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Message-Id: <20240206103201.2013060-3-keescook@chromium.org>
Date: Tue, 6 Feb 2024 02:31:56 -0800
From: Kees Cook <keescook@...omium.org>
To: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Cc: Kees Cook <keescook@...omium.org>,
Marco Elver <elver@...gle.com>,
Eric Biggers <ebiggers@...nel.org>,
Mark Rutland <mark.rutland@....com>,
"Gustavo A. R. Silva" <gustavoars@...nel.org>,
linux-hardening@...r.kernel.org,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Subject: [PATCH v4 3/3] overflow: Introduce wrapping_inc() and wrapping_dec()
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@...vas.dk>
Cc: Marco Elver <elver@...gle.com>
Cc: Eric Biggers <ebiggers@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-hardening@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.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 429c4d61a940..8ab68350f976 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -82,6 +82,22 @@ 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
@@ -113,6 +129,22 @@ 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) { \
--
2.34.1
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