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overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()
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. Reviewed-by: Marco Elver <elver@google.com> Acked-by: Mark Rutland <mark.rutland@arm.com> Signed-off-by: Kees Cook <keescook@chromium.org>
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@ -81,6 +81,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
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__val; \
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})
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/**
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* wrapping_assign_add() - Intentionally perform a wrapping increment assignment
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* @var: variable to be incremented
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* @offset: amount to add
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*
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* Increments @var by @offset with wrap-around. Returns the resulting
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* value of @var. Will not trip any wrap-around sanitizers.
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*
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* Returns the new value of @var.
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*/
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#define wrapping_assign_add(var, offset) \
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({ \
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typeof(var) *__ptr = &(var); \
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*__ptr = wrapping_add(typeof(var), *__ptr, offset); \
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})
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/**
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* check_sub_overflow() - Calculate subtraction with overflow checking
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* @a: minuend; value to subtract from
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@ -111,6 +127,22 @@ static inline bool __must_check __must_check_overflow(bool overflow)
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__val; \
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})
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/**
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* wrapping_assign_sub() - Intentionally perform a wrapping decrement assign
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* @var: variable to be decremented
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* @offset: amount to subtract
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*
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* Decrements @var by @offset with wrap-around. Returns the resulting
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* value of @var. Will not trip any wrap-around sanitizers.
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*
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* Returns the new value of @var.
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*/
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#define wrapping_assign_sub(var, offset) \
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({ \
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typeof(var) *__ptr = &(var); \
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*__ptr = wrapping_sub(typeof(var), *__ptr, offset); \
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})
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/**
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* check_mul_overflow() - Calculate multiplication with overflow checking
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* @a: first factor
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@ -284,6 +284,45 @@ DEFINE_TEST_ARRAY(s64) = {
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"Unexpected wrap " #op " macro side-effect!\n"); \
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} while (0)
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static int global_counter;
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static void bump_counter(void)
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{
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global_counter++;
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}
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static int get_index(void)
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{
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volatile int index = 0;
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bump_counter();
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return index;
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}
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#define check_self_op(fmt, op, sym, a, b) do { \
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typeof(a + 0) _a = a; \
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typeof(b + 0) _b = b; \
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typeof(a + 0) _a_sym = a; \
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typeof(a + 0) _a_orig[1] = { a }; \
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typeof(b + 0) _b_orig = b; \
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typeof(b + 0) _b_bump = b + 1; \
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typeof(a + 0) _r; \
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\
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_a_sym sym _b; \
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_r = wrapping_ ## op(_a, _b); \
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KUNIT_EXPECT_TRUE_MSG(test, _r == _a_sym, \
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"expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n", \
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a, b, _a_sym, _r); \
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KUNIT_EXPECT_TRUE_MSG(test, _a == _a_sym, \
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"expected "fmt" "#op" "fmt" == "fmt", got "fmt"\n", \
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a, b, _a_sym, _a); \
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/* Check for internal macro side-effects. */ \
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global_counter = 0; \
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wrapping_ ## op(_a_orig[get_index()], _b_orig++); \
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KUNIT_EXPECT_EQ_MSG(test, global_counter, 1, \
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"Unexpected wrapping_" #op " macro side-effect on arg1!\n"); \
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KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \
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"Unexpected wrapping_" #op " macro side-effect on arg2!\n"); \
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} while (0)
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#define DEFINE_TEST_FUNC_TYPED(n, t, fmt) \
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static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
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{ \
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@ -293,6 +332,10 @@ static void do_test_ ## n(struct kunit *test, const struct test_ ## n *p) \
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check_one_op(t, fmt, sub, "-", p->a, p->b, p->diff, p->d_of); \
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check_one_op(t, fmt, mul, "*", p->a, p->b, p->prod, p->p_of); \
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check_one_op(t, fmt, mul, "*", p->b, p->a, p->prod, p->p_of); \
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/* wrapping_assign_{add,sub}() */ \
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check_self_op(fmt, assign_add, +=, p->a, p->b); \
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check_self_op(fmt, assign_add, +=, p->b, p->a); \
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check_self_op(fmt, assign_sub, -=, p->a, p->b); \
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} \
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\
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static void n ## _overflow_test(struct kunit *test) { \
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