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
   8 /*
   9  * In the fallback code below, we need to compute the minimum and
  10  * maximum values representable in a given type. These macros may also
  11  * be useful elsewhere, so we provide them outside the
  12  * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block.
  13  *
  14  * It would seem more obvious 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 is_signed_type(type)       (((type)(-1)) < (type)1)
  36 #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
  37 #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
  38 #define type_min(T) ((T)((T)-type_max(T)-(T)1))
  39
  40 /*
  41  * Avoids triggering -Wtype-limits compilation warning,
  42  * while using unsigned data types to check a < 0.
  43  */
  44 #define is_non_negative(a) ((a) > 0 || (a) == 0)
  45 #define is_negative(a) (!(is_non_negative(a)))
  46
  47 /*
  48  * Allows for effectively applying __must_check to a macro so we can have
  49  * both the type-agnostic benefits of the macros while also being able to
  50  * enforce that the return value is, in fact, checked.
  51  */
  52 static inline bool __must_check __must_check_overflow(bool overflow)
  53 {
  54         return unlikely(overflow);
  55 }
  56
  57 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
  58 /*
  59  * For simplicity and code hygiene, the fallback code below insists on
  60  * a, b and *d having the same type (similar to the min() and max()
  61  * macros), whereas gcc's type-generic overflow checkers accept
  62  * different types. Hence we don't just make check_add_overflow an
  63  * alias for __builtin_add_overflow, but add type checks similar to
  64  * below.
  65  */
  66 #define check_add_overflow(a, b, d) __must_check_overflow(({    \
  67         typeof(a) __a = (a);                    \
  68         typeof(b) __b = (b);                    \
  69         typeof(d) __d = (d);                    \
  70         (void) (&__a == &__b);                  \
  71         (void) (&__a == __d);                   \
  72         __builtin_add_overflow(__a, __b, __d);  \
  73 }))
  74
  75 #define check_sub_overflow(a, b, d) __must_check_overflow(({    \
  76         typeof(a) __a = (a);                    \
  77         typeof(b) __b = (b);                    \
  78         typeof(d) __d = (d);                    \
  79         (void) (&__a == &__b);                  \
  80         (void) (&__a == __d);                   \
  81         __builtin_sub_overflow(__a, __b, __d);  \
  82 }))
  83
  84 #define check_mul_overflow(a, b, d) __must_check_overflow(({    \
  85         typeof(a) __a = (a);                    \
  86         typeof(b) __b = (b);                    \
  87         typeof(d) __d = (d);                    \
  88         (void) (&__a == &__b);                  \
  89         (void) (&__a == __d);                   \
  90         __builtin_mul_overflow(__a, __b, __d);  \
  91 }))
  92
  93 #else
  94
  95
  96 /* Checking for unsigned overflow is relatively easy without causing UB. */
  97 #define __unsigned_add_overflow(a, b, d) ({     \
  98         typeof(a) __a = (a);                    \
  99         typeof(b) __b = (b);                    \
 100         typeof(d) __d = (d);                    \
 101         (void) (&__a == &__b);                  \
 102         (void) (&__a == __d);                   \
 103         *__d = __a + __b;                       \
 104         *__d < __a;                             \
 105 })
 106 #define __unsigned_sub_overflow(a, b, d) ({     \
 107         typeof(a) __a = (a);                    \
 108         typeof(b) __b = (b);                    \
 109         typeof(d) __d = (d);                    \
 110         (void) (&__a == &__b);                  \
 111         (void) (&__a == __d);                   \
 112         *__d = __a - __b;                       \
 113         __a < __b;                              \
 114 })
 115 /*
 116  * If one of a or b is a compile-time constant, this avoids a division.
 117  */
 118 #define __unsigned_mul_overflow(a, b, d) ({             \
 119         typeof(a) __a = (a);                            \
 120         typeof(b) __b = (b);                            \
 121         typeof(d) __d = (d);                            \
 122         (void) (&__a == &__b);                          \
 123         (void) (&__a == __d);                           \
 124         *__d = __a * __b;                               \
 125         __builtin_constant_p(__b) ?                     \
 126           __b > 0 && __a > type_max(typeof(__a)) / __b : \
 127           __a > 0 && __b > type_max(typeof(__b)) / __a;  \
 128 })
 129
 130 /*
 131  * For signed types, detecting overflow is much harder, especially if
 132  * we want to avoid UB. But the interface of these macros is such that
 133  * we must provide a result in *d, and in fact we must produce the
 134  * result promised by gcc's builtins, which is simply the possibly
 135  * wrapped-around value. Fortunately, we can just formally do the
 136  * operations in the widest relevant unsigned type (u64) and then
 137  * truncate the result - gcc is smart enough to generate the same code
 138  * with and without the (u64) casts.
 139  */
 140
 141 /*
 142  * Adding two signed integers can overflow only if they have the same
 143  * sign, and overflow has happened iff the result has the opposite
 144  * sign.
 145  */
 146 #define __signed_add_overflow(a, b, d) ({       \
 147         typeof(a) __a = (a);                    \
 148         typeof(b) __b = (b);                    \
 149         typeof(d) __d = (d);                    \
 150         (void) (&__a == &__b);                  \
 151         (void) (&__a == __d);                   \
 152         *__d = (u64)__a + (u64)__b;             \
 153         (((~(__a ^ __b)) & (*__d ^ __a))        \
 154                 & type_min(typeof(__a))) != 0;  \
 155 })
 156
 157 /*
 158  * Subtraction is similar, except that overflow can now happen only
 159  * when the signs are opposite. In this case, overflow has happened if
 160  * the result has the opposite sign of a.
 161  */
 162 #define __signed_sub_overflow(a, b, d) ({       \
 163         typeof(a) __a = (a);                    \
 164         typeof(b) __b = (b);                    \
 165         typeof(d) __d = (d);                    \
 166         (void) (&__a == &__b);                  \
 167         (void) (&__a == __d);                   \
 168         *__d = (u64)__a - (u64)__b;             \
 169         ((((__a ^ __b)) & (*__d ^ __a))         \
 170                 & type_min(typeof(__a))) != 0;  \
 171 })
 172
 173 /*
 174  * Signed multiplication is rather hard. gcc always follows C99, so
 175  * division is truncated towards 0. This means that we can write the
 176  * overflow check like this:
 177  *
 178  * (a > 0 && (b > MAX/a || b < MIN/a)) ||
 179  * (a < -1 && (b > MIN/a || b < MAX/a) ||
 180  * (a == -1 && b == MIN)
 181  *
 182  * The redundant casts of -1 are to silence an annoying -Wtype-limits
 183  * (included in -Wextra) warning: When the type is u8 or u16, the
 184  * __b_c_e in check_mul_overflow obviously selects
 185  * __unsigned_mul_overflow, but unfortunately gcc still parses this
 186  * code and warns about the limited range of __b.
 187  */
 188
 189 #define __signed_mul_overflow(a, b, d) ({                               \
 190         typeof(a) __a = (a);                                            \
 191         typeof(b) __b = (b);                                            \
 192         typeof(d) __d = (d);                                            \
 193         typeof(a) __tmax = type_max(typeof(a));                         \
 194         typeof(a) __tmin = type_min(typeof(a));                         \
 195         (void) (&__a == &__b);                                          \
 196         (void) (&__a == __d);                                           \
 197         *__d = (u64)__a * (u64)__b;                                     \
 198         (__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||        \
 199         (__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
 200         (__b == (typeof(__b))-1 && __a == __tmin);                      \
 201 })
 202
 203
 204 #define check_add_overflow(a, b, d)     __must_check_overflow(          \
 205         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 206                         __signed_add_overflow(a, b, d),                 \
 207                         __unsigned_add_overflow(a, b, d)))
 208
 209 #define check_sub_overflow(a, b, d)     __must_check_overflow(          \
 210         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 211                         __signed_sub_overflow(a, b, d),                 \
 212                         __unsigned_sub_overflow(a, b, d)))
 213
 214 #define check_mul_overflow(a, b, d)     __must_check_overflow(          \
 215         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 216                         __signed_mul_overflow(a, b, d),                 \
 217                         __unsigned_mul_overflow(a, b, d)))
 218
 219 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
 220
 221 /** check_shl_overflow() - Calculate a left-shifted value and check overflow
 222  *
 223  * @a: Value to be shifted
 224  * @s: How many bits left to shift
 225  * @d: Pointer to where to store the result
 226  *
 227  * Computes *@d = (@a << @s)
 228  *
 229  * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
 230  * make sense. Example conditions:
 231  * - 'a << s' causes bits to be lost when stored in *d.
 232  * - 's' is garbage (e.g. negative) or so large that the result of
 233  *   'a << s' is guaranteed to be 0.
 234  * - 'a' is negative.
 235  * - 'a << s' sets the sign bit, if any, in '*d'.
 236  *
 237  * '*d' will hold the results of the attempted shift, but is not
 238  * considered "safe for use" if true is returned.
 239  */
 240 #define check_shl_overflow(a, s, d) __must_check_overflow(({            \
 241         typeof(a) _a = a;                                               \
 242         typeof(s) _s = s;                                               \
 243         typeof(d) _d = d;                                               \
 244         u64 _a_full = _a;                                               \
 245         unsigned int _to_shift =                                        \
 246                 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0;    \
 247         *_d = (_a_full << _to_shift);                                   \
 248         (_to_shift != _s || is_negative(*_d) || is_negative(_a) ||      \
 249         (*_d >> _to_shift) != _a);                                      \
 250 }))
 251
 252 /**
 253  * array_size() - Calculate size of 2-dimensional array.
 254  *
 255  * @a: dimension one
 256  * @b: dimension two
 257  *
 258  * Calculates size of 2-dimensional array: @a * @b.
 259  *
 260  * Returns: number of bytes needed to represent the array or SIZE_MAX on
 261  * overflow.
 262  */
 263 static inline __must_check size_t array_size(size_t a, size_t b)
 264 {
 265         size_t bytes;
 266
 267         if (check_mul_overflow(a, b, &bytes))
 268                 return SIZE_MAX;
 269
 270         return bytes;
 271 }
 272
 273 /**
 274  * array3_size() - Calculate size of 3-dimensional array.
 275  *
 276  * @a: dimension one
 277  * @b: dimension two
 278  * @c: dimension three
 279  *
 280  * Calculates size of 3-dimensional array: @a * @b * @c.
 281  *
 282  * Returns: number of bytes needed to represent the array or SIZE_MAX on
 283  * overflow.
 284  */
 285 static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
 286 {
 287         size_t bytes;
 288
 289         if (check_mul_overflow(a, b, &bytes))
 290                 return SIZE_MAX;
 291         if (check_mul_overflow(bytes, c, &bytes))
 292                 return SIZE_MAX;
 293
 294         return bytes;
 295 }
 296
 297 /*
 298  * Compute a*b+c, returning SIZE_MAX on overflow. Internal helper for
 299  * struct_size() below.
 300  */
 301 static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c)
 302 {
 303         size_t bytes;
 304
 305         if (check_mul_overflow(a, b, &bytes))
 306                 return SIZE_MAX;
 307         if (check_add_overflow(bytes, c, &bytes))
 308                 return SIZE_MAX;
 309
 310         return bytes;
 311 }
 312
 313 /**
 314  * struct_size() - Calculate size of structure with trailing array.
 315  * @p: Pointer to the structure.
 316  * @member: Name of the array member.
 317  * @count: Number of elements in the array.
 318  *
 319  * Calculates size of memory needed for structure @p followed by an
 320  * array of @count number of @member elements.
 321  *
 322  * Return: number of bytes needed or SIZE_MAX on overflow.
 323  */
 324 #define struct_size(p, member, count)                                   \
 325         __ab_c_size(count,                                              \
 326                     sizeof(*(p)->member) + __must_be_array((p)->member),\
 327                     sizeof(*(p)))
 328
 329 /**
 330  * flex_array_size() - Calculate size of a flexible array member
 331  *                     within an enclosing structure.
 332  *
 333  * @p: Pointer to the structure.
 334  * @member: Name of the flexible array member.
 335  * @count: Number of elements in the array.
 336  *
 337  * Calculates size of a flexible array of @count number of @member
 338  * elements, at the end of structure @p.
 339  *
 340  * Return: number of bytes needed or SIZE_MAX on overflow.
 341  */
 342 #define flex_array_size(p, member, count)                               \
 343         array_size(count,                                               \
 344                     sizeof(*(p)->member) + __must_be_array((p)->member))
 345
 346 #endif /* __LINUX_OVERFLOW_H */