overflow.h

  1/* SPDX-License-Identifier: GPL-2.0 OR MIT */
  2#ifndef __LINUX_OVERFLOW_H
  3#define __LINUX_OVERFLOW_H
  4
  5#include <linux/compiler.h>
  6
  7/*
  8 * We need to compute the minimum and maximum values representable in a given
  9 * type. These macros may also be useful elsewhere. It would seem more obvious
 10 * to do something like:
 11 *
 12 * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
 13 * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
 14 *
 15 * Unfortunately, the middle expressions, strictly speaking, have
 16 * undefined behaviour, and at least some versions of gcc warn about
 17 * the type_max expression (but not if -fsanitize=undefined is in
 18 * effect; in that case, the warning is deferred to runtime...).
 19 *
 20 * The slightly excessive casting in type_min is to make sure the
 21 * macros also produce sensible values for the exotic type _Bool. [The
 22 * overflow checkers only almost work for _Bool, but that's
 23 * a-feature-not-a-bug, since people shouldn't be doing arithmetic on
 24 * _Bools. Besides, the gcc builtins don't allow _Bool* as third
 25 * argument.]
 26 *
 27 * Idea stolen from
 28 * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
 29 * credit to Christian Biere.
 30 */
 31#define is_signed_type(type)       (((type)(-1)) < (type)1)
 32#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
 33#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
 34#define type_min(T) ((T)((T)-type_max(T)-(T)1))
 35
 36/*
 37 * For simplicity and code hygiene, the fallback code below insists on
 38 * a, b and *d having the same type (similar to the min() and max()
 39 * macros), whereas gcc's type-generic overflow checkers accept
 40 * different types. Hence we don't just make check_add_overflow an
 41 * alias for __builtin_add_overflow, but add type checks similar to
 42 * below.
 43 */
 44#define check_add_overflow(a, b, d) ({		\
 45	typeof(a) __a = (a);			\
 46	typeof(b) __b = (b);			\
 47	typeof(d) __d = (d);			\
 48	(void) (&__a == &__b);			\
 49	(void) (&__a == __d);			\
 50	__builtin_add_overflow(__a, __b, __d);	\
 51})
 52
 53#define check_sub_overflow(a, b, d) ({		\
 54	typeof(a) __a = (a);			\
 55	typeof(b) __b = (b);			\
 56	typeof(d) __d = (d);			\
 57	(void) (&__a == &__b);			\
 58	(void) (&__a == __d);			\
 59	__builtin_sub_overflow(__a, __b, __d);	\
 60})
 61
 62#define check_mul_overflow(a, b, d) ({		\
 63	typeof(a) __a = (a);			\
 64	typeof(b) __b = (b);			\
 65	typeof(d) __d = (d);			\
 66	(void) (&__a == &__b);			\
 67	(void) (&__a == __d);			\
 68	__builtin_mul_overflow(__a, __b, __d);	\
 69})
 70
 71/**
 72 * array_size() - Calculate size of 2-dimensional array.
 73 *
 74 * @a: dimension one
 75 * @b: dimension two
 76 *
 77 * Calculates size of 2-dimensional array: @a * @b.
 78 *
 79 * Returns: number of bytes needed to represent the array or SIZE_MAX on
 80 * overflow.
 81 */
 82static inline __must_check size_t array_size(size_t a, size_t b)
 83{
 84	size_t bytes;
 85
 86	if (check_mul_overflow(a, b, &bytes))
 87		return SIZE_MAX;
 88
 89	return bytes;
 90}
 91
 92/**
 93 * array3_size() - Calculate size of 3-dimensional array.
 94 *
 95 * @a: dimension one
 96 * @b: dimension two
 97 * @c: dimension three
 98 *
 99 * Calculates size of 3-dimensional array: @a * @b * @c.
100 *
101 * Returns: number of bytes needed to represent the array or SIZE_MAX on
102 * overflow.
103 */
104static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
105{
106	size_t bytes;
107
108	if (check_mul_overflow(a, b, &bytes))
109		return SIZE_MAX;
110	if (check_mul_overflow(bytes, c, &bytes))
111		return SIZE_MAX;
112
113	return bytes;
114}
115
116static inline __must_check size_t __ab_c_size(size_t n, size_t size, size_t c)
117{
118	size_t bytes;
119
120	if (check_mul_overflow(n, size, &bytes))
121		return SIZE_MAX;
122	if (check_add_overflow(bytes, c, &bytes))
123		return SIZE_MAX;
124
125	return bytes;
126}
127
128/**
129 * struct_size() - Calculate size of structure with trailing array.
130 * @p: Pointer to the structure.
131 * @member: Name of the array member.
132 * @n: Number of elements in the array.
133 *
134 * Calculates size of memory needed for structure @p followed by an
135 * array of @n @member elements.
136 *
137 * Return: number of bytes needed or SIZE_MAX on overflow.
138 */
139#define struct_size(p, member, n)					\
140	__ab_c_size(n,							\
141		    sizeof(*(p)->member) + __must_be_array((p)->member),\
142		    sizeof(*(p)))
143
144#endif /* __LINUX_OVERFLOW_H */