[PATCH v7 0/3] overflow: Introduce wrapping helpers

[PATCH v7 0/3] overflow: Introduce wrapping helpers

[Kees Cook]

v7: - further kern-doc cleanups (Mark)
v6: https://lore.kernel.org/all/20240213220844.it.345-kees@kernel.org/
v5: https://lore.kernel.org/all/20240207152317.do.560-kees@kernel.org/
v4: https://lore.kernel.org/all/20240206102354.make.081-kees@kernel.org/
v3: https://lore.kernel.org/all/20240205090854.make.507-kees@kernel.org/
v2: https://lore.kernel.org/all/20240130220218.it.154-kees@kernel.org/
v1: https://lore.kernel.org/all/20240129182845.work.694-kees@kernel.org/

Hi,

In preparation for gaining instrumentation for signed[1], unsigned[2], and
pointer[3] wrap-around, expand the overflow header to include wrap-around
helpers that can be used to annotate arithmetic where wrapped calculations
are expected (e.g. atomics).

After spending time getting the unsigned integer wrap-around sanitizer
running warning-free on a basic x86_64 boot[4], I think the add/sub/mul
helpers first argument being the output type makes the most sense (as
suggested by Rasmus).

-Kees

Link: https://github.com/KSPP/linux/issues/26 [1]
Link: https://github.com/KSPP/linux/issues/27 [2]
Link: https://github.com/KSPP/linux/issues/344 [3]
Link: https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=devel/overflow/enable-unsigned-sanitizer [4]


Kees Cook (3):
  overflow: Adjust check_*_overflow() kern-doc to reflect results
  overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping_mul()
  overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

 include/linux/overflow.h | 101 ++++++++++++++++++++++++++++++++++-----
 lib/overflow_kunit.c     |  67 ++++++++++++++++++++++++--
 2 files changed, 152 insertions(+), 16 deletions(-)

-- 
2.34.1

[PATCH v7 1/3] overflow: Adjust check_*_overflow() kern-doc to reflect results

[Kees Cook]

The check_*_overflow() helpers will return results with potentially
wrapped-around values. These values have always been checked by the
selftests, so avoid the confusing language in the kern-doc. The idea of
"safe for use" was relative to the expectation of whether or not the
caller wants a wrapped value -- the calculation itself will always follow
arithmetic wrapping rules.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: linux-hardening@vger.kernel.org
---
 include/linux/overflow.h | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 7b5cf4a5cd19..ad64d810c8aa 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -57,11 +57,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
  * @b: second addend
  * @d: pointer to store sum
  *
- * Returns 0 on success.
+ * Returns true on wrap-around, false otherwise.
  *
- * *@d holds the results of the attempted addition, but is not considered
- * "safe for use" on a non-zero return value, which indicates that the
- * sum has overflowed or been truncated.
+ * *@d holds the results of the attempted addition, regardless of whether
+ * wrap-around occurred.
  */
 #define check_add_overflow(a, b, d)	\
 	__must_check_overflow(__builtin_add_overflow(a, b, d))
@@ -72,11 +71,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
  * @b: subtrahend; value to subtract from @a
  * @d: pointer to store difference
  *
- * Returns 0 on success.
+ * Returns true on wrap-around, false otherwise.
  *
- * *@d holds the results of the attempted subtraction, but is not considered
- * "safe for use" on a non-zero return value, which indicates that the
- * difference has underflowed or been truncated.
+ * *@d holds the results of the attempted subtraction, regardless of whether
+ * wrap-around occurred.
  */
 #define check_sub_overflow(a, b, d)	\
 	__must_check_overflow(__builtin_sub_overflow(a, b, d))
@@ -87,11 +85,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
  * @b: second factor
  * @d: pointer to store product
  *
- * Returns 0 on success.
+ * Returns true on wrap-around, false otherwise.
  *
- * *@d holds the results of the attempted multiplication, but is not
- * considered "safe for use" on a non-zero return value, which indicates
- * that the product has overflowed or been truncated.
+ * *@d holds the results of the attempted multiplication, regardless of whether
+ * wrap-around occurred.
  */
 #define check_mul_overflow(a, b, d)	\
 	__must_check_overflow(__builtin_mul_overflow(a, b, d))
-- 
2.34.1

Re: [PATCH v7 1/3] overflow: Adjust check_*_overflow() kern-doc to reflect results

[Mark Rutland]

On Wed, Feb 14, 2024 at 11:46:01AM -0800, Kees Cook wrote:
> The check_*_overflow() helpers will return results with potentially
> wrapped-around values. These values have always been checked by the
> selftests, so avoid the confusing language in the kern-doc. The idea of
> "safe for use" was relative to the expectation of whether or not the
> caller wants a wrapped value -- the calculation itself will always follow
> arithmetic wrapping rules.
> 
> Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: linux-hardening@vger.kernel.org

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  include/linux/overflow.h | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 7b5cf4a5cd19..ad64d810c8aa 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -57,11 +57,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>   * @b: second addend
>   * @d: pointer to store sum
>   *
> - * Returns 0 on success.
> + * Returns true on wrap-around, false otherwise.
>   *
> - * *@d holds the results of the attempted addition, but is not considered
> - * "safe for use" on a non-zero return value, which indicates that the
> - * sum has overflowed or been truncated.
> + * *@d holds the results of the attempted addition, regardless of whether
> + * wrap-around occurred.
>   */
>  #define check_add_overflow(a, b, d)	\
>  	__must_check_overflow(__builtin_add_overflow(a, b, d))
> @@ -72,11 +71,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>   * @b: subtrahend; value to subtract from @a
>   * @d: pointer to store difference
>   *
> - * Returns 0 on success.
> + * Returns true on wrap-around, false otherwise.
>   *
> - * *@d holds the results of the attempted subtraction, but is not considered
> - * "safe for use" on a non-zero return value, which indicates that the
> - * difference has underflowed or been truncated.
> + * *@d holds the results of the attempted subtraction, regardless of whether
> + * wrap-around occurred.
>   */
>  #define check_sub_overflow(a, b, d)	\
>  	__must_check_overflow(__builtin_sub_overflow(a, b, d))
> @@ -87,11 +85,10 @@ static inline bool __must_check __must_check_overflow(bool overflow)
>   * @b: second factor
>   * @d: pointer to store product
>   *
> - * Returns 0 on success.
> + * Returns true on wrap-around, false otherwise.
>   *
> - * *@d holds the results of the attempted multiplication, but is not
> - * considered "safe for use" on a non-zero return value, which indicates
> - * that the product has overflowed or been truncated.
> + * *@d holds the results of the attempted multiplication, regardless of whether
> + * wrap-around occurred.
>   */
>  #define check_mul_overflow(a, b, d)	\
>  	__must_check_overflow(__builtin_mul_overflow(a, b, d))
> -- 
> 2.34.1
> 

[PATCH v7 2/3] overflow: Introduce wrapping_add(), wrapping_sub(), and wrapping_mul()

[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:

	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>
---
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
---
 include/linux/overflow.h | 48 ++++++++++++++++++++++++++++++++++++++++
 lib/overflow_kunit.c     | 24 ++++++++++++++++----
 2 files changed, 68 insertions(+), 4 deletions(-)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index ad64d810c8aa..d3ff8e2bec29 100644
--- 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 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);	\
-- 
2.34.1

[PATCH v7 3/3] overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

[Kees Cook]

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.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
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: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-hardening@vger.kernel.org
---
 include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++
 lib/overflow_kunit.c     | 43 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index d3ff8e2bec29..dede374832c9 100644
--- a/include/linux/overflow.h
+++ b/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
diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index d3fdb906d3fe..65e8a72a83bf 100644
--- a/lib/overflow_kunit.c
+++ b/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) {			\
-- 
2.34.1

Re: [PATCH v7 3/3] overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

[Marco Elver]

On Wed, 14 Feb 2024 at 20:46, Kees Cook <keescook@chromium.org> wrote:
>
> 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.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Marco Elver <elver@google.com>

> ---
> 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: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: linux-hardening@vger.kernel.org
> ---
>  include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++
>  lib/overflow_kunit.c     | 43 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 75 insertions(+)
>
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index d3ff8e2bec29..dede374832c9 100644
> --- a/include/linux/overflow.h
> +++ b/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
> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index d3fdb906d3fe..65e8a72a83bf 100644
> --- a/lib/overflow_kunit.c
> +++ b/lib/overflow_kunit.c
[...]
> +       /* 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);                 \
>  }                                                                      \

Merely a curiosity, and am not suggesting this for this patch: I
wonder how much of this could be tested at compile-time. These are
very primitive operations, so I suspect the compiler could either
check these in a static_assert(), or if some of it isn't
constexpr-friendly, after optimizations with a BUILD_BUG.

Re: [PATCH v7 3/3] overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

[Mark Rutland]

On Wed, Feb 14, 2024 at 11:46:03AM -0800, Kees Cook wrote:
> 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.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> 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: "Gustavo A. R. Silva" <gustavoars@kernel.org>
> Cc: linux-hardening@vger.kernel.org

Acked-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
>  include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++
>  lib/overflow_kunit.c     | 43 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 75 insertions(+)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index d3ff8e2bec29..dede374832c9 100644
> --- a/include/linux/overflow.h
> +++ b/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
> diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
> index d3fdb906d3fe..65e8a72a83bf 100644
> --- a/lib/overflow_kunit.c
> +++ b/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) {			\
> -- 
> 2.34.1
>