This allows replacements of the idioms "var += offset" and "var -= offset" with the inc_wrap() and dec_wrap() 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.villem...@prevas.dk> Cc: Mark Rutland <mark.rutl...@arm.com> Cc: "Gustavo A. R. Silva" <gustavo...@kernel.org> Cc: linux-hardening@vger.kernel.org Signed-off-by: Kees Cook <keesc...@chromium.org> --- include/linux/overflow.h | 28 +++++++++++++++++++++++++++ lib/overflow_kunit.c | 42 ++++++++++++++++++++++++++++++++++++++++ 2 files changed, 70 insertions(+) diff --git a/include/linux/overflow.h b/include/linux/overflow.h index 9b8c05bdb788..6a4c18544ab1 100644 --- a/include/linux/overflow.h +++ b/include/linux/overflow.h @@ -82,6 +82,20 @@ static inline bool __must_check __must_check_overflow(bool overflow) __val; \ }) +/** + * inc_wrap() - 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. + */ +#define inc_wrap(var, offset) \ + ({ \ + typeof(var) *__ptr = &(var); \ + *__ptr = add_wrap(typeof(var), *__ptr, offset); \ + }) + /** * check_sub_overflow() - Calculate subtraction with overflow checking * @a: minuend; value to subtract from @@ -113,6 +127,20 @@ static inline bool __must_check __must_check_overflow(bool overflow) __val; \ }) +/** + * dec_wrap() - 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. + */ +#define dec_wrap(var, offset) \ + ({ \ + typeof(var) *__ptr = &(var); \ + *__ptr = sub_wrap(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 064dccd973ad..23e37bce0022 100644 --- a/lib/overflow_kunit.c +++ b/lib/overflow_kunit.c @@ -284,14 +284,56 @@ 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 = op ## _wrap(_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; \ + op ## _wrap(_a_orig[get_index()], _b_orig++); \ + KUNIT_EXPECT_EQ_MSG(test, global_counter, 1, \ + "Unexpected " #op "_wrap() macro side-effect on arg1!\n"); \ + KUNIT_EXPECT_EQ_MSG(test, _b_orig, _b_bump, \ + "Unexpected " #op "_wrap() 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) \ { \ + /* check_{add,sub,mul}_overflow() and {add,sub,mul}_wrap() */ \ 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); \ 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); \ + /* {inc,dec}_wrap() */ \ + 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