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 #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  * castable_to_type - like __same_type(), but also allows for casted literals
 243  *
 244  * @n: variable or constant value
 245  * @T: variable or data type
 246  *
 247  * Unlike the __same_type() macro, this allows a constant value as the
 248  * first argument. If this value would not overflow into an assignment
 249  * of the second argument's type, it returns true. Otherwise, this falls
 250  * back to __same_type().
 251  */
 252 #define castable_to_type(n, T)                                          \
 253         __builtin_choose_expr(__is_constexpr(n),                        \
 254                               !__overflows_type_constexpr(n, T),        \
 255                               __same_type(n, T))
 256
 257 /**
 258  * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
 259  * @factor1: first factor
 260  * @factor2: second factor
 261  *
 262  * Returns: calculate @factor1 * @factor2, both promoted to size_t,
 263  * with any overflow causing the return value to be SIZE_MAX. The
 264  * lvalue must be size_t to avoid implicit type conversion.
 265  */
 266 static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
 267 {
 268         size_t bytes;
 269
 270         if (check_mul_overflow(factor1, factor2, &bytes))
 271                 return SIZE_MAX;
 272
 273         return bytes;
 274 }
 275
 276 /**
 277  * size_add() - Calculate size_t addition with saturation at SIZE_MAX
 278  * @addend1: first addend
 279  * @addend2: second addend
 280  *
 281  * Returns: calculate @addend1 + @addend2, both promoted to size_t,
 282  * with any overflow causing the return value to be SIZE_MAX. The
 283  * lvalue must be size_t to avoid implicit type conversion.
 284  */
 285 static inline size_t __must_check size_add(size_t addend1, size_t addend2)
 286 {
 287         size_t bytes;
 288
 289         if (check_add_overflow(addend1, addend2, &bytes))
 290                 return SIZE_MAX;
 291
 292         return bytes;
 293 }
 294
 295 /**
 296  * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
 297  * @minuend: value to subtract from
 298  * @subtrahend: value to subtract from @minuend
 299  *
 300  * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
 301  * with any overflow causing the return value to be SIZE_MAX. For
 302  * composition with the size_add() and size_mul() helpers, neither
 303  * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
 304  * The lvalue must be size_t to avoid implicit type conversion.
 305  */
 306 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
 307 {
 308         size_t bytes;
 309
 310         if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
 311             check_sub_overflow(minuend, subtrahend, &bytes))
 312                 return SIZE_MAX;
 313
 314         return bytes;
 315 }
 316
 317 /**
 318  * array_size() - Calculate size of 2-dimensional array.
 319  * @a: dimension one
 320  * @b: dimension two
 321  *
 322  * Calculates size of 2-dimensional array: @a * @b.
 323  *
 324  * Returns: number of bytes needed to represent the array or SIZE_MAX on
 325  * overflow.
 326  */
 327 #define array_size(a, b)        size_mul(a, b)
 328
 329 /**
 330  * array3_size() - Calculate size of 3-dimensional array.
 331  * @a: dimension one
 332  * @b: dimension two
 333  * @c: dimension three
 334  *
 335  * Calculates size of 3-dimensional array: @a * @b * @c.
 336  *
 337  * Returns: number of bytes needed to represent the array or SIZE_MAX on
 338  * overflow.
 339  */
 340 #define array3_size(a, b, c)    size_mul(size_mul(a, b), c)
 341
 342 /**
 343  * flex_array_size() - Calculate size of a flexible array member
 344  *                     within an enclosing structure.
 345  * @p: Pointer to the structure.
 346  * @member: Name of the flexible array member.
 347  * @count: Number of elements in the array.
 348  *
 349  * Calculates size of a flexible array of @count number of @member
 350  * elements, at the end of structure @p.
 351  *
 352  * Return: number of bytes needed or SIZE_MAX on overflow.
 353  */
 354 #define flex_array_size(p, member, count)                               \
 355         __builtin_choose_expr(__is_constexpr(count),                    \
 356                 (count) * sizeof(*(p)->member) + __must_be_array((p)->member),  \
 357                 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
 358
 359 /**
 360  * struct_size() - Calculate size of structure with trailing flexible array.
 361  * @p: Pointer to the structure.
 362  * @member: Name of the array member.
 363  * @count: Number of elements in the array.
 364  *
 365  * Calculates size of memory needed for structure of @p followed by an
 366  * array of @count number of @member elements.
 367  *
 368  * Return: number of bytes needed or SIZE_MAX on overflow.
 369  */
 370 #define struct_size(p, member, count)                                   \
 371         __builtin_choose_expr(__is_constexpr(count),                    \
 372                 sizeof(*(p)) + flex_array_size(p, member, count),       \
 373                 size_add(sizeof(*(p)), flex_array_size(p, member, count)))
 374
 375 /**
 376  * struct_size_t() - Calculate size of structure with trailing flexible array
 377  * @type: structure type name.
 378  * @member: Name of the array member.
 379  * @count: Number of elements in the array.
 380  *
 381  * Calculates size of memory needed for structure @type followed by an
 382  * array of @count number of @member elements. Prefer using struct_size()
 383  * when possible instead, to keep calculations associated with a specific
 384  * instance variable of type @type.
 385  *
 386  * Return: number of bytes needed or SIZE_MAX on overflow.
 387  */
 388 #define struct_size_t(type, member, count)                                      \
 389         struct_size((type *)NULL, member, count)
 390
 391 /**
 392  * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
 393  * Enables caller macro to pass (different) initializer.
 394  *
 395  * @type: structure type name, including "struct" keyword.
 396  * @name: Name for a variable to define.
 397  * @member: Name of the array member.
 398  * @count: Number of elements in the array; must be compile-time const.
 399  * @initializer: initializer expression (could be empty for no init).
 400  */
 401 #define _DEFINE_FLEX(type, name, member, count, initializer...)                 \
 402         _Static_assert(__builtin_constant_p(count),                             \
 403                        "onstack flex array members require compile-time const count"); \
 404         union {                                                                 \
 405                 u8 bytes[struct_size_t(type, member, count)];                   \
 406                 type obj;                                                       \
 407         } name##_u initializer;                                                 \
 408         type *name = (type *)&name##_u
 409
 410 /**
 411  * DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing
 412  * flexible array member, when it does not have a __counted_by annotation.
 413  *
 414  * @type: structure type name, including "struct" keyword.
 415  * @name: Name for a variable to define.
 416  * @member: Name of the array member.
 417  * @count: Number of elements in the array; must be compile-time const.
 418  *
 419  * Define a zeroed, on-stack, instance of @type structure with a trailing
 420  * flexible array member.
 421  * Use __struct_size(@name) to get compile-time size of it afterwards.
 422  */
 423 #define DEFINE_RAW_FLEX(type, name, member, count)      \
 424         _DEFINE_FLEX(type, name, member, count, = {})
 425
 426 /**
 427  * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing
 428  * flexible array member.
 429  *
 430  * @TYPE: structure type name, including "struct" keyword.
 431  * @NAME: Name for a variable to define.
 432  * @MEMBER: Name of the array member.
 433  * @COUNTER: Name of the __counted_by member.
 434  * @COUNT: Number of elements in the array; must be compile-time const.
 435  *
 436  * Define a zeroed, on-stack, instance of @TYPE structure with a trailing
 437  * flexible array member.
 438  * Use __struct_size(@NAME) to get compile-time size of it afterwards.
 439  */
 440 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \
 441         _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
 442
 443 #endif /* __LINUX_OVERFLOW_H */