base/numerics/safe_math.h

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef BASE_SAFE_MATH_H_
   6 #define BASE_SAFE_MATH_H_
   7
   8 #include "base/numerics/safe_math_impl.h"
   9
  10 namespace base {
  11
  12 namespace internal {
  13
  14 // CheckedNumeric implements all the logic and operators for detecting integer
  15 // boundary conditions such as overflow, underflow, and invalid conversions.
  16 // The CheckedNumeric type implicitly converts from floating point and integer
  17 // data types, and contains overloads for basic arithmetic operations (i.e.: +,
  18 // -, *, /, %).
  19 //
  20 // The following methods convert from CheckedNumeric to standard numeric values:
  21 // IsValid() - Returns true if the underlying numeric value is valid (i.e. has
  22 //             has not wrapped and is not the result of an invalid conversion).
  23 // ValueOrDie() - Returns the underlying value. If the state is not valid this
  24 //                call will crash on a CHECK.
  25 // ValueOrDefault() - Returns the current value, or the supplied default if the
  26 //                    state is not valid.
  27 // ValueFloating() - Returns the underlying floating point value (valid only
  28 //                   only for floating point CheckedNumeric types).
  29 //
  30 // Bitwise operations are explicitly not supported, because correct
  31 // handling of some cases (e.g. sign manipulation) is ambiguous. Comparison
  32 // operations are explicitly not supported because they could result in a crash
  33 // on a CHECK condition. You should use patterns like the following for these
  34 // operations:
  35 // Bitwise operation:
  36 //     CheckedNumeric<int> checked_int = untrusted_input_value;
  37 //     int x = checked_int.ValueOrDefault(0) | kFlagValues;
  38 // Comparison:
  39 //   CheckedNumeric<size_t> checked_size;
  40 //   CheckedNumeric<int> checked_size = untrusted_input_value;
  41 //   checked_size = checked_size + HEADER LENGTH;
  42 //   if (checked_size.IsValid() && checked_size.ValueOrDie() < buffer_size)
  43 //     Do stuff...
  44 template <typename T>
  45 class CheckedNumeric {
  46  public:
  47   typedef T type;
  48
  49   CheckedNumeric() {}
  50
  51   // Copy constructor.
  52   template <typename Src>
  53   CheckedNumeric(const CheckedNumeric<Src>& rhs)
  54       : state_(rhs.ValueUnsafe(), rhs.validity()) {}
  55
  56   template <typename Src>
  57   CheckedNumeric(Src value, RangeConstraint validity)
  58       : state_(value, validity) {}
  59
  60   // This is not an explicit constructor because we implicitly upgrade regular
  61   // numerics to CheckedNumerics to make them easier to use.
  62   template <typename Src>
  63   CheckedNumeric(Src value)
  64       : state_(value) {
  65     static_assert(std::numeric_limits<Src>::is_specialized,
  66                   "Argument must be numeric.");
  67   }
  68
  69   // IsValid() is the public API to test if a CheckedNumeric is currently valid.
  70   bool IsValid() const { return validity() == RANGE_VALID; }
  71
  72   // ValueOrDie() The primary accessor for the underlying value. If the current
  73   // state is not valid it will CHECK and crash.
  74   T ValueOrDie() const {
  75     CHECK(IsValid());
  76     return state_.value();
  77   }
  78
  79   // ValueOrDefault(T default_value) A convenience method that returns the
  80   // current value if the state is valid, and the supplied default_value for
  81   // any other state.
  82   T ValueOrDefault(T default_value) const {
  83     return IsValid() ? state_.value() : default_value;
  84   }
  85
  86   // ValueFloating() - Since floating point values include their validity state,
  87   // we provide an easy method for extracting them directly, without a risk of
  88   // crashing on a CHECK.
  89   T ValueFloating() const {
  90     static_assert(std::numeric_limits<T>::is_iec559, "Argument must be float.");
  91     return CheckedNumeric<T>::cast(*this).ValueUnsafe();
  92   }
  93
  94   // validity() - DO NOT USE THIS IN EXTERNAL CODE - It is public right now for
  95   // tests and to avoid a big matrix of friend operator overloads. But the
  96   // values it returns are likely to change in the future.
  97   // Returns: current validity state (i.e. valid, overflow, underflow, nan).
  98   // TODO(jschuh): crbug.com/332611 Figure out and implement semantics for
  99   // saturation/wrapping so we can expose this state consistently and implement
 100   // saturated arithmetic.
 101   RangeConstraint validity() const { return state_.validity(); }
 102
 103   // ValueUnsafe() - DO NOT USE THIS IN EXTERNAL CODE - It is public right now
 104   // for tests and to avoid a big matrix of friend operator overloads. But the
 105   // values it returns are likely to change in the future.
 106   // Returns: the raw numeric value, regardless of the current state.
 107   // TODO(jschuh): crbug.com/332611 Figure out and implement semantics for
 108   // saturation/wrapping so we can expose this state consistently and implement
 109   // saturated arithmetic.
 110   T ValueUnsafe() const { return state_.value(); }
 111
 112   // Prototypes for the supported arithmetic operator overloads.
 113   template <typename Src> CheckedNumeric& operator+=(Src rhs);
 114   template <typename Src> CheckedNumeric& operator-=(Src rhs);
 115   template <typename Src> CheckedNumeric& operator*=(Src rhs);
 116   template <typename Src> CheckedNumeric& operator/=(Src rhs);
 117   template <typename Src> CheckedNumeric& operator%=(Src rhs);
 118
 119   CheckedNumeric operator-() const {
 120     RangeConstraint validity;
 121     T value = CheckedNeg(state_.value(), &validity);
 122     // Negation is always valid for floating point.
 123     if (std::numeric_limits<T>::is_iec559)
 124       return CheckedNumeric<T>(value);
 125
 126     validity = GetRangeConstraint(state_.validity() | validity);
 127     return CheckedNumeric<T>(value, validity);
 128   }
 129
 130   CheckedNumeric Abs() const {
 131     RangeConstraint validity;
 132     T value = CheckedAbs(state_.value(), &validity);
 133     // Absolute value is always valid for floating point.
 134     if (std::numeric_limits<T>::is_iec559)
 135       return CheckedNumeric<T>(value);
 136
 137     validity = GetRangeConstraint(state_.validity() | validity);
 138     return CheckedNumeric<T>(value, validity);
 139   }
 140
 141   CheckedNumeric& operator++() {
 142     *this += 1;
 143     return *this;
 144   }
 145
 146   CheckedNumeric operator++(int) {
 147     CheckedNumeric value = *this;
 148     *this += 1;
 149     return value;
 150   }
 151
 152   CheckedNumeric& operator--() {
 153     *this -= 1;
 154     return *this;
 155   }
 156
 157   CheckedNumeric operator--(int) {
 158     CheckedNumeric value = *this;
 159     *this -= 1;
 160     return value;
 161   }
 162
 163   // These static methods behave like a convenience cast operator targeting
 164   // the desired CheckedNumeric type. As an optimization, a reference is
 165   // returned when Src is the same type as T.
 166   template <typename Src>
 167   static CheckedNumeric<T> cast(
 168       Src u,
 169       typename enable_if<std::numeric_limits<Src>::is_specialized, int>::type =
 170           0) {
 171     return u;
 172   }
 173
 174   template <typename Src>
 175   static CheckedNumeric<T> cast(
 176       const CheckedNumeric<Src>& u,
 177       typename enable_if<!is_same<Src, T>::value, int>::type = 0) {
 178     return u;
 179   }
 180
 181   static const CheckedNumeric<T>& cast(const CheckedNumeric<T>& u) { return u; }
 182
 183  private:
 184   CheckedNumericState<T> state_;
 185 };
 186
 187 // This is the boilerplate for the standard arithmetic operator overloads. A
 188 // macro isn't the prettiest solution, but it beats rewriting these five times.
 189 // Some details worth noting are:
 190 //  * We apply the standard arithmetic promotions.
 191 //  * We skip range checks for floating points.
 192 //  * We skip range checks for destination integers with sufficient range.
 193 // TODO(jschuh): extract these out into templates.
 194 #define BASE_NUMERIC_ARITHMETIC_OPERATORS(NAME, OP, COMPOUND_OP)              \
 195   /* Binary arithmetic operator for CheckedNumerics of the same type. */      \
 196   template <typename T>                                                       \
 197   CheckedNumeric<typename ArithmeticPromotion<T>::type> operator OP(          \
 198       const CheckedNumeric<T>& lhs, const CheckedNumeric<T>& rhs) {           \
 199     typedef typename ArithmeticPromotion<T>::type Promotion;                  \
 200     /* Floating point always takes the fast path */                           \
 201     if (std::numeric_limits<T>::is_iec559)                                    \
 202       return CheckedNumeric<T>(lhs.ValueUnsafe() OP rhs.ValueUnsafe());       \
 203     if (IsIntegerArithmeticSafe<Promotion, T, T>::value)                      \
 204       return CheckedNumeric<Promotion>(                                       \
 205           lhs.ValueUnsafe() OP rhs.ValueUnsafe(),                             \
 206           GetRangeConstraint(rhs.validity() | lhs.validity()));               \
 207     RangeConstraint validity = RANGE_VALID;                                   \
 208     T result = static_cast<T>(Checked##NAME(                                  \
 209         static_cast<Promotion>(lhs.ValueUnsafe()),                            \
 210         static_cast<Promotion>(rhs.ValueUnsafe()),                            \
 211         &validity));                                                          \
 212     return CheckedNumeric<Promotion>(                                         \
 213         result,                                                               \
 214         GetRangeConstraint(validity | lhs.validity() | rhs.validity()));      \
 215   }                                                                           \
 216   /* Assignment arithmetic operator implementation from CheckedNumeric. */    \
 217   template <typename T>                                                       \
 218   template <typename Src>                                                     \
 219   CheckedNumeric<T>& CheckedNumeric<T>::operator COMPOUND_OP(Src rhs) {       \
 220     *this = CheckedNumeric<T>::cast(*this) OP CheckedNumeric<Src>::cast(rhs); \
 221     return *this;                                                             \
 222   }                                                                           \
 223   /* Binary arithmetic operator for CheckedNumeric of different type. */      \
 224   template <typename T, typename Src>                                         \
 225   CheckedNumeric<typename ArithmeticPromotion<T, Src>::type> operator OP(     \
 226       const CheckedNumeric<Src>& lhs, const CheckedNumeric<T>& rhs) {         \
 227     typedef typename ArithmeticPromotion<T, Src>::type Promotion;             \
 228     if (IsIntegerArithmeticSafe<Promotion, T, Src>::value)                    \
 229       return CheckedNumeric<Promotion>(                                       \
 230           lhs.ValueUnsafe() OP rhs.ValueUnsafe(),                             \
 231           GetRangeConstraint(rhs.validity() | lhs.validity()));               \
 232     return CheckedNumeric<Promotion>::cast(lhs)                               \
 233         OP CheckedNumeric<Promotion>::cast(rhs);                              \
 234   }                                                                           \
 235   /* Binary arithmetic operator for left CheckedNumeric and right numeric. */ \
 236   template <typename T, typename Src>                                         \
 237   CheckedNumeric<typename ArithmeticPromotion<T, Src>::type> operator OP(     \
 238       const CheckedNumeric<T>& lhs, Src rhs) {                                \
 239     typedef typename ArithmeticPromotion<T, Src>::type Promotion;             \
 240     if (IsIntegerArithmeticSafe<Promotion, T, Src>::value)                    \
 241       return CheckedNumeric<Promotion>(lhs.ValueUnsafe() OP rhs,              \
 242                                        lhs.validity());                       \
 243     return CheckedNumeric<Promotion>::cast(lhs)                               \
 244         OP CheckedNumeric<Promotion>::cast(rhs);                              \
 245   }                                                                           \
 246   /* Binary arithmetic operator for right numeric and left CheckedNumeric. */ \
 247   template <typename T, typename Src>                                         \
 248   CheckedNumeric<typename ArithmeticPromotion<T, Src>::type> operator OP(     \
 249       Src lhs, const CheckedNumeric<T>& rhs) {                                \
 250     typedef typename ArithmeticPromotion<T, Src>::type Promotion;             \
 251     if (IsIntegerArithmeticSafe<Promotion, T, Src>::value)                    \
 252       return CheckedNumeric<Promotion>(lhs OP rhs.ValueUnsafe(),              \
 253                                        rhs.validity());                       \
 254     return CheckedNumeric<Promotion>::cast(lhs)                               \
 255         OP CheckedNumeric<Promotion>::cast(rhs);                              \
 256   }
 257
 258 BASE_NUMERIC_ARITHMETIC_OPERATORS(Add, +, += )
 259 BASE_NUMERIC_ARITHMETIC_OPERATORS(Sub, -, -= )
 260 BASE_NUMERIC_ARITHMETIC_OPERATORS(Mul, *, *= )
 261 BASE_NUMERIC_ARITHMETIC_OPERATORS(Div, /, /= )
 262 BASE_NUMERIC_ARITHMETIC_OPERATORS(Mod, %, %= )
 263
 264 #undef BASE_NUMERIC_ARITHMETIC_OPERATORS
 265
 266 }  // namespace internal
 267
 268 using internal::CheckedNumeric;
 269
 270 }  // namespace base
 271
 272 #endif  // BASE_SAFE_MATH_H_