From 4b21d25bf519c9487935a664886956bb18f04f6d Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Mon, 24 Oct 2022 23:11:25 +0300 Subject: overflow: Introduce overflows_type() and castable_to_type() Implement a robust overflows_type() macro to test if a variable or constant value would overflow another variable or type. This can be used as a constant expression for static_assert() (which requires a constant expression[1][2]) when used on constant values. This must be constructed manually, since __builtin_add_overflow() does not produce a constant expression[3]. Additionally adds castable_to_type(), similar to __same_type(), but for checking if a constant value would overflow if cast to a given type. Add unit tests for overflows_type(), __same_type(), and castable_to_type() to the existing KUnit "overflow" test: [16:03:33] ================== overflow (21 subtests) ================== ... [16:03:33] [PASSED] overflows_type_test [16:03:33] [PASSED] same_type_test [16:03:33] [PASSED] castable_to_type_test [16:03:33] ==================== [PASSED] overflow ===================== [16:03:33] ============================================================ [16:03:33] Testing complete. Ran 21 tests: passed: 21 [16:03:33] Elapsed time: 24.022s total, 0.002s configuring, 22.598s building, 0.767s running [1] https://en.cppreference.com/w/c/language/_Static_assert [2] C11 standard (ISO/IEC 9899:2011): 6.7.10 Static assertions [3] https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html 6.56 Built-in Functions to Perform Arithmetic with Overflow Checking Built-in Function: bool __builtin_add_overflow (type1 a, type2 b, Cc: Luc Van Oostenryck Cc: Nathan Chancellor Cc: Nick Desaulniers Cc: Tom Rix Cc: Daniel Latypov Cc: Vitor Massaru Iha Cc: "Gustavo A. R. Silva" Cc: Jani Nikula Cc: Mauro Carvalho Chehab Cc: linux-hardening@vger.kernel.org Cc: llvm@lists.linux.dev Co-developed-by: Gwan-gyeong Mun Signed-off-by: Gwan-gyeong Mun Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20221024201125.1416422-1-gwan-gyeong.mun@intel.com --- include/linux/overflow.h | 47 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 47 insertions(+) (limited to 'include/linux/overflow.h') diff --git a/include/linux/overflow.h b/include/linux/overflow.h index 1d3be1a2204c..0e33b5cbdb9f 100644 --- a/include/linux/overflow.h +++ b/include/linux/overflow.h @@ -128,6 +128,53 @@ static inline bool __must_check __must_check_overflow(bool overflow) (*_d >> _to_shift) != _a); \ })) +#define __overflows_type_constexpr(x, T) ( \ + is_unsigned_type(typeof(x)) ? \ + (x) > type_max(typeof(T)) : \ + is_unsigned_type(typeof(T)) ? \ + (x) < 0 || (x) > type_max(typeof(T)) : \ + (x) < type_min(typeof(T)) || (x) > type_max(typeof(T))) + +#define __overflows_type(x, T) ({ \ + typeof(T) v = 0; \ + check_add_overflow((x), v, &v); \ +}) + +/** + * overflows_type - helper for checking the overflows between value, variables, + * or data type + * + * @n: source constant value or variable to be checked + * @T: destination variable or data type proposed to store @x + * + * Compares the @x expression for whether or not it can safely fit in + * the storage of the type in @T. @x and @T can have different types. + * If @x is a constant expression, this will also resolve to a constant + * expression. + * + * Returns: true if overflow can occur, false otherwise. + */ +#define overflows_type(n, T) \ + __builtin_choose_expr(__is_constexpr(n), \ + __overflows_type_constexpr(n, T), \ + __overflows_type(n, T)) + +/** + * castable_to_type - like __same_type(), but also allows for casted literals + * + * @n: variable or constant value + * @T: variable or data type + * + * Unlike the __same_type() macro, this allows a constant value as the + * first argument. If this value would not overflow into an assignment + * of the second argument's type, it returns true. Otherwise, this falls + * back to __same_type(). + */ +#define castable_to_type(n, T) \ + __builtin_choose_expr(__is_constexpr(n), \ + !__overflows_type_constexpr(n, T), \ + __same_type(n, T)) + /** * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX * @factor1: first factor -- cgit