entropy_pool.cc (6718B)
1 // Copyright 2017 The Abseil Authors. 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); 4 // you may not use this file except in compliance with the License. 5 // You may obtain a copy of the License at 6 // 7 // https://www.apache.org/licenses/LICENSE-2.0 8 // 9 // Unless required by applicable law or agreed to in writing, software 10 // distributed under the License is distributed on an "AS IS" BASIS, 11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 // See the License for the specific language governing permissions and 13 // limitations under the License. 14 15 #include "absl/random/internal/entropy_pool.h" 16 17 #include <algorithm> 18 #include <atomic> 19 #include <cstdint> 20 #include <cstring> 21 #include <iterator> 22 23 #include "absl/base/attributes.h" 24 #include "absl/base/call_once.h" 25 #include "absl/base/config.h" 26 #include "absl/base/internal/spinlock.h" 27 #include "absl/base/optimization.h" 28 #include "absl/base/thread_annotations.h" 29 #include "absl/random/internal/randen.h" 30 #include "absl/random/internal/randen_traits.h" 31 #include "absl/random/internal/seed_material.h" 32 #include "absl/random/seed_gen_exception.h" 33 #include "absl/types/span.h" 34 35 using absl::base_internal::SpinLock; 36 using absl::base_internal::SpinLockHolder; 37 38 namespace absl { 39 ABSL_NAMESPACE_BEGIN 40 namespace random_internal { 41 namespace { 42 43 // RandenPoolEntry is a thread-safe pseudorandom bit generator, implementing a 44 // single generator within a RandenPool<T>. It is an internal implementation 45 // detail, and does not aim to conform to [rand.req.urng]. 46 // 47 // NOTE: There are alignment issues when used on ARM, for instance. 48 // See the allocation code in PoolAlignedAlloc(). 49 class RandenPoolEntry { 50 public: 51 static constexpr size_t kState = RandenTraits::kStateBytes / sizeof(uint32_t); 52 static constexpr size_t kCapacity = 53 RandenTraits::kCapacityBytes / sizeof(uint32_t); 54 55 void Init(absl::Span<const uint32_t> data) { 56 SpinLockHolder l(&mu_); // Always uncontested. 57 std::copy(data.begin(), data.end(), std::begin(state_)); 58 next_ = kState; 59 } 60 61 // Copy bytes into out. 62 void Fill(uint8_t* out, size_t bytes) ABSL_LOCKS_EXCLUDED(mu_); 63 64 inline void MaybeRefill() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) { 65 if (next_ >= kState) { 66 next_ = kCapacity; 67 impl_.Generate(state_); 68 } 69 } 70 71 inline size_t available() const ABSL_SHARED_LOCKS_REQUIRED(mu_) { 72 return kState - next_; 73 } 74 75 private: 76 // Randen URBG state. 77 uint32_t state_[kState] ABSL_GUARDED_BY(mu_); // First to satisfy alignment. 78 SpinLock mu_; 79 const Randen impl_; 80 size_t next_ ABSL_GUARDED_BY(mu_); 81 }; 82 83 void RandenPoolEntry::Fill(uint8_t* out, size_t bytes) { 84 SpinLockHolder l(&mu_); 85 while (bytes > 0) { 86 MaybeRefill(); 87 size_t remaining = available() * sizeof(state_[0]); 88 size_t to_copy = std::min(bytes, remaining); 89 std::memcpy(out, &state_[next_], to_copy); 90 out += to_copy; 91 bytes -= to_copy; 92 next_ += (to_copy + sizeof(state_[0]) - 1) / sizeof(state_[0]); 93 } 94 } 95 96 // Number of pooled urbg entries. 97 static constexpr size_t kPoolSize = 8; 98 99 // Shared pool entries. 100 static absl::once_flag pool_once; 101 ABSL_CACHELINE_ALIGNED static RandenPoolEntry* shared_pools[kPoolSize]; 102 103 // Returns an id in the range [0 ... kPoolSize), which indexes into the 104 // pool of random engines. 105 // 106 // Each thread to access the pool is assigned a sequential ID (without reuse) 107 // from the pool-id space; the id is cached in a thread_local variable. 108 // This id is assigned based on the arrival-order of the thread to the 109 // GetPoolID call; this has no binary, CL, or runtime stability because 110 // on subsequent runs the order within the same program may be significantly 111 // different. However, as other thread IDs are not assigned sequentially, 112 // this is not expected to matter. 113 size_t GetPoolID() { 114 static_assert(kPoolSize >= 1, 115 "At least one urbg instance is required for PoolURBG"); 116 117 ABSL_CONST_INIT static std::atomic<uint64_t> sequence{0}; 118 119 #ifdef ABSL_HAVE_THREAD_LOCAL 120 static thread_local size_t my_pool_id = kPoolSize; 121 if (ABSL_PREDICT_FALSE(my_pool_id == kPoolSize)) { 122 my_pool_id = (sequence++ % kPoolSize); 123 } 124 return my_pool_id; 125 #else 126 static pthread_key_t tid_key = [] { 127 pthread_key_t tmp_key; 128 int err = pthread_key_create(&tmp_key, nullptr); 129 if (err) { 130 ABSL_RAW_LOG(FATAL, "pthread_key_create failed with %d", err); 131 } 132 return tmp_key; 133 }(); 134 135 // Store the value in the pthread_{get/set}specific. However an uninitialized 136 // value is 0, so add +1 to distinguish from the null value. 137 uintptr_t my_pool_id = 138 reinterpret_cast<uintptr_t>(pthread_getspecific(tid_key)); 139 if (ABSL_PREDICT_FALSE(my_pool_id == 0)) { 140 // No allocated ID, allocate the next value, cache it, and return. 141 my_pool_id = (sequence++ % kPoolSize) + 1; 142 int err = pthread_setspecific(tid_key, reinterpret_cast<void*>(my_pool_id)); 143 if (err) { 144 ABSL_RAW_LOG(FATAL, "pthread_setspecific failed with %d", err); 145 } 146 } 147 return my_pool_id - 1; 148 #endif 149 } 150 151 // Allocate a RandenPoolEntry with at least 32-byte alignment, which is required 152 // by ARM platform code. 153 RandenPoolEntry* PoolAlignedAlloc() { 154 constexpr size_t kAlignment = 155 ABSL_CACHELINE_SIZE > 32 ? ABSL_CACHELINE_SIZE : 32; 156 157 // Not all the platforms that we build for have std::aligned_alloc, however 158 // since we never free these objects, we can over allocate and munge the 159 // pointers to the correct alignment. 160 uintptr_t x = reinterpret_cast<uintptr_t>( 161 new char[sizeof(RandenPoolEntry) + kAlignment]); 162 auto y = x % kAlignment; 163 void* aligned = reinterpret_cast<void*>(y == 0 ? x : (x + kAlignment - y)); 164 return new (aligned) RandenPoolEntry(); 165 } 166 167 // Allocate and initialize kPoolSize objects of type RandenPoolEntry. 168 void InitPoolURBG() { 169 static constexpr size_t kSeedSize = 170 RandenTraits::kStateBytes / sizeof(uint32_t); 171 // Read OS entropy once, and use it to initialize each pool entry. 172 uint32_t seed_material[kPoolSize * kSeedSize]; 173 if (!ReadSeedMaterialFromOSEntropy(absl::MakeSpan(seed_material))) { 174 ThrowSeedGenException(); 175 } 176 for (size_t i = 0; i < kPoolSize; i++) { 177 shared_pools[i] = PoolAlignedAlloc(); 178 shared_pools[i]->Init( 179 absl::MakeSpan(&seed_material[i * kSeedSize], kSeedSize)); 180 } 181 } 182 183 // Returns the pool entry for the current thread. 184 RandenPoolEntry* GetPoolForCurrentThread() { 185 absl::call_once(pool_once, InitPoolURBG); 186 return shared_pools[GetPoolID()]; 187 } 188 189 } // namespace 190 191 void GetEntropyFromRandenPool(void* dest, size_t bytes) { 192 auto* pool = GetPoolForCurrentThread(); 193 pool->Fill(reinterpret_cast<uint8_t*>(dest), bytes); 194 } 195 196 } // namespace random_internal 197 ABSL_NAMESPACE_END 198 } // namespace absl