4,2/3] overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping_mul()

Message ID 20240206103201.2013060-2-keescook@chromium.org
State New
Headers
Series overflow: Introduce wrapping helpers |

Commit Message

Kees Cook Feb. 6, 2024, 10:31 a.m. UTC 
  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.

Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Cc: Marco Elver <elver@google.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-hardening@vger.kernel.org
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
 lib/overflow_kunit.c     | 24 +++++++++++++++---
 2 files changed, 74 insertions(+), 4 deletions(-)

Comments

Gustavo A. R. Silva Feb. 6, 2024, 4:54 p.m. UTC | #1 
On 2/6/24 04:31, Kees Cook wrote:
> 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.
> 
> Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
> Cc: Marco Elver <elver@google.com>
> Cc: Eric Biggers <ebiggers@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-hardening@vger.kernel.org
> Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>   include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
>   lib/overflow_kunit.c     | 24 +++++++++++++++---
>   2 files changed, 74 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 4e741ebb8005..429c4d61a940 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -64,6 +64,24 @@ 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;					\
> +		if (__builtin_add_overflow(a, b, &__val)) {	\
> +			/* do nothing */			\
> +		}						\
> +		__val;						\

mmh... now that __builtin_*_overflow() is directly used, I guess
we don't need to _check_ for overflow anymore.

Thanks
--
Gustavo
Kees Cook Feb. 7, 2024, 9:35 a.m. UTC | #2 
On Tue, Feb 06, 2024 at 10:54:06AM -0600, Gustavo A. R. Silva wrote:
> 
> 
> On 2/6/24 04:31, Kees Cook wrote:
> > 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.
> > 
> > Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
> > Cc: Marco Elver <elver@google.com>
> > Cc: Eric Biggers <ebiggers@kernel.org>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: linux-hardening@vger.kernel.org
> > Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >   include/linux/overflow.h | 54 ++++++++++++++++++++++++++++++++++++++++
> >   lib/overflow_kunit.c     | 24 +++++++++++++++---
> >   2 files changed, 74 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> > index 4e741ebb8005..429c4d61a940 100644
> > --- a/include/linux/overflow.h
> > +++ b/include/linux/overflow.h
> > @@ -64,6 +64,24 @@ 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;					\
> > +		if (__builtin_add_overflow(a, b, &__val)) {	\
> > +			/* do nothing */			\
> > +		}						\
> > +		__val;						\
> 
> mmh... now that __builtin_*_overflow() is directly used, I guess
> we don't need to _check_ for overflow anymore.

/me slaps his forehead

Yes indeed! I will adjust it.
diff mbox series

Patch

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 4e741ebb8005..429c4d61a940 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -64,6 +64,24 @@  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;					\
+		if (__builtin_add_overflow(a, b, &__val)) {	\
+			/* do nothing */			\
+		}						\
+		__val;						\
+	})
+
 /**
  * check_sub_overflow() - Calculate subtraction with overflow checking
  * @a: minuend; value to subtract from
@@ -77,6 +95,24 @@  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))
 
+/**
+ * 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;					\
+		if (__builtin_sub_overflow(a, b, &__val)) {	\
+			/* do nothing */			\
+		}						\
+		__val;						\
+	})
+
 /**
  * check_mul_overflow() - Calculate multiplication with overflow checking
  * @a: first factor
@@ -90,6 +126,24 @@  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))
 
+/**
+ * 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;					\
+		if (__builtin_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 c527f6b75789..d3fdb906d3fe 100644
--- 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);	\