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
   7 /*
   8  * In the fallback code below, we need to compute the minimum and
   9  * maximum values representable in a given type. These macros may also
  10  * be useful elsewhere, so we provide them outside the
  11  * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block.
  12  *
  13  * It would seem more obvious to do something like
  14  *
  15  * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
  16  * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
  17  *
  18  * Unfortunately, the middle expressions, strictly speaking, have
  19  * undefined behaviour, and at least some versions of gcc warn about
  20  * the type_max expression (but not if -fsanitize=undefined is in
  21  * effect; in that case, the warning is deferred to runtime...).
  22  *
  23  * The slightly excessive casting in type_min is to make sure the
  24  * macros also produce sensible values for the exotic type _Bool. [The
  25  * overflow checkers only almost work for _Bool, but that's
  26  * a-feature-not-a-bug, since people shouldn't be doing arithmetic on
  27  * _Bools. Besides, the gcc builtins don't allow _Bool* as third
  28  * argument.]
  29  *
  30  * Idea stolen from
  31  * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
  32  * credit to Christian Biere.
  33  */
  34 #define is_signed_type(type)       (((type)(-1)) < (type)1)
  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 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
  47 /*
  48  * For simplicity and code hygiene, the fallback code below insists on
  49  * a, b and *d having the same type (similar to the min() and max()
  50  * macros), whereas gcc's type-generic overflow checkers accept
  51  * different types. Hence we don't just make check_add_overflow an
  52  * alias for __builtin_add_overflow, but add type checks similar to
  53  * below.
  54  */
  55 #define check_add_overflow(a, b, d) ({          \
  56         typeof(a) __a = (a);                    \
  57         typeof(b) __b = (b);                    \
  58         typeof(d) __d = (d);                    \
  59         (void) (&__a == &__b);                  \
  60         (void) (&__a == __d);                   \
  61         __builtin_add_overflow(__a, __b, __d);  \
  62 })
  63
  64 #define check_sub_overflow(a, b, d) ({          \
  65         typeof(a) __a = (a);                    \
  66         typeof(b) __b = (b);                    \
  67         typeof(d) __d = (d);                    \
  68         (void) (&__a == &__b);                  \
  69         (void) (&__a == __d);                   \
  70         __builtin_sub_overflow(__a, __b, __d);  \
  71 })
  72
  73 #define check_mul_overflow(a, b, d) ({          \
  74         typeof(a) __a = (a);                    \
  75         typeof(b) __b = (b);                    \
  76         typeof(d) __d = (d);                    \
  77         (void) (&__a == &__b);                  \
  78         (void) (&__a == __d);                   \
  79         __builtin_mul_overflow(__a, __b, __d);  \
  80 })
  81
  82 #else
  83
  84
  85 /* Checking for unsigned overflow is relatively easy without causing UB. */
  86 #define __unsigned_add_overflow(a, b, d) ({     \
  87         typeof(a) __a = (a);                    \
  88         typeof(b) __b = (b);                    \
  89         typeof(d) __d = (d);                    \
  90         (void) (&__a == &__b);                  \
  91         (void) (&__a == __d);                   \
  92         *__d = __a + __b;                       \
  93         *__d < __a;                             \
  94 })
  95 #define __unsigned_sub_overflow(a, b, d) ({     \
  96         typeof(a) __a = (a);                    \
  97         typeof(b) __b = (b);                    \
  98         typeof(d) __d = (d);                    \
  99         (void) (&__a == &__b);                  \
 100         (void) (&__a == __d);                   \
 101         *__d = __a - __b;                       \
 102         __a < __b;                              \
 103 })
 104 /*
 105  * If one of a or b is a compile-time constant, this avoids a division.
 106  */
 107 #define __unsigned_mul_overflow(a, b, d) ({             \
 108         typeof(a) __a = (a);                            \
 109         typeof(b) __b = (b);                            \
 110         typeof(d) __d = (d);                            \
 111         (void) (&__a == &__b);                          \
 112         (void) (&__a == __d);                           \
 113         *__d = __a * __b;                               \
 114         __builtin_constant_p(__b) ?                     \
 115           __b > 0 && __a > type_max(typeof(__a)) / __b : \
 116           __a > 0 && __b > type_max(typeof(__b)) / __a;  \
 117 })
 118
 119 /*
 120  * For signed types, detecting overflow is much harder, especially if
 121  * we want to avoid UB. But the interface of these macros is such that
 122  * we must provide a result in *d, and in fact we must produce the
 123  * result promised by gcc's builtins, which is simply the possibly
 124  * wrapped-around value. Fortunately, we can just formally do the
 125  * operations in the widest relevant unsigned type (u64) and then
 126  * truncate the result - gcc is smart enough to generate the same code
 127  * with and without the (u64) casts.
 128  */
 129
 130 /*
 131  * Adding two signed integers can overflow only if they have the same
 132  * sign, and overflow has happened iff the result has the opposite
 133  * sign.
 134  */
 135 #define __signed_add_overflow(a, b, d) ({       \
 136         typeof(a) __a = (a);                    \
 137         typeof(b) __b = (b);                    \
 138         typeof(d) __d = (d);                    \
 139         (void) (&__a == &__b);                  \
 140         (void) (&__a == __d);                   \
 141         *__d = (u64)__a + (u64)__b;             \
 142         (((~(__a ^ __b)) & (*__d ^ __a))        \
 143                 & type_min(typeof(__a))) != 0;  \
 144 })
 145
 146 /*
 147  * Subtraction is similar, except that overflow can now happen only
 148  * when the signs are opposite. In this case, overflow has happened if
 149  * the result has the opposite sign of a.
 150  */
 151 #define __signed_sub_overflow(a, b, d) ({       \
 152         typeof(a) __a = (a);                    \
 153         typeof(b) __b = (b);                    \
 154         typeof(d) __d = (d);                    \
 155         (void) (&__a == &__b);                  \
 156         (void) (&__a == __d);                   \
 157         *__d = (u64)__a - (u64)__b;             \
 158         ((((__a ^ __b)) & (*__d ^ __a))         \
 159                 & type_min(typeof(__a))) != 0;  \
 160 })
 161
 162 /*
 163  * Signed multiplication is rather hard. gcc always follows C99, so
 164  * division is truncated towards 0. This means that we can write the
 165  * overflow check like this:
 166  *
 167  * (a > 0 && (b > MAX/a || b < MIN/a)) ||
 168  * (a < -1 && (b > MIN/a || b < MAX/a) ||
 169  * (a == -1 && b == MIN)
 170  *
 171  * The redundant casts of -1 are to silence an annoying -Wtype-limits
 172  * (included in -Wextra) warning: When the type is u8 or u16, the
 173  * __b_c_e in check_mul_overflow obviously selects
 174  * __unsigned_mul_overflow, but unfortunately gcc still parses this
 175  * code and warns about the limited range of __b.
 176  */
 177
 178 #define __signed_mul_overflow(a, b, d) ({                               \
 179         typeof(a) __a = (a);                                            \
 180         typeof(b) __b = (b);                                            \
 181         typeof(d) __d = (d);                                            \
 182         typeof(a) __tmax = type_max(typeof(a));                         \
 183         typeof(a) __tmin = type_min(typeof(a));                         \
 184         (void) (&__a == &__b);                                          \
 185         (void) (&__a == __d);                                           \
 186         *__d = (u64)__a * (u64)__b;                                     \
 187         (__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||        \
 188         (__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
 189         (__b == (typeof(__b))-1 && __a == __tmin);                      \
 190 })
 191
 192
 193 #define check_add_overflow(a, b, d)                                     \
 194         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 195                         __signed_add_overflow(a, b, d),                 \
 196                         __unsigned_add_overflow(a, b, d))
 197
 198 #define check_sub_overflow(a, b, d)                                     \
 199         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 200                         __signed_sub_overflow(a, b, d),                 \
 201                         __unsigned_sub_overflow(a, b, d))
 202
 203 #define check_mul_overflow(a, b, d)                                     \
 204         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 205                         __signed_mul_overflow(a, b, d),                 \
 206                         __unsigned_mul_overflow(a, b, d))
 207
 208
 209 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
 210
 211 /** check_shl_overflow() - Calculate a left-shifted value and check overflow
 212  *
 213  * @a: Value to be shifted
 214  * @s: How many bits left to shift
 215  * @d: Pointer to where to store the result
 216  *
 217  * Computes *@d = (@a << @s)
 218  *
 219  * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
 220  * make sense. Example conditions:
 221  * - 'a << s' causes bits to be lost when stored in *d.
 222  * - 's' is garbage (e.g. negative) or so large that the result of
 223  *   'a << s' is guaranteed to be 0.
 224  * - 'a' is negative.
 225  * - 'a << s' sets the sign bit, if any, in '*d'.
 226  *
 227  * '*d' will hold the results of the attempted shift, but is not
 228  * considered "safe for use" if false is returned.
 229  */
 230 #define check_shl_overflow(a, s, d) ({                                  \
 231         typeof(a) _a = a;                                               \
 232         typeof(s) _s = s;                                               \
 233         typeof(d) _d = d;                                               \
 234         u64 _a_full = _a;                                               \
 235         unsigned int _to_shift =                                        \
 236                 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0;    \
 237         *_d = (_a_full << _to_shift);                                   \
 238         (_to_shift != _s || is_negative(*_d) || is_negative(_a) ||      \
 239         (*_d >> _to_shift) != _a);                                      \
 240 })
 241
 242 /**
 243  * array_size() - Calculate size of 2-dimensional array.
 244  *
 245  * @a: dimension one
 246  * @b: dimension two
 247  *
 248  * Calculates size of 2-dimensional array: @a * @b.
 249  *
 250  * Returns: number of bytes needed to represent the array or SIZE_MAX on
 251  * overflow.
 252  */
 253 static inline __must_check size_t array_size(size_t a, size_t b)
 254 {
 255         size_t bytes;
 256
 257         if (check_mul_overflow(a, b, &bytes))
 258                 return SIZE_MAX;
 259
 260         return bytes;
 261 }
 262
 263 /**
 264  * array3_size() - Calculate size of 3-dimensional array.
 265  *
 266  * @a: dimension one
 267  * @b: dimension two
 268  * @c: dimension three
 269  *
 270  * Calculates size of 3-dimensional array: @a * @b * @c.
 271  *
 272  * Returns: number of bytes needed to represent the array or SIZE_MAX on
 273  * overflow.
 274  */
 275 static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
 276 {
 277         size_t bytes;
 278
 279         if (check_mul_overflow(a, b, &bytes))
 280                 return SIZE_MAX;
 281         if (check_mul_overflow(bytes, c, &bytes))
 282                 return SIZE_MAX;
 283
 284         return bytes;
 285 }
 286
 287 /*
 288  * Compute a*b+c, returning SIZE_MAX on overflow. Internal helper for
 289  * struct_size() below.
 290  */
 291 static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c)
 292 {
 293         size_t bytes;
 294
 295         if (check_mul_overflow(a, b, &bytes))
 296                 return SIZE_MAX;
 297         if (check_add_overflow(bytes, c, &bytes))
 298                 return SIZE_MAX;
 299
 300         return bytes;
 301 }
 302
 303 /**
 304  * struct_size() - Calculate size of structure with trailing array.
 305  * @p: Pointer to the structure.
 306  * @member: Name of the array member.
 307  * @n: Number of elements in the array.
 308  *
 309  * Calculates size of memory needed for structure @p followed by an
 310  * array of @n @member elements.
 311  *
 312  * Return: number of bytes needed or SIZE_MAX on overflow.
 313  */
 314 #define struct_size(p, member, n)                                       \
 315         __ab_c_size(n,                                                  \
 316                     sizeof(*(p)->member) + __must_be_array((p)->member),\
 317                     sizeof(*(p)))
 318
 319 #endif /* __LINUX_OVERFLOW_H */