datapath/linux/compat/include/linux/overflow.h

   1 /* SPDX-License-Identifier: GPL-2.0 OR MIT */
   2 #ifdef HAVE_OVERFLOW_H
   3 #include_next <linux/overflow.h>
   4 #else
   5 #ifndef __LINUX_OVERFLOW_H
   6 #define __LINUX_OVERFLOW_H
   7
   8 #include <linux/compiler.h>
   9
  10 /*
  11  * In the fallback code below, we need to compute the minimum and
  12  * maximum values representable in a given type. These macros may also
  13  * be useful elsewhere, so we provide them outside the
  14  * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block.
  15  *
  16  * It would seem more obvious to do something like
  17  *
  18  * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
  19  * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
  20  *
  21  * Unfortunately, the middle expressions, strictly speaking, have
  22  * undefined behaviour, and at least some versions of gcc warn about
  23  * the type_max expression (but not if -fsanitize=undefined is in
  24  * effect; in that case, the warning is deferred to runtime...).
  25  *
  26  * The slightly excessive casting in type_min is to make sure the
  27  * macros also produce sensible values for the exotic type _Bool. [The
  28  * overflow checkers only almost work for _Bool, but that's
  29  * a-feature-not-a-bug, since people shouldn't be doing arithmetic on
  30  * _Bools. Besides, the gcc builtins don't allow _Bool* as third
  31  * argument.]
  32  *
  33  * Idea stolen from
  34  * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
  35  * credit to Christian Biere.
  36  */
  37 #define is_signed_type(type)       (((type)(-1)) < (type)1)
  38 #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
  39 #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
  40 #define type_min(T) ((T)((T)-type_max(T)-(T)1))
  41
  42
  43 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
  44 /*
  45  * For simplicity and code hygiene, the fallback code below insists on
  46  * a, b and *d having the same type (similar to the min() and max()
  47  * macros), whereas gcc's type-generic overflow checkers accept
  48  * different types. Hence we don't just make check_add_overflow an
  49  * alias for __builtin_add_overflow, but add type checks similar to
  50  * below.
  51  */
  52 #define check_add_overflow(a, b, d) ({          \
  53         typeof(a) __a = (a);                    \
  54         typeof(b) __b = (b);                    \
  55         typeof(d) __d = (d);                    \
  56         (void) (&__a == &__b);                  \
  57         (void) (&__a == __d);                   \
  58         __builtin_add_overflow(__a, __b, __d);  \
  59 })
  60
  61 #define check_sub_overflow(a, b, d) ({          \
  62         typeof(a) __a = (a);                    \
  63         typeof(b) __b = (b);                    \
  64         typeof(d) __d = (d);                    \
  65         (void) (&__a == &__b);                  \
  66         (void) (&__a == __d);                   \
  67         __builtin_sub_overflow(__a, __b, __d);  \
  68 })
  69
  70 #define check_mul_overflow(a, b, d) ({          \
  71         typeof(a) __a = (a);                    \
  72         typeof(b) __b = (b);                    \
  73         typeof(d) __d = (d);                    \
  74         (void) (&__a == &__b);                  \
  75         (void) (&__a == __d);                   \
  76         __builtin_mul_overflow(__a, __b, __d);  \
  77 })
  78
  79 #else
  80
  81
  82 /* Checking for unsigned overflow is relatively easy without causing UB. */
  83 #define __unsigned_add_overflow(a, b, d) ({     \
  84         typeof(a) __a = (a);                    \
  85         typeof(b) __b = (b);                    \
  86         typeof(d) __d = (d);                    \
  87         (void) (&__a == &__b);                  \
  88         (void) (&__a == __d);                   \
  89         *__d = __a + __b;                       \
  90         *__d < __a;                             \
  91 })
  92 #define __unsigned_sub_overflow(a, b, d) ({     \
  93         typeof(a) __a = (a);                    \
  94         typeof(b) __b = (b);                    \
  95         typeof(d) __d = (d);                    \
  96         (void) (&__a == &__b);                  \
  97         (void) (&__a == __d);                   \
  98         *__d = __a - __b;                       \
  99         __a < __b;                              \
 100 })
 101 /*
 102  * If one of a or b is a compile-time constant, this avoids a division.
 103  */
 104 #define __unsigned_mul_overflow(a, b, d) ({             \
 105         typeof(a) __a = (a);                            \
 106         typeof(b) __b = (b);                            \
 107         typeof(d) __d = (d);                            \
 108         (void) (&__a == &__b);                          \
 109         (void) (&__a == __d);                           \
 110         *__d = __a * __b;                               \
 111         __builtin_constant_p(__b) ?                     \
 112           __b > 0 && __a > type_max(typeof(__a)) / __b : \
 113           __a > 0 && __b > type_max(typeof(__b)) / __a;  \
 114 })
 115
 116 /*
 117  * For signed types, detecting overflow is much harder, especially if
 118  * we want to avoid UB. But the interface of these macros is such that
 119  * we must provide a result in *d, and in fact we must produce the
 120  * result promised by gcc's builtins, which is simply the possibly
 121  * wrapped-around value. Fortunately, we can just formally do the
 122  * operations in the widest relevant unsigned type (u64) and then
 123  * truncate the result - gcc is smart enough to generate the same code
 124  * with and without the (u64) casts.
 125  */
 126
 127 /*
 128  * Adding two signed integers can overflow only if they have the same
 129  * sign, and overflow has happened iff the result has the opposite
 130  * sign.
 131  */
 132 #define __signed_add_overflow(a, b, d) ({       \
 133         typeof(a) __a = (a);                    \
 134         typeof(b) __b = (b);                    \
 135         typeof(d) __d = (d);                    \
 136         (void) (&__a == &__b);                  \
 137         (void) (&__a == __d);                   \
 138         *__d = (u64)__a + (u64)__b;             \
 139         (((~(__a ^ __b)) & (*__d ^ __a))        \
 140                 & type_min(typeof(__a))) != 0;  \
 141 })
 142
 143 /*
 144  * Subtraction is similar, except that overflow can now happen only
 145  * when the signs are opposite. In this case, overflow has happened if
 146  * the result has the opposite sign of a.
 147  */
 148 #define __signed_sub_overflow(a, b, d) ({       \
 149         typeof(a) __a = (a);                    \
 150         typeof(b) __b = (b);                    \
 151         typeof(d) __d = (d);                    \
 152         (void) (&__a == &__b);                  \
 153         (void) (&__a == __d);                   \
 154         *__d = (u64)__a - (u64)__b;             \
 155         ((((__a ^ __b)) & (*__d ^ __a))         \
 156                 & type_min(typeof(__a))) != 0;  \
 157 })
 158
 159 /*
 160  * Signed multiplication is rather hard. gcc always follows C99, so
 161  * division is truncated towards 0. This means that we can write the
 162  * overflow check like this:
 163  *
 164  * (a > 0 && (b > MAX/a || b < MIN/a)) ||
 165  * (a < -1 && (b > MIN/a || b < MAX/a) ||
 166  * (a == -1 && b == MIN)
 167  *
 168  * The redundant casts of -1 are to silence an annoying -Wtype-limits
 169  * (included in -Wextra) warning: When the type is u8 or u16, the
 170  * __b_c_e in check_mul_overflow obviously selects
 171  * __unsigned_mul_overflow, but unfortunately gcc still parses this
 172  * code and warns about the limited range of __b.
 173  */
 174
 175 #define __signed_mul_overflow(a, b, d) ({                               \
 176         typeof(a) __a = (a);                                            \
 177         typeof(b) __b = (b);                                            \
 178         typeof(d) __d = (d);                                            \
 179         typeof(a) __tmax = type_max(typeof(a));                         \
 180         typeof(a) __tmin = type_min(typeof(a));                         \
 181         (void) (&__a == &__b);                                          \
 182         (void) (&__a == __d);                                           \
 183         *__d = (u64)__a * (u64)__b;                                     \
 184         (__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||        \
 185         (__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
 186         (__b == (typeof(__b))-1 && __a == __tmin);                      \
 187 })
 188
 189
 190 #define check_add_overflow(a, b, d)                                     \
 191         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 192                         __signed_add_overflow(a, b, d),                 \
 193                         __unsigned_add_overflow(a, b, d))
 194
 195 #define check_sub_overflow(a, b, d)                                     \
 196         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 197                         __signed_sub_overflow(a, b, d),                 \
 198                         __unsigned_sub_overflow(a, b, d))
 199
 200 #define check_mul_overflow(a, b, d)                                     \
 201         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 202                         __signed_mul_overflow(a, b, d),                 \
 203                         __unsigned_mul_overflow(a, b, d))
 204
 205
 206 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
 207
 208 /** check_shl_overflow() - Calculate a left-shifted value and check overflow
 209  *
 210  * @a: Value to be shifted
 211  * @s: How many bits left to shift
 212  * @d: Pointer to where to store the result
 213  *
 214  * Computes *@d = (@a << @s)
 215  *
 216  * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
 217  * make sense. Example conditions:
 218  * - 'a << s' causes bits to be lost when stored in *d.
 219  * - 's' is garbage (e.g. negative) or so large that the result of
 220  *   'a << s' is guaranteed to be 0.
 221  * - 'a' is negative.
 222  * - 'a << s' sets the sign bit, if any, in '*d'.
 223  *
 224  * '*d' will hold the results of the attempted shift, but is not
 225  * considered "safe for use" if false is returned.
 226  */
 227 #define check_shl_overflow(a, s, d) ({                                  \
 228         typeof(a) _a = a;                                               \
 229         typeof(s) _s = s;                                               \
 230         typeof(d) _d = d;                                               \
 231         u64 _a_full = _a;                                               \
 232         unsigned int _to_shift =                                        \
 233                 _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0;                \
 234         *_d = (_a_full << _to_shift);                                   \
 235         (_to_shift != _s || *_d < 0 || _a < 0 ||                        \
 236                 (*_d >> _to_shift) != _a);                              \
 237 })
 238
 239 /**
 240  * array_size() - Calculate size of 2-dimensional array.
 241  *
 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 static inline __must_check size_t array_size(size_t a, size_t b)
 251 {
 252         size_t bytes;
 253
 254         if (check_mul_overflow(a, b, &bytes))
 255                 return SIZE_MAX;
 256
 257         return bytes;
 258 }
 259
 260 /**
 261  * array3_size() - Calculate size of 3-dimensional array.
 262  *
 263  * @a: dimension one
 264  * @b: dimension two
 265  * @c: dimension three
 266  *
 267  * Calculates size of 3-dimensional array: @a * @b * @c.
 268  *
 269  * Returns: number of bytes needed to represent the array or SIZE_MAX on
 270  * overflow.
 271  */
 272 static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
 273 {
 274         size_t bytes;
 275
 276         if (check_mul_overflow(a, b, &bytes))
 277                 return SIZE_MAX;
 278         if (check_mul_overflow(bytes, c, &bytes))
 279                 return SIZE_MAX;
 280
 281         return bytes;
 282 }
 283
 284 static inline __must_check size_t __ab_c_size(size_t n, size_t size, size_t c)
 285 {
 286         size_t bytes;
 287
 288         if (check_mul_overflow(n, size, &bytes))
 289                 return SIZE_MAX;
 290         if (check_add_overflow(bytes, c, &bytes))
 291                 return SIZE_MAX;
 292
 293         return bytes;
 294 }
 295
 296 /**
 297  * struct_size() - Calculate size of structure with trailing array.
 298  * @p: Pointer to the structure.
 299  * @member: Name of the array member.
 300  * @n: Number of elements in the array.
 301  *
 302  * Calculates size of memory needed for structure @p followed by an
 303  * array of @n @member elements.
 304  *
 305  * Return: number of bytes needed or SIZE_MAX on overflow.
 306  */
 307 #define struct_size(p, member, n)                                       \
 308         __ab_c_size(n,                                                  \
 309                     sizeof(*(p)->member) + __must_be_array((p)->member),\
 310                     sizeof(*(p)))
 311
 312 #endif /* __LINUX_OVERFLOW_H */
 313 #endif /* HAVE_OVERFLOW_H */