include/linux/overflow.h

/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */

#ifndef __LINUX_OVERFLOW_H
#define __LINUX_OVERFLOW_H

#define is_signed_type(type)	(((type)(-1)) < (type)1)
#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
#define type_min(T) ((T)((T)-type_max(T)-(T)1))

#ifndef unlikely
#define unlikely(x)	__builtin_expect(!!(x), 0)
#endif

#ifdef __GNUC__
#define GCC_VERSION (__GNUC__ * 10000           \
                     + __GNUC_MINOR__ * 100     \
                     + __GNUC_PATCHLEVEL__)
#if GCC_VERSION >= 50100
#define COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 1
#endif
#endif

#ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW

#define check_mul_overflow(a, b, d) ({		\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	__builtin_mul_overflow(__a, __b, __d);	\
})

#else

/*
 * If one of a or b is a compile-time constant, this avoids a division.
 */
#define __unsigned_mul_overflow(a, b, d) ({		\
	typeof(a) __a = (a);				\
	typeof(b) __b = (b);				\
	typeof(d) __d = (d);				\
	(void) (&__a == &__b);				\
	(void) (&__a == __d);				\
	*__d = __a * __b;				\
	__builtin_constant_p(__b) ?			\
	  __b > 0 && __a > type_max(typeof(__a)) / __b : \
	  __a > 0 && __b > type_max(typeof(__b)) / __a;	 \
})

/*
 * Signed multiplication is rather hard. gcc always follows C99, so
 * division is truncated towards 0. This means that we can write the
 * overflow check like this:
 *
 * (a > 0 && (b > MAX/a || b < MIN/a)) ||
 * (a < -1 && (b > MIN/a || b < MAX/a) ||
 * (a == -1 && b == MIN)
 *
 * The redundant casts of -1 are to silence an annoying -Wtype-limits
 * (included in -Wextra) warning: When the type is u8 or u16, the
 * __b_c_e in check_mul_overflow obviously selects
 * __unsigned_mul_overflow, but unfortunately gcc still parses this
 * code and warns about the limited range of __b.
 */

#define __signed_mul_overflow(a, b, d) ({				\
	typeof(a) __a = (a);						\
	typeof(b) __b = (b);						\
	typeof(d) __d = (d);						\
	typeof(a) __tmax = type_max(typeof(a));				\
	typeof(a) __tmin = type_min(typeof(a));				\
	(void) (&__a == &__b);						\
	(void) (&__a == __d);						\
	*__d = (__u64)__a * (__u64)__b;					\
	(__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||	\
	(__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
	(__b == (typeof(__b))-1 && __a == __tmin);			\
})

#define check_mul_overflow(a, b, d)					\
	__builtin_choose_expr(is_signed_type(typeof(a)),		\
			__signed_mul_overflow(a, b, d),			\
			__unsigned_mul_overflow(a, b, d))


#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */

#endif