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overflow: Introduce wrapping_assign_add() and wrapping_assign_sub()

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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>
This commit is contained in:
Kees Cook 2024-01-26 22:09:50 -08:00
parent d70de8054c
commit 08d45ee84b
2 changed files with 75 additions and 0 deletions
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32
include/linux/overflow.h
View File

@ -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

43
lib/overflow_kunit.c
View File

@ -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) { \