diff mbox series

[v5,3/3] overflow: Introduce wrapping_inc() and wrapping_dec()

Message ID 20240207152422.3855315-3-keescook@chromium.org (mailing list archive)
State Superseded
Commit a0e452100f6c16cbcdac369628735f578d31ebf3
Headers show
Series overflow: Introduce wrapping helpers | expand

Commit Message

Kees Cook Feb. 7, 2024, 3:24 p.m. UTC
This allows replacements of the idioms "var += offset" and "var -= offset"
with the wrapping_inc() and wrapping_dec() helpers respectively. They
will avoid wrap-around sanitizer instrumentation.

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

Comments

Andy Shevchenko Feb. 7, 2024, 3:31 p.m. UTC | #1
On Wed, Feb 7, 2024 at 5:24 PM Kees Cook <keescook@chromium.org> wrote:
>
> This allows replacements of the idioms "var += offset" and "var -= offset"
> with the wrapping_inc() and wrapping_dec() helpers respectively. They
> will avoid wrap-around sanitizer instrumentation.
>
> Add to the selftests to validate behavior and lack of side-effects.

I would expect inc/dec to add/subtract 1. So, to me the terminology is
confusing. The list of the C/C++ operators on Wikipedia describes
above as

+= Assignment by sum
-= Assignment by difference


Hence the proposal is to have

wrapping_sum()
wrapping_diff()
Kees Cook Feb. 7, 2024, 4:08 p.m. UTC | #2
On Wed, Feb 07, 2024 at 05:31:54PM +0200, Andy Shevchenko wrote:
> On Wed, Feb 7, 2024 at 5:24 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > This allows replacements of the idioms "var += offset" and "var -= offset"
> > with the wrapping_inc() and wrapping_dec() helpers respectively. They
> > will avoid wrap-around sanitizer instrumentation.
> >
> > Add to the selftests to validate behavior and lack of side-effects.
> 
> I would expect inc/dec to add/subtract 1. So, to me the terminology is
> confusing. The list of the C/C++ operators on Wikipedia describes
> above as
> 
> += Assignment by sum
> -= Assignment by difference
> 
> 
> Hence the proposal is to have
> 
> wrapping_sum()
> wrapping_diff()

I think maybe the missing phrase is "assign", which is what makes
argument 1 "special" -- it's being changed. Perhaps:

wrapping_add_assign()
wrapping_sub_assign()

?
Andy Shevchenko Feb. 7, 2024, 4:34 p.m. UTC | #3
On Wed, Feb 07, 2024 at 08:08:14AM -0800, Kees Cook wrote:
> On Wed, Feb 07, 2024 at 05:31:54PM +0200, Andy Shevchenko wrote:
> > On Wed, Feb 7, 2024 at 5:24 PM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > This allows replacements of the idioms "var += offset" and "var -= offset"
> > > with the wrapping_inc() and wrapping_dec() helpers respectively. They
> > > will avoid wrap-around sanitizer instrumentation.
> > >
> > > Add to the selftests to validate behavior and lack of side-effects.
> > 
> > I would expect inc/dec to add/subtract 1. So, to me the terminology is
> > confusing. The list of the C/C++ operators on Wikipedia describes
> > above as
> > 
> > += Assignment by sum
> > -= Assignment by difference
> > 
> > Hence the proposal is to have
> > 
> > wrapping_sum()
> > wrapping_diff()
> 
> I think maybe the missing phrase is "assign", which is what makes
> argument 1 "special" -- it's being changed. Perhaps:
> 
> wrapping_add_assign()
> wrapping_sub_assign()
> 
> ?

WFM!
diff mbox series

Patch

diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 849a49fb496e..f4b18007fd2b 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -80,6 +80,22 @@  static inline bool __must_check __must_check_overflow(bool overflow)
 		__val;						\
 	})
 
+/**
+ * wrapping_inc() - Intentionally perform a wrapping increment
+ * @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_inc(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
@@ -109,6 +125,22 @@  static inline bool __must_check __must_check_overflow(bool overflow)
 		__val;						\
 	})
 
+/**
+ * wrapping_dec() - Intentionally perform a wrapping decrement
+ * @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_dec(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..6cadce8f8a47 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_{inc,dec}() */					\
+	check_self_op(fmt, inc, +=, p->a, p->b);			\
+	check_self_op(fmt, inc, +=, p->b, p->a);			\
+	check_self_op(fmt, dec, -=, p->a, p->b);			\
 }									\
 									\
 static void n ## _overflow_test(struct kunit *test) {			\