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_max(t) __type_max(typeof(t)) 36 #define __type_min(T) ((T)((T)-type_max(T)-(T)1)) 37 #define type_min(t) __type_min(typeof(t)) 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 true on wrap-around, false otherwise. 63 * 64 * *@d holds the results of the attempted addition, regardless of whether 65 * wrap-around occurred. 66 */ 67 #define check_add_overflow(a, b, d) \ 68 __must_check_overflow(__builtin_add_overflow(a, b, d)) 69 70 /** 71 * wrapping_add() - Intentionally perform a wrapping addition 72 * @type: type for result of calculation 73 * @a: first addend 74 * @b: second addend 75 * 76 * Return the potentially wrapped-around addition without 77 * tripping any wrap-around sanitizers that may be enabled. 78 */ 79 #define wrapping_add(type, a, b) \ 80 ({ \ 81 type __val; \ 82 __builtin_add_overflow(a, b, &__val); \ 83 __val; \ 84 }) 85 86 /** 87 * wrapping_assign_add() - Intentionally perform a wrapping increment assignment 88 * @var: variable to be incremented 89 * @offset: amount to add 90 * 91 * Increments @var by @offset with wrap-around. Returns the resulting 92 * value of @var. Will not trip any wrap-around sanitizers. 93 * 94 * Returns the new value of @var. 95 */ 96 #define wrapping_assign_add(var, offset) \ 97 ({ \ 98 typeof(var) *__ptr = &(var); \ 99 *__ptr = wrapping_add(typeof(var), *__ptr, offset); \ 100 }) 101 102 /** 103 * check_sub_overflow() - Calculate subtraction with overflow checking 104 * @a: minuend; value to subtract from 105 * @b: subtrahend; value to subtract from @a 106 * @d: pointer to store difference 107 * 108 * Returns true on wrap-around, false otherwise. 109 * 110 * *@d holds the results of the attempted subtraction, regardless of whether 111 * wrap-around occurred. 112 */ 113 #define check_sub_overflow(a, b, d) \ 114 __must_check_overflow(__builtin_sub_overflow(a, b, d)) 115 116 /** 117 * wrapping_sub() - Intentionally perform a wrapping subtraction 118 * @type: type for result of calculation 119 * @a: minuend; value to subtract from 120 * @b: subtrahend; value to subtract from @a 121 * 122 * Return the potentially wrapped-around subtraction without 123 * tripping any wrap-around sanitizers that may be enabled. 124 */ 125 #define wrapping_sub(type, a, b) \ 126 ({ \ 127 type __val; \ 128 __builtin_sub_overflow(a, b, &__val); \ 129 __val; \ 130 }) 131 132 /** 133 * wrapping_assign_sub() - Intentionally perform a wrapping decrement assign 134 * @var: variable to be decremented 135 * @offset: amount to subtract 136 * 137 * Decrements @var by @offset with wrap-around. Returns the resulting 138 * value of @var. Will not trip any wrap-around sanitizers. 139 * 140 * Returns the new value of @var. 141 */ 142 #define wrapping_assign_sub(var, offset) \ 143 ({ \ 144 typeof(var) *__ptr = &(var); \ 145 *__ptr = wrapping_sub(typeof(var), *__ptr, offset); \ 146 }) 147 148 /** 149 * check_mul_overflow() - Calculate multiplication with overflow checking 150 * @a: first factor 151 * @b: second factor 152 * @d: pointer to store product 153 * 154 * Returns true on wrap-around, false otherwise. 155 * 156 * *@d holds the results of the attempted multiplication, regardless of whether 157 * wrap-around occurred. 158 */ 159 #define check_mul_overflow(a, b, d) \ 160 __must_check_overflow(__builtin_mul_overflow(a, b, d)) 161 162 /** 163 * wrapping_mul() - Intentionally perform a wrapping multiplication 164 * @type: type for result of calculation 165 * @a: first factor 166 * @b: second factor 167 * 168 * Return the potentially wrapped-around multiplication without 169 * tripping any wrap-around sanitizers that may be enabled. 170 */ 171 #define wrapping_mul(type, a, b) \ 172 ({ \ 173 type __val; \ 174 __builtin_mul_overflow(a, b, &__val); \ 175 __val; \ 176 }) 177 178 /** 179 * check_shl_overflow() - Calculate a left-shifted value and check overflow 180 * @a: Value to be shifted 181 * @s: How many bits left to shift 182 * @d: Pointer to where to store the result 183 * 184 * Computes *@d = (@a << @s) 185 * 186 * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't 187 * make sense. Example conditions: 188 * 189 * - '@a << @s' causes bits to be lost when stored in *@d. 190 * - '@s' is garbage (e.g. negative) or so large that the result of 191 * '@a << @s' is guaranteed to be 0. 192 * - '@a' is negative. 193 * - '@a << @s' sets the sign bit, if any, in '*@d'. 194 * 195 * '*@d' will hold the results of the attempted shift, but is not 196 * considered "safe for use" if true is returned. 197 */ 198 #define check_shl_overflow(a, s, d) __must_check_overflow(({ \ 199 typeof(a) _a = a; \ 200 typeof(s) _s = s; \ 201 typeof(d) _d = d; \ 202 unsigned long long _a_full = _a; \ 203 unsigned int _to_shift = \ 204 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ 205 *_d = (_a_full << _to_shift); \ 206 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ 207 (*_d >> _to_shift) != _a); \ 208 })) 209 210 #define __overflows_type_constexpr(x, T) ( \ 211 is_unsigned_type(typeof(x)) ? \ 212 (x) > type_max(T) : \ 213 is_unsigned_type(typeof(T)) ? \ 214 (x) < 0 || (x) > type_max(T) : \ 215 (x) < type_min(T) || (x) > type_max(T)) 216 217 #define __overflows_type(x, T) ({ \ 218 typeof(T) v = 0; \ 219 check_add_overflow((x), v, &v); \ 220 }) 221 222 /** 223 * overflows_type - helper for checking the overflows between value, variables, 224 * or data type 225 * 226 * @n: source constant value or variable to be checked 227 * @T: destination variable or data type proposed to store @x 228 * 229 * Compares the @x expression for whether or not it can safely fit in 230 * the storage of the type in @T. @x and @T can have different types. 231 * If @x is a constant expression, this will also resolve to a constant 232 * expression. 233 * 234 * Returns: true if overflow can occur, false otherwise. 235 */ 236 #define overflows_type(n, T) \ 237 __builtin_choose_expr(__is_constexpr(n), \ 238 __overflows_type_constexpr(n, T), \ 239 __overflows_type(n, T)) 240 241 /** 242 * range_overflows() - Check if a range is out of bounds 243 * @start: Start of the range. 244 * @size: Size of the range. 245 * @max: Exclusive upper boundary. 246 * 247 * A strict check to determine if the range [@start, @start + @size) is 248 * invalid with respect to the allowable range [0, @max). Any range 249 * starting at or beyond @max is considered an overflow, even if @size is 0. 250 * 251 * Returns: true if the range is out of bounds. 252 */ 253 #define range_overflows(start, size, max) ({ \ 254 typeof(start) start__ = (start); \ 255 typeof(size) size__ = (size); \ 256 typeof(max) max__ = (max); \ 257 (void)(&start__ == &size__); \ 258 (void)(&start__ == &max__); \ 259 start__ >= max__ || size__ > max__ - start__; \ 260 }) 261 262 /** 263 * range_overflows_t() - Check if a range is out of bounds 264 * @type: Data type to use. 265 * @start: Start of the range. 266 * @size: Size of the range. 267 * @max: Exclusive upper boundary. 268 * 269 * Same as range_overflows() but forcing the parameters to @type. 270 * 271 * Returns: true if the range is out of bounds. 272 */ 273 #define range_overflows_t(type, start, size, max) \ 274 range_overflows((type)(start), (type)(size), (type)(max)) 275 276 /** 277 * range_end_overflows() - Check if a range's endpoint is out of bounds 278 * @start: Start of the range. 279 * @size: Size of the range. 280 * @max: Exclusive upper boundary. 281 * 282 * Checks only if the endpoint of a range (@start + @size) exceeds @max. 283 * Unlike range_overflows(), a zero-sized range at the boundary (@start == @max) 284 * is not considered an overflow. Useful for iterator-style checks. 285 * 286 * Returns: true if the endpoint exceeds the boundary. 287 */ 288 #define range_end_overflows(start, size, max) ({ \ 289 typeof(start) start__ = (start); \ 290 typeof(size) size__ = (size); \ 291 typeof(max) max__ = (max); \ 292 (void)(&start__ == &size__); \ 293 (void)(&start__ == &max__); \ 294 start__ > max__ || size__ > max__ - start__; \ 295 }) 296 297 /** 298 * range_end_overflows_t() - Check if a range's endpoint is out of bounds 299 * @type: Data type to use. 300 * @start: Start of the range. 301 * @size: Size of the range. 302 * @max: Exclusive upper boundary. 303 * 304 * Same as range_end_overflows() but forcing the parameters to @type. 305 * 306 * Returns: true if the endpoint exceeds the boundary. 307 */ 308 #define range_end_overflows_t(type, start, size, max) \ 309 range_end_overflows((type)(start), (type)(size), (type)(max)) 310 311 /** 312 * castable_to_type - like __same_type(), but also allows for casted literals 313 * 314 * @n: variable or constant value 315 * @T: variable or data type 316 * 317 * Unlike the __same_type() macro, this allows a constant value as the 318 * first argument. If this value would not overflow into an assignment 319 * of the second argument's type, it returns true. Otherwise, this falls 320 * back to __same_type(). 321 */ 322 #define castable_to_type(n, T) \ 323 __builtin_choose_expr(__is_constexpr(n), \ 324 !__overflows_type_constexpr(n, T), \ 325 __same_type(n, T)) 326 327 /** 328 * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX 329 * @factor1: first factor 330 * @factor2: second factor 331 * 332 * Returns: calculate @factor1 * @factor2, both promoted to size_t, 333 * with any overflow causing the return value to be SIZE_MAX. The 334 * lvalue must be size_t to avoid implicit type conversion. 335 */ 336 static inline size_t __must_check size_mul(size_t factor1, size_t factor2) 337 { 338 size_t bytes; 339 340 if (check_mul_overflow(factor1, factor2, &bytes)) 341 return SIZE_MAX; 342 343 return bytes; 344 } 345 346 /** 347 * size_add() - Calculate size_t addition with saturation at SIZE_MAX 348 * @addend1: first addend 349 * @addend2: second addend 350 * 351 * Returns: calculate @addend1 + @addend2, both promoted to size_t, 352 * with any overflow causing the return value to be SIZE_MAX. The 353 * lvalue must be size_t to avoid implicit type conversion. 354 */ 355 static inline size_t __must_check size_add(size_t addend1, size_t addend2) 356 { 357 size_t bytes; 358 359 if (check_add_overflow(addend1, addend2, &bytes)) 360 return SIZE_MAX; 361 362 return bytes; 363 } 364 365 /** 366 * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX 367 * @minuend: value to subtract from 368 * @subtrahend: value to subtract from @minuend 369 * 370 * Returns: calculate @minuend - @subtrahend, both promoted to size_t, 371 * with any overflow causing the return value to be SIZE_MAX. For 372 * composition with the size_add() and size_mul() helpers, neither 373 * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX). 374 * The lvalue must be size_t to avoid implicit type conversion. 375 */ 376 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) 377 { 378 size_t bytes; 379 380 if (minuend == SIZE_MAX || subtrahend == SIZE_MAX || 381 check_sub_overflow(minuend, subtrahend, &bytes)) 382 return SIZE_MAX; 383 384 return bytes; 385 } 386 387 /** 388 * array_size() - Calculate size of 2-dimensional array. 389 * @a: dimension one 390 * @b: dimension two 391 * 392 * Calculates size of 2-dimensional array: @a * @b. 393 * 394 * Returns: number of bytes needed to represent the array or SIZE_MAX on 395 * overflow. 396 */ 397 #define array_size(a, b) size_mul(a, b) 398 399 /** 400 * array3_size() - Calculate size of 3-dimensional array. 401 * @a: dimension one 402 * @b: dimension two 403 * @c: dimension three 404 * 405 * Calculates size of 3-dimensional array: @a * @b * @c. 406 * 407 * Returns: number of bytes needed to represent the array or SIZE_MAX on 408 * overflow. 409 */ 410 #define array3_size(a, b, c) size_mul(size_mul(a, b), c) 411 412 /** 413 * flex_array_size() - Calculate size of a flexible array member 414 * within an enclosing structure. 415 * @p: Pointer to the structure. 416 * @member: Name of the flexible array member. 417 * @count: Number of elements in the array. 418 * 419 * Calculates size of a flexible array of @count number of @member 420 * elements, at the end of structure @p. 421 * 422 * Return: number of bytes needed or SIZE_MAX on overflow. 423 */ 424 #define flex_array_size(p, member, count) \ 425 __builtin_choose_expr(__is_constexpr(count), \ 426 (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \ 427 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member))) 428 429 /** 430 * struct_size() - Calculate size of structure with trailing flexible array. 431 * @p: Pointer to the structure. 432 * @member: Name of the array member. 433 * @count: Number of elements in the array. 434 * 435 * Calculates size of memory needed for structure of @p followed by an 436 * array of @count number of @member elements. 437 * 438 * Return: number of bytes needed or SIZE_MAX on overflow. 439 */ 440 #define struct_size(p, member, count) \ 441 __builtin_choose_expr(__is_constexpr(count), \ 442 sizeof(*(p)) + flex_array_size(p, member, count), \ 443 size_add(sizeof(*(p)), flex_array_size(p, member, count))) 444 445 /** 446 * struct_size_t() - Calculate size of structure with trailing flexible array 447 * @type: structure type name. 448 * @member: Name of the array member. 449 * @count: Number of elements in the array. 450 * 451 * Calculates size of memory needed for structure @type followed by an 452 * array of @count number of @member elements. Prefer using struct_size() 453 * when possible instead, to keep calculations associated with a specific 454 * instance variable of type @type. 455 * 456 * Return: number of bytes needed or SIZE_MAX on overflow. 457 */ 458 #define struct_size_t(type, member, count) \ 459 struct_size((type *)NULL, member, count) 460 461 /** 462 * __DEFINE_FLEX() - helper macro for DEFINE_FLEX() family. 463 * Enables caller macro to pass arbitrary trailing expressions 464 * 465 * @type: structure type name, including "struct" keyword. 466 * @name: Name for a variable to define. 467 * @member: Name of the array member. 468 * @count: Number of elements in the array; must be compile-time const. 469 * @trailer: Trailing expressions for attributes and/or initializers. 470 */ 471 #define __DEFINE_FLEX(type, name, member, count, trailer...) \ 472 _Static_assert(__builtin_constant_p(count), \ 473 "onstack flex array members require compile-time const count"); \ 474 union { \ 475 u8 bytes[struct_size_t(type, member, count)]; \ 476 type obj; \ 477 } name##_u trailer; \ 478 type *name = (type *)&name##_u 479 480 /** 481 * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family. 482 * Enables caller macro to pass (different) initializer. 483 * 484 * @type: structure type name, including "struct" keyword. 485 * @name: Name for a variable to define. 486 * @member: Name of the array member. 487 * @count: Number of elements in the array; must be compile-time const. 488 * @initializer: Initializer expression (e.g., pass `= { }` at minimum). 489 */ 490 #define _DEFINE_FLEX(type, name, member, count, initializer...) \ 491 __DEFINE_FLEX(type, name, member, count, = { .obj initializer }) 492 493 /** 494 * DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing 495 * flexible array member, when it does not have a __counted_by annotation. 496 * 497 * @type: structure type name, including "struct" keyword. 498 * @name: Name for a variable to define. 499 * @member: Name of the array member. 500 * @count: Number of elements in the array; must be compile-time const. 501 * 502 * Define a zeroed, on-stack, instance of @type structure with a trailing 503 * flexible array member. 504 * Use __struct_size(@name) to get compile-time size of it afterwards. 505 * Use __member_size(@name->member) to get compile-time size of @name members. 506 * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of 507 * elements in array @member. 508 */ 509 #define DEFINE_RAW_FLEX(type, name, member, count) \ 510 __DEFINE_FLEX(type, name, member, count, = { }) 511 512 /** 513 * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing 514 * flexible array member. 515 * 516 * @TYPE: structure type name, including "struct" keyword. 517 * @NAME: Name for a variable to define. 518 * @MEMBER: Name of the array member. 519 * @COUNTER: Name of the __counted_by member. 520 * @COUNT: Number of elements in the array; must be compile-time const. 521 * 522 * Define a zeroed, on-stack, instance of @TYPE structure with a trailing 523 * flexible array member. 524 * Use __struct_size(@NAME) to get compile-time size of it afterwards. 525 * Use __member_size(@NAME->member) to get compile-time size of @NAME members. 526 * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of 527 * elements in array @member. 528 */ 529 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \ 530 _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, }) 531 532 /** 533 * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family. 534 * Returns the number of elements in @array. 535 * 536 * @name: Name for a variable defined in DEFINE_RAW_FLEX()/DEFINE_FLEX(). 537 * @array: Name of the array member. 538 */ 539 #define STACK_FLEX_ARRAY_SIZE(name, array) \ 540 (__member_size((name)->array) / sizeof(*(name)->array) + \ 541 __must_be_array((name)->array)) 542 543 #endif /* __LINUX_OVERFLOW_H */ 544