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Diffstat (limited to 'mfbt/HashFunctions.h')
| -rw-r--r-- | mfbt/HashFunctions.h | 389 |
1 files changed, 389 insertions, 0 deletions
diff --git a/mfbt/HashFunctions.h b/mfbt/HashFunctions.h new file mode 100644 index 000000000..eeb192c36 --- /dev/null +++ b/mfbt/HashFunctions.h @@ -0,0 +1,389 @@ +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */ +/* vim: set ts=8 sts=2 et sw=2 tw=80: */ +/* This Source Code Form is subject to the terms of the Mozilla Public + * License, v. 2.0. If a copy of the MPL was not distributed with this + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */ + +/* Utilities for hashing. */ + +/* + * This file exports functions for hashing data down to a 32-bit value, + * including: + * + * - HashString Hash a char* or char16_t/wchar_t* of known or unknown + * length. + * + * - HashBytes Hash a byte array of known length. + * + * - HashGeneric Hash one or more values. Currently, we support uint32_t, + * types which can be implicitly cast to uint32_t, data + * pointers, and function pointers. + * + * - AddToHash Add one or more values to the given hash. This supports the + * same list of types as HashGeneric. + * + * + * You can chain these functions together to hash complex objects. For example: + * + * class ComplexObject + * { + * char* mStr; + * uint32_t mUint1, mUint2; + * void (*mCallbackFn)(); + * + * public: + * uint32_t hash() + * { + * uint32_t hash = HashString(mStr); + * hash = AddToHash(hash, mUint1, mUint2); + * return AddToHash(hash, mCallbackFn); + * } + * }; + * + * If you want to hash an nsAString or nsACString, use the HashString functions + * in nsHashKeys.h. + */ + +#ifndef mozilla_HashFunctions_h +#define mozilla_HashFunctions_h + +#include "mozilla/Assertions.h" +#include "mozilla/Attributes.h" +#include "mozilla/Char16.h" +#include "mozilla/MathAlgorithms.h" +#include "mozilla/Types.h" + +#include <stdint.h> + +#ifdef __cplusplus +namespace mozilla { + +/** + * The golden ratio as a 32-bit fixed-point value. + */ +static const uint32_t kGoldenRatioU32 = 0x9E3779B9U; + +inline uint32_t +RotateBitsLeft32(uint32_t aValue, uint8_t aBits) +{ + MOZ_ASSERT(aBits < 32); + return (aValue << aBits) | (aValue >> (32 - aBits)); +} + +namespace detail { + +inline uint32_t +AddU32ToHash(uint32_t aHash, uint32_t aValue) +{ + /* + * This is the meat of all our hash routines. This hash function is not + * particularly sophisticated, but it seems to work well for our mostly + * plain-text inputs. Implementation notes follow. + * + * Our use of the golden ratio here is arbitrary; we could pick almost any + * number which: + * + * * is odd (because otherwise, all our hash values will be even) + * + * * has a reasonably-even mix of 1's and 0's (consider the extreme case + * where we multiply by 0x3 or 0xeffffff -- this will not produce good + * mixing across all bits of the hash). + * + * The rotation length of 5 is also arbitrary, although an odd number is again + * preferable so our hash explores the whole universe of possible rotations. + * + * Finally, we multiply by the golden ratio *after* xor'ing, not before. + * Otherwise, if |aHash| is 0 (as it often is for the beginning of a + * message), the expression + * + * (kGoldenRatioU32 * RotateBitsLeft(aHash, 5)) |xor| aValue + * + * evaluates to |aValue|. + * + * (Number-theoretic aside: Because any odd number |m| is relatively prime to + * our modulus (2^32), the list + * + * [x * m (mod 2^32) for 0 <= x < 2^32] + * + * has no duplicate elements. This means that multiplying by |m| does not + * cause us to skip any possible hash values. + * + * It's also nice if |m| has large-ish order mod 2^32 -- that is, if the + * smallest k such that m^k == 1 (mod 2^32) is large -- so we can safely + * multiply our hash value by |m| a few times without negating the + * multiplicative effect. Our golden ratio constant has order 2^29, which is + * more than enough for our purposes.) + */ + return kGoldenRatioU32 * (RotateBitsLeft32(aHash, 5) ^ aValue); +} + +/** + * AddUintptrToHash takes sizeof(uintptr_t) as a template parameter. + */ +template<size_t PtrSize> +inline uint32_t +AddUintptrToHash(uint32_t aHash, uintptr_t aValue); + +template<> +inline uint32_t +AddUintptrToHash<4>(uint32_t aHash, uintptr_t aValue) +{ + return AddU32ToHash(aHash, static_cast<uint32_t>(aValue)); +} + +template<> +inline uint32_t +AddUintptrToHash<8>(uint32_t aHash, uintptr_t aValue) +{ + /* + * The static cast to uint64_t below is necessary because this function + * sometimes gets compiled on 32-bit platforms (yes, even though it's a + * template and we never call this particular override in a 32-bit build). If + * we do aValue >> 32 on a 32-bit machine, we're shifting a 32-bit uintptr_t + * right 32 bits, and the compiler throws an error. + */ + uint32_t v1 = static_cast<uint32_t>(aValue); + uint32_t v2 = static_cast<uint32_t>(static_cast<uint64_t>(aValue) >> 32); + return AddU32ToHash(AddU32ToHash(aHash, v1), v2); +} + +} /* namespace detail */ + +/** + * AddToHash takes a hash and some values and returns a new hash based on the + * inputs. + * + * Currently, we support hashing uint32_t's, values which we can implicitly + * convert to uint32_t, data pointers, and function pointers. + */ +template<typename A> +MOZ_MUST_USE inline uint32_t +AddToHash(uint32_t aHash, A aA) +{ + /* + * Try to convert |A| to uint32_t implicitly. If this works, great. If not, + * we'll error out. + */ + return detail::AddU32ToHash(aHash, aA); +} + +template<typename A> +MOZ_MUST_USE inline uint32_t +AddToHash(uint32_t aHash, A* aA) +{ + /* + * You might think this function should just take a void*. But then we'd only + * catch data pointers and couldn't handle function pointers. + */ + + static_assert(sizeof(aA) == sizeof(uintptr_t), "Strange pointer!"); + + return detail::AddUintptrToHash<sizeof(uintptr_t)>(aHash, uintptr_t(aA)); +} + +template<> +MOZ_MUST_USE inline uint32_t +AddToHash(uint32_t aHash, uintptr_t aA) +{ + return detail::AddUintptrToHash<sizeof(uintptr_t)>(aHash, aA); +} + +template<typename A, typename... Args> +MOZ_MUST_USE uint32_t +AddToHash(uint32_t aHash, A aArg, Args... aArgs) +{ + return AddToHash(AddToHash(aHash, aArg), aArgs...); +} + +/** + * The HashGeneric class of functions let you hash one or more values. + * + * If you want to hash together two values x and y, calling HashGeneric(x, y) is + * much better than calling AddToHash(x, y), because AddToHash(x, y) assumes + * that x has already been hashed. + */ +template<typename... Args> +MOZ_MUST_USE inline uint32_t +HashGeneric(Args... aArgs) +{ + return AddToHash(0, aArgs...); +} + +namespace detail { + +template<typename T> +uint32_t +HashUntilZero(const T* aStr) +{ + uint32_t hash = 0; + for (T c; (c = *aStr); aStr++) { + hash = AddToHash(hash, c); + } + return hash; +} + +template<typename T> +uint32_t +HashKnownLength(const T* aStr, size_t aLength) +{ + uint32_t hash = 0; + for (size_t i = 0; i < aLength; i++) { + hash = AddToHash(hash, aStr[i]); + } + return hash; +} + +} /* namespace detail */ + +/** + * The HashString overloads below do just what you'd expect. + * + * If you have the string's length, you might as well call the overload which + * includes the length. It may be marginally faster. + */ +MOZ_MUST_USE inline uint32_t +HashString(const char* aStr) +{ + return detail::HashUntilZero(reinterpret_cast<const unsigned char*>(aStr)); +} + +MOZ_MUST_USE inline uint32_t +HashString(const char* aStr, size_t aLength) +{ + return detail::HashKnownLength(reinterpret_cast<const unsigned char*>(aStr), aLength); +} + +MOZ_MUST_USE +inline uint32_t +HashString(const unsigned char* aStr, size_t aLength) +{ + return detail::HashKnownLength(aStr, aLength); +} + +MOZ_MUST_USE inline uint32_t +HashString(const char16_t* aStr) +{ + return detail::HashUntilZero(aStr); +} + +MOZ_MUST_USE inline uint32_t +HashString(const char16_t* aStr, size_t aLength) +{ + return detail::HashKnownLength(aStr, aLength); +} + +/* + * On Windows, wchar_t is not the same as char16_t, even though it's + * the same width! + */ +#ifdef WIN32 +MOZ_MUST_USE inline uint32_t +HashString(const wchar_t* aStr) +{ + return detail::HashUntilZero(aStr); +} + +MOZ_MUST_USE inline uint32_t +HashString(const wchar_t* aStr, size_t aLength) +{ + return detail::HashKnownLength(aStr, aLength); +} +#endif + +/** + * Hash some number of bytes. + * + * This hash walks word-by-word, rather than byte-by-byte, so you won't get the + * same result out of HashBytes as you would out of HashString. + */ +MOZ_MUST_USE extern MFBT_API uint32_t +HashBytes(const void* bytes, size_t aLength); + +/** + * A pseudorandom function mapping 32-bit integers to 32-bit integers. + * + * This is for when you're feeding private data (like pointer values or credit + * card numbers) to a non-crypto hash function (like HashBytes) and then using + * the hash code for something that untrusted parties could observe (like a JS + * Map). Plug in a HashCodeScrambler before that last step to avoid leaking the + * private data. + * + * By itself, this does not prevent hash-flooding DoS attacks, because an + * attacker can still generate many values with exactly equal hash codes by + * attacking the non-crypto hash function alone. Equal hash codes will, of + * course, still be equal however much you scramble them. + * + * The algorithm is SipHash-1-3. See <https://131002.net/siphash/>. + */ +class HashCodeScrambler +{ + struct SipHasher; + + uint64_t mK0, mK1; + +public: + /** Creates a new scrambler with the given 128-bit key. */ + constexpr HashCodeScrambler(uint64_t aK0, uint64_t aK1) : mK0(aK0), mK1(aK1) {} + + /** + * Scramble a hash code. Always produces the same result for the same + * combination of key and hash code. + */ + uint32_t scramble(uint32_t aHashCode) const + { + SipHasher hasher(mK0, mK1); + return uint32_t(hasher.sipHash(aHashCode)); + } + +private: + struct SipHasher + { + SipHasher(uint64_t aK0, uint64_t aK1) + { + // 1. Initialization. + mV0 = aK0 ^ UINT64_C(0x736f6d6570736575); + mV1 = aK1 ^ UINT64_C(0x646f72616e646f6d); + mV2 = aK0 ^ UINT64_C(0x6c7967656e657261); + mV3 = aK1 ^ UINT64_C(0x7465646279746573); + } + + uint64_t sipHash(uint64_t aM) + { + // 2. Compression. + mV3 ^= aM; + sipRound(); + mV0 ^= aM; + + // 3. Finalization. + mV2 ^= 0xff; + for (int i = 0; i < 3; i++) + sipRound(); + return mV0 ^ mV1 ^ mV2 ^ mV3; + } + + void sipRound() + { + mV0 += mV1; + mV1 = RotateLeft(mV1, 13); + mV1 ^= mV0; + mV0 = RotateLeft(mV0, 32); + mV2 += mV3; + mV3 = RotateLeft(mV3, 16); + mV3 ^= mV2; + mV0 += mV3; + mV3 = RotateLeft(mV3, 21); + mV3 ^= mV0; + mV2 += mV1; + mV1 = RotateLeft(mV1, 17); + mV1 ^= mV2; + mV2 = RotateLeft(mV2, 32); + } + + uint64_t mV0, mV1, mV2, mV3; + }; +}; + +} /* namespace mozilla */ +#endif /* __cplusplus */ + +#endif /* mozilla_HashFunctions_h */ |