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 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
  41 /*
  42  * For simplicity and code hygiene, the fallback code below insists on
  43  * a, b and *d having the same type (similar to the min() and max()
  44  * macros), whereas gcc's type-generic overflow checkers accept
  45  * different types. Hence we don't just make check_add_overflow an
  46  * alias for __builtin_add_overflow, but add type checks similar to
  47  * below.
  48  */
  49 #define check_add_overflow(a, b, d) ({          \
  50         typeof(a) __a = (a);                    \
  51         typeof(b) __b = (b);                    \
  52         typeof(d) __d = (d);                    \
  53         (void) (&__a == &__b);                  \
  54         (void) (&__a == __d);                   \
  55         __builtin_add_overflow(__a, __b, __d);  \
  56 })
  57
  58 #define check_sub_overflow(a, b, d) ({          \
  59         typeof(a) __a = (a);                    \
  60         typeof(b) __b = (b);                    \
  61         typeof(d) __d = (d);                    \
  62         (void) (&__a == &__b);                  \
  63         (void) (&__a == __d);                   \
  64         __builtin_sub_overflow(__a, __b, __d);  \
  65 })
  66
  67 #define check_mul_overflow(a, b, d) ({          \
  68         typeof(a) __a = (a);                    \
  69         typeof(b) __b = (b);                    \
  70         typeof(d) __d = (d);                    \
  71         (void) (&__a == &__b);                  \
  72         (void) (&__a == __d);                   \
  73         __builtin_mul_overflow(__a, __b, __d);  \
  74 })
  75
  76 #else
  77
  78
  79 /* Checking for unsigned overflow is relatively easy without causing UB. */
  80 #define __unsigned_add_overflow(a, b, d) ({     \
  81         typeof(a) __a = (a);                    \
  82         typeof(b) __b = (b);                    \
  83         typeof(d) __d = (d);                    \
  84         (void) (&__a == &__b);                  \
  85         (void) (&__a == __d);                   \
  86         *__d = __a + __b;                       \
  87         *__d < __a;                             \
  88 })
  89 #define __unsigned_sub_overflow(a, b, d) ({     \
  90         typeof(a) __a = (a);                    \
  91         typeof(b) __b = (b);                    \
  92         typeof(d) __d = (d);                    \
  93         (void) (&__a == &__b);                  \
  94         (void) (&__a == __d);                   \
  95         *__d = __a - __b;                       \
  96         __a < __b;                              \
  97 })
  98 /*
  99  * If one of a or b is a compile-time constant, this avoids a division.
 100  */
 101 #define __unsigned_mul_overflow(a, b, d) ({             \
 102         typeof(a) __a = (a);                            \
 103         typeof(b) __b = (b);                            \
 104         typeof(d) __d = (d);                            \
 105         (void) (&__a == &__b);                          \
 106         (void) (&__a == __d);                           \
 107         *__d = __a * __b;                               \
 108         __builtin_constant_p(__b) ?                     \
 109           __b > 0 && __a > type_max(typeof(__a)) / __b : \
 110           __a > 0 && __b > type_max(typeof(__b)) / __a;  \
 111 })
 112
 113 /*
 114  * For signed types, detecting overflow is much harder, especially if
 115  * we want to avoid UB. But the interface of these macros is such that
 116  * we must provide a result in *d, and in fact we must produce the
 117  * result promised by gcc's builtins, which is simply the possibly
 118  * wrapped-around value. Fortunately, we can just formally do the
 119  * operations in the widest relevant unsigned type (u64) and then
 120  * truncate the result - gcc is smart enough to generate the same code
 121  * with and without the (u64) casts.
 122  */
 123
 124 /*
 125  * Adding two signed integers can overflow only if they have the same
 126  * sign, and overflow has happened iff the result has the opposite
 127  * sign.
 128  */
 129 #define __signed_add_overflow(a, b, d) ({       \
 130         typeof(a) __a = (a);                    \
 131         typeof(b) __b = (b);                    \
 132         typeof(d) __d = (d);                    \
 133         (void) (&__a == &__b);                  \
 134         (void) (&__a == __d);                   \
 135         *__d = (u64)__a + (u64)__b;             \
 136         (((~(__a ^ __b)) & (*__d ^ __a))        \
 137                 & type_min(typeof(__a))) != 0;  \
 138 })
 139
 140 /*
 141  * Subtraction is similar, except that overflow can now happen only
 142  * when the signs are opposite. In this case, overflow has happened if
 143  * the result has the opposite sign of a.
 144  */
 145 #define __signed_sub_overflow(a, b, d) ({       \
 146         typeof(a) __a = (a);                    \
 147         typeof(b) __b = (b);                    \
 148         typeof(d) __d = (d);                    \
 149         (void) (&__a == &__b);                  \
 150         (void) (&__a == __d);                   \
 151         *__d = (u64)__a - (u64)__b;             \
 152         ((((__a ^ __b)) & (*__d ^ __a))         \
 153                 & type_min(typeof(__a))) != 0;  \
 154 })
 155
 156 /*
 157  * Signed multiplication is rather hard. gcc always follows C99, so
 158  * division is truncated towards 0. This means that we can write the
 159  * overflow check like this:
 160  *
 161  * (a > 0 && (b > MAX/a || b < MIN/a)) ||
 162  * (a < -1 && (b > MIN/a || b < MAX/a) ||
 163  * (a == -1 && b == MIN)
 164  *
 165  * The redundant casts of -1 are to silence an annoying -Wtype-limits
 166  * (included in -Wextra) warning: When the type is u8 or u16, the
 167  * __b_c_e in check_mul_overflow obviously selects
 168  * __unsigned_mul_overflow, but unfortunately gcc still parses this
 169  * code and warns about the limited range of __b.
 170  */
 171
 172 #define __signed_mul_overflow(a, b, d) ({                               \
 173         typeof(a) __a = (a);                                            \
 174         typeof(b) __b = (b);                                            \
 175         typeof(d) __d = (d);                                            \
 176         typeof(a) __tmax = type_max(typeof(a));                         \
 177         typeof(a) __tmin = type_min(typeof(a));                         \
 178         (void) (&__a == &__b);                                          \
 179         (void) (&__a == __d);                                           \
 180         *__d = (u64)__a * (u64)__b;                                     \
 181         (__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||        \
 182         (__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
 183         (__b == (typeof(__b))-1 && __a == __tmin);                      \
 184 })
 185
 186
 187 #define check_add_overflow(a, b, d)                                     \
 188         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 189                         __signed_add_overflow(a, b, d),                 \
 190                         __unsigned_add_overflow(a, b, d))
 191
 192 #define check_sub_overflow(a, b, d)                                     \
 193         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 194                         __signed_sub_overflow(a, b, d),                 \
 195                         __unsigned_sub_overflow(a, b, d))
 196
 197 #define check_mul_overflow(a, b, d)                                     \
 198         __builtin_choose_expr(is_signed_type(typeof(a)),                \
 199                         __signed_mul_overflow(a, b, d),                 \
 200                         __unsigned_mul_overflow(a, b, d))
 201
 202
 203 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
 204
 205 #endif /* __LINUX_OVERFLOW_H */