tools/include/linux/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  */
  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  */
 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
 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 */