1 /* SPDX-License-Identifier: GPL-2.0 OR MIT */ 2 #ifndef _LINUXKPI_LINUX_OVERFLOW_H 3 #define _LINUXKPI_LINUX_OVERFLOW_H 4 5 #include <linux/compiler.h> 6 #include <linux/limits.h> 7 #ifdef __linux__ 8 #include <linux/const.h> 9 #endif 10 11 /* 12 * We need to compute the minimum and maximum values representable in a given 13 * type. These macros may also be useful elsewhere. It would seem more obvious 14 * to do something like: 15 * 16 * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0) 17 * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0) 18 * 19 * Unfortunately, the middle expressions, strictly speaking, have 20 * undefined behaviour, and at least some versions of gcc warn about 21 * the type_max expression (but not if -fsanitize=undefined is in 22 * effect; in that case, the warning is deferred to runtime...). 23 * 24 * The slightly excessive casting in type_min is to make sure the 25 * macros also produce sensible values for the exotic type _Bool. [The 26 * overflow checkers only almost work for _Bool, but that's 27 * a-feature-not-a-bug, since people shouldn't be doing arithmetic on 28 * _Bools. Besides, the gcc builtins don't allow _Bool* as third 29 * argument.] 30 * 31 * Idea stolen from 32 * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html - 33 * credit to Christian Biere. 34 */ 35 #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type))) 36 #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T))) 37 #define type_min(T) ((T)((T)-type_max(T)-(T)1)) 38 39 /* 40 * Avoids triggering -Wtype-limits compilation warning, 41 * while using unsigned data types to check a < 0. 42 */ 43 #define is_non_negative(a) ((a) > 0 || (a) == 0) 44 #define is_negative(a) (!(is_non_negative(a))) 45 46 /* 47 * Allows for effectively applying __must_check to a macro so we can have 48 * both the type-agnostic benefits of the macros while also being able to 49 * enforce that the return value is, in fact, checked. 50 */ 51 static inline bool __must_check __must_check_overflow(bool overflow) 52 { 53 return unlikely(overflow); 54 } 55 56 /** 57 * check_add_overflow() - Calculate addition with overflow checking 58 * @a: first addend 59 * @b: second addend 60 * @d: pointer to store sum 61 * 62 * Returns 0 on success. 63 * 64 * *@d holds the results of the attempted addition, but is not considered 65 * "safe for use" on a non-zero return value, which indicates that the 66 * sum has overflowed or been truncated. 67 */ 68 #define check_add_overflow(a, b, d) \ 69 __must_check_overflow(__builtin_add_overflow(a, b, d)) 70 71 /** 72 * check_sub_overflow() - Calculate subtraction with overflow checking 73 * @a: minuend; value to subtract from 74 * @b: subtrahend; value to subtract from @a 75 * @d: pointer to store difference 76 * 77 * Returns 0 on success. 78 * 79 * *@d holds the results of the attempted subtraction, but is not considered 80 * "safe for use" on a non-zero return value, which indicates that the 81 * difference has underflowed or been truncated. 82 */ 83 #define check_sub_overflow(a, b, d) \ 84 __must_check_overflow(__builtin_sub_overflow(a, b, d)) 85 86 /** 87 * check_mul_overflow() - Calculate multiplication with overflow checking 88 * @a: first factor 89 * @b: second factor 90 * @d: pointer to store product 91 * 92 * Returns 0 on success. 93 * 94 * *@d holds the results of the attempted multiplication, but is not 95 * considered "safe for use" on a non-zero return value, which indicates 96 * that the product has overflowed or been truncated. 97 */ 98 #define check_mul_overflow(a, b, d) \ 99 __must_check_overflow(__builtin_mul_overflow(a, b, d)) 100 101 /** 102 * check_shl_overflow() - Calculate a left-shifted value and check overflow 103 * @a: Value to be shifted 104 * @s: How many bits left to shift 105 * @d: Pointer to where to store the result 106 * 107 * Computes *@d = (@a << @s) 108 * 109 * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't 110 * make sense. Example conditions: 111 * 112 * - '@a << @s' causes bits to be lost when stored in *@d. 113 * - '@s' is garbage (e.g. negative) or so large that the result of 114 * '@a << @s' is guaranteed to be 0. 115 * - '@a' is negative. 116 * - '@a << @s' sets the sign bit, if any, in '*@d'. 117 * 118 * '*@d' will hold the results of the attempted shift, but is not 119 * considered "safe for use" if true is returned. 120 */ 121 #define check_shl_overflow(a, s, d) __must_check_overflow(({ \ 122 typeof(a) _a = a; \ 123 typeof(s) _s = s; \ 124 typeof(d) _d = d; \ 125 u64 _a_full = _a; \ 126 unsigned int _to_shift = \ 127 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ 128 *_d = (_a_full << _to_shift); \ 129 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ 130 (*_d >> _to_shift) != _a); \ 131 })) 132 133 #define __overflows_type_constexpr(x, T) ( \ 134 is_unsigned_type(typeof(x)) ? \ 135 (x) > type_max(typeof(T)) : \ 136 is_unsigned_type(typeof(T)) ? \ 137 (x) < 0 || (x) > type_max(typeof(T)) : \ 138 (x) < type_min(typeof(T)) || (x) > type_max(typeof(T))) 139 140 #define __overflows_type(x, T) ({ \ 141 typeof(T) v = 0; \ 142 check_add_overflow((x), v, &v); \ 143 }) 144 145 /** 146 * overflows_type - helper for checking the overflows between value, variables, 147 * or data type 148 * 149 * @n: source constant value or variable to be checked 150 * @T: destination variable or data type proposed to store @x 151 * 152 * Compares the @x expression for whether or not it can safely fit in 153 * the storage of the type in @T. @x and @T can have different types. 154 * If @x is a constant expression, this will also resolve to a constant 155 * expression. 156 * 157 * Returns: true if overflow can occur, false otherwise. 158 */ 159 #define overflows_type(n, T) \ 160 __builtin_choose_expr(__is_constexpr(n), \ 161 __overflows_type_constexpr(n, T), \ 162 __overflows_type(n, T)) 163 164 /** 165 * castable_to_type - like __same_type(), but also allows for casted literals 166 * 167 * @n: variable or constant value 168 * @T: variable or data type 169 * 170 * Unlike the __same_type() macro, this allows a constant value as the 171 * first argument. If this value would not overflow into an assignment 172 * of the second argument's type, it returns true. Otherwise, this falls 173 * back to __same_type(). 174 */ 175 #define castable_to_type(n, T) \ 176 __builtin_choose_expr(__is_constexpr(n), \ 177 !__overflows_type_constexpr(n, T), \ 178 __same_type(n, T)) 179 180 /** 181 * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX 182 * @factor1: first factor 183 * @factor2: second factor 184 * 185 * Returns: calculate @factor1 * @factor2, both promoted to size_t, 186 * with any overflow causing the return value to be SIZE_MAX. The 187 * lvalue must be size_t to avoid implicit type conversion. 188 */ 189 static inline size_t __must_check size_mul(size_t factor1, size_t factor2) 190 { 191 size_t bytes; 192 193 if (check_mul_overflow(factor1, factor2, &bytes)) 194 return SIZE_MAX; 195 196 return bytes; 197 } 198 199 /** 200 * size_add() - Calculate size_t addition with saturation at SIZE_MAX 201 * @addend1: first addend 202 * @addend2: second addend 203 * 204 * Returns: calculate @addend1 + @addend2, both promoted to size_t, 205 * with any overflow causing the return value to be SIZE_MAX. The 206 * lvalue must be size_t to avoid implicit type conversion. 207 */ 208 static inline size_t __must_check size_add(size_t addend1, size_t addend2) 209 { 210 size_t bytes; 211 212 if (check_add_overflow(addend1, addend2, &bytes)) 213 return SIZE_MAX; 214 215 return bytes; 216 } 217 218 /** 219 * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX 220 * @minuend: value to subtract from 221 * @subtrahend: value to subtract from @minuend 222 * 223 * Returns: calculate @minuend - @subtrahend, both promoted to size_t, 224 * with any overflow causing the return value to be SIZE_MAX. For 225 * composition with the size_add() and size_mul() helpers, neither 226 * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX). 227 * The lvalue must be size_t to avoid implicit type conversion. 228 */ 229 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) 230 { 231 size_t bytes; 232 233 if (minuend == SIZE_MAX || subtrahend == SIZE_MAX || 234 check_sub_overflow(minuend, subtrahend, &bytes)) 235 return SIZE_MAX; 236 237 return bytes; 238 } 239 240 /** 241 * array_size() - Calculate size of 2-dimensional array. 242 * @a: dimension one 243 * @b: dimension two 244 * 245 * Calculates size of 2-dimensional array: @a * @b. 246 * 247 * Returns: number of bytes needed to represent the array or SIZE_MAX on 248 * overflow. 249 */ 250 #define array_size(a, b) size_mul(a, b) 251 252 /** 253 * array3_size() - Calculate size of 3-dimensional array. 254 * @a: dimension one 255 * @b: dimension two 256 * @c: dimension three 257 * 258 * Calculates size of 3-dimensional array: @a * @b * @c. 259 * 260 * Returns: number of bytes needed to represent the array or SIZE_MAX on 261 * overflow. 262 */ 263 #define array3_size(a, b, c) size_mul(size_mul(a, b), c) 264 265 /** 266 * flex_array_size() - Calculate size of a flexible array member 267 * within an enclosing structure. 268 * @p: Pointer to the structure. 269 * @member: Name of the flexible array member. 270 * @count: Number of elements in the array. 271 * 272 * Calculates size of a flexible array of @count number of @member 273 * elements, at the end of structure @p. 274 * 275 * Return: number of bytes needed or SIZE_MAX on overflow. 276 */ 277 #define flex_array_size(p, member, count) \ 278 __builtin_choose_expr(__is_constexpr(count), \ 279 (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \ 280 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member))) 281 282 /** 283 * struct_size() - Calculate size of structure with trailing flexible array. 284 * @p: Pointer to the structure. 285 * @member: Name of the array member. 286 * @count: Number of elements in the array. 287 * 288 * Calculates size of memory needed for structure of @p followed by an 289 * array of @count number of @member elements. 290 * 291 * Return: number of bytes needed or SIZE_MAX on overflow. 292 */ 293 #define struct_size(p, member, count) \ 294 __builtin_choose_expr(__is_constexpr(count), \ 295 sizeof(*(p)) + flex_array_size(p, member, count), \ 296 size_add(sizeof(*(p)), flex_array_size(p, member, count))) 297 298 /** 299 * struct_size_t() - Calculate size of structure with trailing flexible array 300 * @type: structure type name. 301 * @member: Name of the array member. 302 * @count: Number of elements in the array. 303 * 304 * Calculates size of memory needed for structure @type followed by an 305 * array of @count number of @member elements. Prefer using struct_size() 306 * when possible instead, to keep calculations associated with a specific 307 * instance variable of type @type. 308 * 309 * Return: number of bytes needed or SIZE_MAX on overflow. 310 */ 311 #define struct_size_t(type, member, count) \ 312 struct_size((type *)NULL, member, count) 313 314 /** 315 * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family. 316 * Enables caller macro to pass (different) initializer. 317 * 318 * @type: structure type name, including "struct" keyword. 319 * @name: Name for a variable to define. 320 * @member: Name of the array member. 321 * @count: Number of elements in the array; must be compile-time const. 322 * @initializer: initializer expression (could be empty for no init). 323 */ 324 #define _DEFINE_FLEX(type, name, member, count, initializer) \ 325 _Static_assert(__builtin_constant_p(count), \ 326 "onstack flex array members require compile-time const count"); \ 327 union { \ 328 u8 bytes[struct_size_t(type, member, count)]; \ 329 type obj; \ 330 } name##_u initializer; \ 331 type *name = (type *)&name##_u 332 333 /** 334 * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing 335 * flexible array member. 336 * 337 * @type: structure type name, including "struct" keyword. 338 * @name: Name for a variable to define. 339 * @member: Name of the array member. 340 * @count: Number of elements in the array; must be compile-time const. 341 * 342 * Define a zeroed, on-stack, instance of @type structure with a trailing 343 * flexible array member. 344 * Use __struct_size(@name) to get compile-time size of it afterwards. 345 */ 346 #define DEFINE_FLEX(type, name, member, count) \ 347 _DEFINE_FLEX(type, name, member, count, = {}) 348 349 #endif /* _LINUXKPI_LINUX_OVERFLOW_H */ 350