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  */
array_size(size_t a,size_t b)82 static 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  */
array3_size(size_t a,size_t b,size_t c)104 static 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 
__ab_c_size(size_t n,size_t size,size_t c)116 static 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 */
145