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 __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type))) 34 #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T))) 35 #define type_min(T) ((T)((T)-type_max(T)-(T)1)) 36 37 /* 38 * Avoids triggering -Wtype-limits compilation warning, 39 * while using unsigned data types to check a < 0. 40 */ 41 #define is_non_negative(a) ((a) > 0 || (a) == 0) 42 #define is_negative(a) (!(is_non_negative(a))) 43 44 /* 45 * Allows for effectively applying __must_check to a macro so we can have 46 * both the type-agnostic benefits of the macros while also being able to 47 * enforce that the return value is, in fact, checked. 48 */ 49 static inline bool __must_check __must_check_overflow(bool overflow) 50 { 51 return unlikely(overflow); 52 } 53 54 /** 55 * check_add_overflow() - Calculate addition with overflow checking 56 * @a: first addend 57 * @b: second addend 58 * @d: pointer to store sum 59 * 60 * Returns 0 on success. 61 * 62 * *@d holds the results of the attempted addition, but is not considered 63 * "safe for use" on a non-zero return value, which indicates that the 64 * sum has overflowed or been truncated. 65 */ 66 #define check_add_overflow(a, b, d) \ 67 __must_check_overflow(__builtin_add_overflow(a, b, d)) 68 69 /** 70 * check_sub_overflow() - Calculate subtraction with overflow checking 71 * @a: minuend; value to subtract from 72 * @b: subtrahend; value to subtract from @a 73 * @d: pointer to store difference 74 * 75 * Returns 0 on success. 76 * 77 * *@d holds the results of the attempted subtraction, but is not considered 78 * "safe for use" on a non-zero return value, which indicates that the 79 * difference has underflowed or been truncated. 80 */ 81 #define check_sub_overflow(a, b, d) \ 82 __must_check_overflow(__builtin_sub_overflow(a, b, d)) 83 84 /** 85 * check_mul_overflow() - Calculate multiplication with overflow checking 86 * @a: first factor 87 * @b: second factor 88 * @d: pointer to store product 89 * 90 * Returns 0 on success. 91 * 92 * *@d holds the results of the attempted multiplication, but is not 93 * considered "safe for use" on a non-zero return value, which indicates 94 * that the product has overflowed or been truncated. 95 */ 96 #define check_mul_overflow(a, b, d) \ 97 __must_check_overflow(__builtin_mul_overflow(a, b, d)) 98 99 /** 100 * check_shl_overflow() - Calculate a left-shifted value and check overflow 101 * @a: Value to be shifted 102 * @s: How many bits left to shift 103 * @d: Pointer to where to store the result 104 * 105 * Computes *@d = (@a << @s) 106 * 107 * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't 108 * make sense. Example conditions: 109 * 110 * - '@a << @s' causes bits to be lost when stored in *@d. 111 * - '@s' is garbage (e.g. negative) or so large that the result of 112 * '@a << @s' is guaranteed to be 0. 113 * - '@a' is negative. 114 * - '@a << @s' sets the sign bit, if any, in '*@d'. 115 * 116 * '*@d' will hold the results of the attempted shift, but is not 117 * considered "safe for use" if true is returned. 118 */ 119 #define check_shl_overflow(a, s, d) __must_check_overflow(({ \ 120 typeof(a) _a = a; \ 121 typeof(s) _s = s; \ 122 typeof(d) _d = d; \ 123 u64 _a_full = _a; \ 124 unsigned int _to_shift = \ 125 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ 126 *_d = (_a_full << _to_shift); \ 127 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ 128 (*_d >> _to_shift) != _a); \ 129 })) 130 131 #define __overflows_type_constexpr(x, T) ( \ 132 is_unsigned_type(typeof(x)) ? \ 133 (x) > type_max(typeof(T)) : \ 134 is_unsigned_type(typeof(T)) ? \ 135 (x) < 0 || (x) > type_max(typeof(T)) : \ 136 (x) < type_min(typeof(T)) || (x) > type_max(typeof(T))) 137 138 #define __overflows_type(x, T) ({ \ 139 typeof(T) v = 0; \ 140 check_add_overflow((x), v, &v); \ 141 }) 142 143 /** 144 * overflows_type - helper for checking the overflows between value, variables, 145 * or data type 146 * 147 * @n: source constant value or variable to be checked 148 * @T: destination variable or data type proposed to store @x 149 * 150 * Compares the @x expression for whether or not it can safely fit in 151 * the storage of the type in @T. @x and @T can have different types. 152 * If @x is a constant expression, this will also resolve to a constant 153 * expression. 154 * 155 * Returns: true if overflow can occur, false otherwise. 156 */ 157 #define overflows_type(n, T) \ 158 __builtin_choose_expr(__is_constexpr(n), \ 159 __overflows_type_constexpr(n, T), \ 160 __overflows_type(n, T)) 161 162 /** 163 * castable_to_type - like __same_type(), but also allows for casted literals 164 * 165 * @n: variable or constant value 166 * @T: variable or data type 167 * 168 * Unlike the __same_type() macro, this allows a constant value as the 169 * first argument. If this value would not overflow into an assignment 170 * of the second argument's type, it returns true. Otherwise, this falls 171 * back to __same_type(). 172 */ 173 #define castable_to_type(n, T) \ 174 __builtin_choose_expr(__is_constexpr(n), \ 175 !__overflows_type_constexpr(n, T), \ 176 __same_type(n, T)) 177 178 /** 179 * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX 180 * @factor1: first factor 181 * @factor2: second factor 182 * 183 * Returns: calculate @factor1 * @factor2, both promoted to size_t, 184 * with any overflow causing the return value to be SIZE_MAX. The 185 * lvalue must be size_t to avoid implicit type conversion. 186 */ 187 static inline size_t __must_check size_mul(size_t factor1, size_t factor2) 188 { 189 size_t bytes; 190 191 if (check_mul_overflow(factor1, factor2, &bytes)) 192 return SIZE_MAX; 193 194 return bytes; 195 } 196 197 /** 198 * size_add() - Calculate size_t addition with saturation at SIZE_MAX 199 * @addend1: first addend 200 * @addend2: second addend 201 * 202 * Returns: calculate @addend1 + @addend2, both promoted to size_t, 203 * with any overflow causing the return value to be SIZE_MAX. The 204 * lvalue must be size_t to avoid implicit type conversion. 205 */ 206 static inline size_t __must_check size_add(size_t addend1, size_t addend2) 207 { 208 size_t bytes; 209 210 if (check_add_overflow(addend1, addend2, &bytes)) 211 return SIZE_MAX; 212 213 return bytes; 214 } 215 216 /** 217 * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX 218 * @minuend: value to subtract from 219 * @subtrahend: value to subtract from @minuend 220 * 221 * Returns: calculate @minuend - @subtrahend, both promoted to size_t, 222 * with any overflow causing the return value to be SIZE_MAX. For 223 * composition with the size_add() and size_mul() helpers, neither 224 * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX). 225 * The lvalue must be size_t to avoid implicit type conversion. 226 */ 227 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) 228 { 229 size_t bytes; 230 231 if (minuend == SIZE_MAX || subtrahend == SIZE_MAX || 232 check_sub_overflow(minuend, subtrahend, &bytes)) 233 return SIZE_MAX; 234 235 return bytes; 236 } 237 238 /** 239 * array_size() - Calculate size of 2-dimensional array. 240 * @a: dimension one 241 * @b: dimension two 242 * 243 * Calculates size of 2-dimensional array: @a * @b. 244 * 245 * Returns: number of bytes needed to represent the array or SIZE_MAX on 246 * overflow. 247 */ 248 #define array_size(a, b) size_mul(a, b) 249 250 /** 251 * array3_size() - Calculate size of 3-dimensional array. 252 * @a: dimension one 253 * @b: dimension two 254 * @c: dimension three 255 * 256 * Calculates size of 3-dimensional array: @a * @b * @c. 257 * 258 * Returns: number of bytes needed to represent the array or SIZE_MAX on 259 * overflow. 260 */ 261 #define array3_size(a, b, c) size_mul(size_mul(a, b), c) 262 263 /** 264 * flex_array_size() - Calculate size of a flexible array member 265 * within an enclosing structure. 266 * @p: Pointer to the structure. 267 * @member: Name of the flexible array member. 268 * @count: Number of elements in the array. 269 * 270 * Calculates size of a flexible array of @count number of @member 271 * elements, at the end of structure @p. 272 * 273 * Return: number of bytes needed or SIZE_MAX on overflow. 274 */ 275 #define flex_array_size(p, member, count) \ 276 __builtin_choose_expr(__is_constexpr(count), \ 277 (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \ 278 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member))) 279 280 /** 281 * struct_size() - Calculate size of structure with trailing flexible array. 282 * @p: Pointer to the structure. 283 * @member: Name of the array member. 284 * @count: Number of elements in the array. 285 * 286 * Calculates size of memory needed for structure of @p followed by an 287 * array of @count number of @member elements. 288 * 289 * Return: number of bytes needed or SIZE_MAX on overflow. 290 */ 291 #define struct_size(p, member, count) \ 292 __builtin_choose_expr(__is_constexpr(count), \ 293 sizeof(*(p)) + flex_array_size(p, member, count), \ 294 size_add(sizeof(*(p)), flex_array_size(p, member, count))) 295 296 /** 297 * struct_size_t() - Calculate size of structure with trailing flexible array 298 * @type: structure type name. 299 * @member: Name of the array member. 300 * @count: Number of elements in the array. 301 * 302 * Calculates size of memory needed for structure @type followed by an 303 * array of @count number of @member elements. Prefer using struct_size() 304 * when possible instead, to keep calculations associated with a specific 305 * instance variable of type @type. 306 * 307 * Return: number of bytes needed or SIZE_MAX on overflow. 308 */ 309 #define struct_size_t(type, member, count) \ 310 struct_size((type *)NULL, member, count) 311 312 #endif /* __LINUX_OVERFLOW_H */ 313