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 #include <linux/limits.h> 7 #include <linux/const.h> 8 9 /* 10 * We need to compute the minimum and maximum values representable in a given 11 * type. These macros may also be useful elsewhere. It would seem more obvious 12 * to do something like: 13 * 14 * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0) 15 * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0) 16 * 17 * Unfortunately, the middle expressions, strictly speaking, have 18 * undefined behaviour, and at least some versions of gcc warn about 19 * the type_max expression (but not if -fsanitize=undefined is in 20 * effect; in that case, the warning is deferred to runtime...). 21 * 22 * The slightly excessive casting in type_min is to make sure the 23 * macros also produce sensible values for the exotic type _Bool. [The 24 * overflow checkers only almost work for _Bool, but that's 25 * a-feature-not-a-bug, since people shouldn't be doing arithmetic on 26 * _Bools. Besides, the gcc builtins don't allow _Bool* as third 27 * argument.] 28 * 29 * Idea stolen from 30 * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html - 31 * credit to Christian Biere. 32 */ 33 #define is_signed_type(type) (((type)(-1)) < (type)1) 34 #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type))) 35 #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T))) 36 #define type_min(T) ((T)((T)-type_max(T)-(T)1)) 37 38 /* 39 * Avoids triggering -Wtype-limits compilation warning, 40 * while using unsigned data types to check a < 0. 41 */ 42 #define is_non_negative(a) ((a) > 0 || (a) == 0) 43 #define is_negative(a) (!(is_non_negative(a))) 44 45 /* 46 * Allows for effectively applying __must_check to a macro so we can have 47 * both the type-agnostic benefits of the macros while also being able to 48 * enforce that the return value is, in fact, checked. 49 */ 50 static inline bool __must_check __must_check_overflow(bool overflow) 51 { 52 return unlikely(overflow); 53 } 54 55 /* 56 * For simplicity and code hygiene, the fallback code below insists on 57 * a, b and *d having the same type (similar to the min() and max() 58 * macros), whereas gcc's type-generic overflow checkers accept 59 * different types. Hence we don't just make check_add_overflow an 60 * alias for __builtin_add_overflow, but add type checks similar to 61 * below. 62 */ 63 #define check_add_overflow(a, b, d) __must_check_overflow(({ \ 64 typeof(a) __a = (a); \ 65 typeof(b) __b = (b); \ 66 typeof(d) __d = (d); \ 67 (void) (&__a == &__b); \ 68 (void) (&__a == __d); \ 69 __builtin_add_overflow(__a, __b, __d); \ 70 })) 71 72 #define check_sub_overflow(a, b, d) __must_check_overflow(({ \ 73 typeof(a) __a = (a); \ 74 typeof(b) __b = (b); \ 75 typeof(d) __d = (d); \ 76 (void) (&__a == &__b); \ 77 (void) (&__a == __d); \ 78 __builtin_sub_overflow(__a, __b, __d); \ 79 })) 80 81 #define check_mul_overflow(a, b, d) __must_check_overflow(({ \ 82 typeof(a) __a = (a); \ 83 typeof(b) __b = (b); \ 84 typeof(d) __d = (d); \ 85 (void) (&__a == &__b); \ 86 (void) (&__a == __d); \ 87 __builtin_mul_overflow(__a, __b, __d); \ 88 })) 89 90 /** check_shl_overflow() - Calculate a left-shifted value and check overflow 91 * 92 * @a: Value to be shifted 93 * @s: How many bits left to shift 94 * @d: Pointer to where to store the result 95 * 96 * Computes *@d = (@a << @s) 97 * 98 * Returns true if '*d' cannot hold the result or when 'a << s' doesn't 99 * make sense. Example conditions: 100 * - 'a << s' causes bits to be lost when stored in *d. 101 * - 's' is garbage (e.g. negative) or so large that the result of 102 * 'a << s' is guaranteed to be 0. 103 * - 'a' is negative. 104 * - 'a << s' sets the sign bit, if any, in '*d'. 105 * 106 * '*d' will hold the results of the attempted shift, but is not 107 * considered "safe for use" if true is returned. 108 */ 109 #define check_shl_overflow(a, s, d) __must_check_overflow(({ \ 110 typeof(a) _a = a; \ 111 typeof(s) _s = s; \ 112 typeof(d) _d = d; \ 113 u64 _a_full = _a; \ 114 unsigned int _to_shift = \ 115 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ 116 *_d = (_a_full << _to_shift); \ 117 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ 118 (*_d >> _to_shift) != _a); \ 119 })) 120 121 /** 122 * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX 123 * 124 * @factor1: first factor 125 * @factor2: second factor 126 * 127 * Returns: calculate @factor1 * @factor2, both promoted to size_t, 128 * with any overflow causing the return value to be SIZE_MAX. The 129 * lvalue must be size_t to avoid implicit type conversion. 130 */ 131 static inline size_t __must_check size_mul(size_t factor1, size_t factor2) 132 { 133 size_t bytes; 134 135 if (check_mul_overflow(factor1, factor2, &bytes)) 136 return SIZE_MAX; 137 138 return bytes; 139 } 140 141 /** 142 * size_add() - Calculate size_t addition with saturation at SIZE_MAX 143 * 144 * @addend1: first addend 145 * @addend2: second addend 146 * 147 * Returns: calculate @addend1 + @addend2, both promoted to size_t, 148 * with any overflow causing the return value to be SIZE_MAX. The 149 * lvalue must be size_t to avoid implicit type conversion. 150 */ 151 static inline size_t __must_check size_add(size_t addend1, size_t addend2) 152 { 153 size_t bytes; 154 155 if (check_add_overflow(addend1, addend2, &bytes)) 156 return SIZE_MAX; 157 158 return bytes; 159 } 160 161 /** 162 * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX 163 * 164 * @minuend: value to subtract from 165 * @subtrahend: value to subtract from @minuend 166 * 167 * Returns: calculate @minuend - @subtrahend, both promoted to size_t, 168 * with any overflow causing the return value to be SIZE_MAX. For 169 * composition with the size_add() and size_mul() helpers, neither 170 * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX). 171 * The lvalue must be size_t to avoid implicit type conversion. 172 */ 173 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) 174 { 175 size_t bytes; 176 177 if (minuend == SIZE_MAX || subtrahend == SIZE_MAX || 178 check_sub_overflow(minuend, subtrahend, &bytes)) 179 return SIZE_MAX; 180 181 return bytes; 182 } 183 184 /** 185 * array_size() - Calculate size of 2-dimensional array. 186 * 187 * @a: dimension one 188 * @b: dimension two 189 * 190 * Calculates size of 2-dimensional array: @a * @b. 191 * 192 * Returns: number of bytes needed to represent the array or SIZE_MAX on 193 * overflow. 194 */ 195 #define array_size(a, b) size_mul(a, b) 196 197 /** 198 * array3_size() - Calculate size of 3-dimensional array. 199 * 200 * @a: dimension one 201 * @b: dimension two 202 * @c: dimension three 203 * 204 * Calculates size of 3-dimensional array: @a * @b * @c. 205 * 206 * Returns: number of bytes needed to represent the array or SIZE_MAX on 207 * overflow. 208 */ 209 #define array3_size(a, b, c) size_mul(size_mul(a, b), c) 210 211 /** 212 * flex_array_size() - Calculate size of a flexible array member 213 * within an enclosing structure. 214 * 215 * @p: Pointer to the structure. 216 * @member: Name of the flexible array member. 217 * @count: Number of elements in the array. 218 * 219 * Calculates size of a flexible array of @count number of @member 220 * elements, at the end of structure @p. 221 * 222 * Return: number of bytes needed or SIZE_MAX on overflow. 223 */ 224 #define flex_array_size(p, member, count) \ 225 __builtin_choose_expr(__is_constexpr(count), \ 226 (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \ 227 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member))) 228 229 /** 230 * struct_size() - Calculate size of structure with trailing flexible array. 231 * 232 * @p: Pointer to the structure. 233 * @member: Name of the array member. 234 * @count: Number of elements in the array. 235 * 236 * Calculates size of memory needed for structure @p followed by an 237 * array of @count number of @member elements. 238 * 239 * Return: number of bytes needed or SIZE_MAX on overflow. 240 */ 241 #define struct_size(p, member, count) \ 242 __builtin_choose_expr(__is_constexpr(count), \ 243 sizeof(*(p)) + flex_array_size(p, member, count), \ 244 size_add(sizeof(*(p)), flex_array_size(p, member, count))) 245 246 #endif /* __LINUX_OVERFLOW_H */ 247