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1 : : // Copyright (c) 2022 The Bitcoin Core developers
2 : : // Distributed under the MIT software license, see the accompanying
3 : : // file COPYING or http://www.opensource.org/licenses/mit-license.php.
4 : :
5 : : #include <node/eviction.h>
6 : :
7 : : #include <algorithm>
8 : : #include <array>
9 : : #include <chrono>
10 : : #include <cstdint>
11 : : #include <functional>
12 : : #include <map>
13 : : #include <vector>
14 : :
15 : :
16 : 0 : static bool ReverseCompareNodeMinPingTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
17 : : {
18 : 0 : return a.m_min_ping_time > b.m_min_ping_time;
19 : : }
20 : :
21 : 0 : static bool ReverseCompareNodeTimeConnected(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
22 : : {
23 : 0 : return a.m_connected > b.m_connected;
24 : : }
25 : :
26 : 0 : static bool CompareNetGroupKeyed(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b) {
27 : 0 : return a.nKeyedNetGroup < b.nKeyedNetGroup;
28 : : }
29 : :
30 : 0 : static bool CompareNodeBlockTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
31 : : {
32 : : // There is a fall-through here because it is common for a node to have many peers which have not yet relayed a block.
33 [ # # ]: 0 : if (a.m_last_block_time != b.m_last_block_time) return a.m_last_block_time < b.m_last_block_time;
34 [ # # ]: 0 : if (a.fRelevantServices != b.fRelevantServices) return b.fRelevantServices;
35 : 0 : return a.m_connected > b.m_connected;
36 : 0 : }
37 : :
38 : 0 : static bool CompareNodeTXTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
39 : : {
40 : : // There is a fall-through here because it is common for a node to have more than a few peers that have not yet relayed txn.
41 [ # # ]: 0 : if (a.m_last_tx_time != b.m_last_tx_time) return a.m_last_tx_time < b.m_last_tx_time;
42 [ # # ]: 0 : if (a.m_relay_txs != b.m_relay_txs) return b.m_relay_txs;
43 [ # # ]: 0 : if (a.fBloomFilter != b.fBloomFilter) return a.fBloomFilter;
44 : 0 : return a.m_connected > b.m_connected;
45 : 0 : }
46 : :
47 : : // Pick out the potential block-relay only peers, and sort them by last block time.
48 : 0 : static bool CompareNodeBlockRelayOnlyTime(const NodeEvictionCandidate &a, const NodeEvictionCandidate &b)
49 : : {
50 [ # # ]: 0 : if (a.m_relay_txs != b.m_relay_txs) return a.m_relay_txs;
51 [ # # ]: 0 : if (a.m_last_block_time != b.m_last_block_time) return a.m_last_block_time < b.m_last_block_time;
52 [ # # ]: 0 : if (a.fRelevantServices != b.fRelevantServices) return b.fRelevantServices;
53 : 0 : return a.m_connected > b.m_connected;
54 : 0 : }
55 : :
56 : : /**
57 : : * Sort eviction candidates by network/localhost and connection uptime.
58 : : * Candidates near the beginning are more likely to be evicted, and those
59 : : * near the end are more likely to be protected, e.g. less likely to be evicted.
60 : : * - First, nodes that are not `is_local` and that do not belong to `network`,
61 : : * sorted by increasing uptime (from most recently connected to connected longer).
62 : : * - Then, nodes that are `is_local` or belong to `network`, sorted by increasing uptime.
63 : : */
64 : : struct CompareNodeNetworkTime {
65 : : const bool m_is_local;
66 : : const Network m_network;
67 : 0 : CompareNodeNetworkTime(bool is_local, Network network) : m_is_local(is_local), m_network(network) {}
68 : 0 : bool operator()(const NodeEvictionCandidate& a, const NodeEvictionCandidate& b) const
69 : : {
70 [ # # ][ # # ]: 0 : if (m_is_local && a.m_is_local != b.m_is_local) return b.m_is_local;
71 [ # # ]: 0 : if ((a.m_network == m_network) != (b.m_network == m_network)) return b.m_network == m_network;
72 : 0 : return a.m_connected > b.m_connected;
73 : 0 : };
74 : : };
75 : :
76 : : //! Sort an array by the specified comparator, then erase the last K elements where predicate is true.
77 : : template <typename T, typename Comparator>
78 : 0 : static void EraseLastKElements(
79 : : std::vector<T>& elements, Comparator comparator, size_t k,
80 : 0 : std::function<bool(const NodeEvictionCandidate&)> predicate = [](const NodeEvictionCandidate& n) { return true; })
81 : : {
82 : 0 : std::sort(elements.begin(), elements.end(), comparator);
83 : 0 : size_t eraseSize = std::min(k, elements.size());
84 [ # # ][ # # ]: 0 : elements.erase(std::remove_if(elements.end() - eraseSize, elements.end(), predicate), elements.end());
[ # # ][ # # ]
85 : 0 : }
86 : :
87 : 0 : void ProtectNoBanConnections(std::vector<NodeEvictionCandidate>& eviction_candidates)
88 : : {
89 : 0 : eviction_candidates.erase(std::remove_if(eviction_candidates.begin(), eviction_candidates.end(),
90 : 0 : [](NodeEvictionCandidate const& n) {
91 : 0 : return n.m_noban;
92 : : }),
93 : 0 : eviction_candidates.end());
94 : 0 : }
95 : :
96 : 0 : void ProtectOutboundConnections(std::vector<NodeEvictionCandidate>& eviction_candidates)
97 : : {
98 : 0 : eviction_candidates.erase(std::remove_if(eviction_candidates.begin(), eviction_candidates.end(),
99 : 0 : [](NodeEvictionCandidate const& n) {
100 : 0 : return n.m_conn_type != ConnectionType::INBOUND;
101 : : }),
102 : 0 : eviction_candidates.end());
103 : 0 : }
104 : :
105 : 0 : void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& eviction_candidates)
106 : : {
107 : : // Protect the half of the remaining nodes which have been connected the longest.
108 : : // This replicates the non-eviction implicit behavior, and precludes attacks that start later.
109 : : // To favorise the diversity of our peer connections, reserve up to half of these protected
110 : : // spots for Tor/onion, localhost, I2P, and CJDNS peers, even if they're not longest uptime
111 : : // overall. This helps protect these higher-latency peers that tend to be otherwise
112 : : // disadvantaged under our eviction criteria.
113 : 0 : const size_t initial_size = eviction_candidates.size();
114 : 0 : const size_t total_protect_size{initial_size / 2};
115 : :
116 : : // Disadvantaged networks to protect. In the case of equal counts, earlier array members
117 : : // have the first opportunity to recover unused slots from the previous iteration.
118 : : struct Net { bool is_local; Network id; size_t count; };
119 : 0 : std::array<Net, 4> networks{
120 : : {{false, NET_CJDNS, 0}, {false, NET_I2P, 0}, {/*localhost=*/true, NET_MAX, 0}, {false, NET_ONION, 0}}};
121 : :
122 : : // Count and store the number of eviction candidates per network.
123 [ # # ]: 0 : for (Net& n : networks) {
124 : 0 : n.count = std::count_if(eviction_candidates.cbegin(), eviction_candidates.cend(),
125 : 0 : [&n](const NodeEvictionCandidate& c) {
126 [ # # ]: 0 : return n.is_local ? c.m_is_local : c.m_network == n.id;
127 : : });
128 : : }
129 : : // Sort `networks` by ascending candidate count, to give networks having fewer candidates
130 : : // the first opportunity to recover unused protected slots from the previous iteration.
131 : 0 : std::stable_sort(networks.begin(), networks.end(), [](Net a, Net b) { return a.count < b.count; });
132 : :
133 : : // Protect up to 25% of the eviction candidates by disadvantaged network.
134 : 0 : const size_t max_protect_by_network{total_protect_size / 2};
135 : 0 : size_t num_protected{0};
136 : :
137 [ # # ]: 0 : while (num_protected < max_protect_by_network) {
138 : : // Count the number of disadvantaged networks from which we have peers to protect.
139 : 0 : auto num_networks = std::count_if(networks.begin(), networks.end(), [](const Net& n) { return n.count; });
140 [ # # ]: 0 : if (num_networks == 0) {
141 : 0 : break;
142 : : }
143 : 0 : const size_t disadvantaged_to_protect{max_protect_by_network - num_protected};
144 : 0 : const size_t protect_per_network{std::max(disadvantaged_to_protect / num_networks, static_cast<size_t>(1))};
145 : : // Early exit flag if there are no remaining candidates by disadvantaged network.
146 : 0 : bool protected_at_least_one{false};
147 : :
148 [ # # ]: 0 : for (Net& n : networks) {
149 [ # # ]: 0 : if (n.count == 0) continue;
150 : 0 : const size_t before = eviction_candidates.size();
151 [ # # ][ # # ]: 0 : EraseLastKElements(eviction_candidates, CompareNodeNetworkTime(n.is_local, n.id),
152 : 0 : protect_per_network, [&n](const NodeEvictionCandidate& c) {
153 [ # # ]: 0 : return n.is_local ? c.m_is_local : c.m_network == n.id;
154 : : });
155 : 0 : const size_t after = eviction_candidates.size();
156 [ # # ]: 0 : if (before > after) {
157 : 0 : protected_at_least_one = true;
158 : 0 : const size_t delta{before - after};
159 : 0 : num_protected += delta;
160 [ # # ]: 0 : if (num_protected >= max_protect_by_network) {
161 : 0 : break;
162 : : }
163 : 0 : n.count -= delta;
164 : 0 : }
165 : : }
166 [ # # ]: 0 : if (!protected_at_least_one) {
167 : 0 : break;
168 : : }
169 : : }
170 : :
171 : : // Calculate how many we removed, and update our total number of peers that
172 : : // we want to protect based on uptime accordingly.
173 [ # # ]: 0 : assert(num_protected == initial_size - eviction_candidates.size());
174 : 0 : const size_t remaining_to_protect{total_protect_size - num_protected};
175 [ # # ]: 0 : EraseLastKElements(eviction_candidates, ReverseCompareNodeTimeConnected, remaining_to_protect);
176 : 0 : }
177 : :
178 : 0 : [[nodiscard]] std::optional<NodeId> SelectNodeToEvict(std::vector<NodeEvictionCandidate>&& vEvictionCandidates)
179 : : {
180 : : // Protect connections with certain characteristics
181 : :
182 : 0 : ProtectNoBanConnections(vEvictionCandidates);
183 : :
184 : 0 : ProtectOutboundConnections(vEvictionCandidates);
185 : :
186 : : // Deterministically select 4 peers to protect by netgroup.
187 : : // An attacker cannot predict which netgroups will be protected
188 [ # # ]: 0 : EraseLastKElements(vEvictionCandidates, CompareNetGroupKeyed, 4);
189 : : // Protect the 8 nodes with the lowest minimum ping time.
190 : : // An attacker cannot manipulate this metric without physically moving nodes closer to the target.
191 [ # # ]: 0 : EraseLastKElements(vEvictionCandidates, ReverseCompareNodeMinPingTime, 8);
192 : : // Protect 4 nodes that most recently sent us novel transactions accepted into our mempool.
193 : : // An attacker cannot manipulate this metric without performing useful work.
194 [ # # ]: 0 : EraseLastKElements(vEvictionCandidates, CompareNodeTXTime, 4);
195 : : // Protect up to 8 non-tx-relay peers that have sent us novel blocks.
196 [ # # ][ # # ]: 0 : EraseLastKElements(vEvictionCandidates, CompareNodeBlockRelayOnlyTime, 8,
197 [ # # ]: 0 : [](const NodeEvictionCandidate& n) { return !n.m_relay_txs && n.fRelevantServices; });
198 : :
199 : : // Protect 4 nodes that most recently sent us novel blocks.
200 : : // An attacker cannot manipulate this metric without performing useful work.
201 [ # # ]: 0 : EraseLastKElements(vEvictionCandidates, CompareNodeBlockTime, 4);
202 : :
203 : : // Protect some of the remaining eviction candidates by ratios of desirable
204 : : // or disadvantaged characteristics.
205 : 0 : ProtectEvictionCandidatesByRatio(vEvictionCandidates);
206 : :
207 [ # # ]: 0 : if (vEvictionCandidates.empty()) return std::nullopt;
208 : :
209 : : // If any remaining peers are preferred for eviction consider only them.
210 : : // This happens after the other preferences since if a peer is really the best by other criteria (esp relaying blocks)
211 : : // then we probably don't want to evict it no matter what.
212 [ # # ]: 0 : if (std::any_of(vEvictionCandidates.begin(),vEvictionCandidates.end(),[](NodeEvictionCandidate const &n){return n.prefer_evict;})) {
213 : 0 : vEvictionCandidates.erase(std::remove_if(vEvictionCandidates.begin(),vEvictionCandidates.end(),
214 : 0 : [](NodeEvictionCandidate const &n){return !n.prefer_evict;}),vEvictionCandidates.end());
215 : 0 : }
216 : :
217 : : // Identify the network group with the most connections and youngest member.
218 : : // (vEvictionCandidates is already sorted by reverse connect time)
219 : : uint64_t naMostConnections;
220 : 0 : unsigned int nMostConnections = 0;
221 : 0 : std::chrono::seconds nMostConnectionsTime{0};
222 : 0 : std::map<uint64_t, std::vector<NodeEvictionCandidate> > mapNetGroupNodes;
223 [ # # ]: 0 : for (const NodeEvictionCandidate &node : vEvictionCandidates) {
224 [ # # ]: 0 : std::vector<NodeEvictionCandidate> &group = mapNetGroupNodes[node.nKeyedNetGroup];
225 [ # # ]: 0 : group.push_back(node);
226 : 0 : const auto grouptime{group[0].m_connected};
227 : :
228 [ # # ][ # # ]: 0 : if (group.size() > nMostConnections || (group.size() == nMostConnections && grouptime > nMostConnectionsTime)) {
[ # # ][ # # ]
229 : 0 : nMostConnections = group.size();
230 : 0 : nMostConnectionsTime = grouptime;
231 : 0 : naMostConnections = node.nKeyedNetGroup;
232 : 0 : }
233 : : }
234 : :
235 : : // Reduce to the network group with the most connections
236 [ # # ]: 0 : vEvictionCandidates = std::move(mapNetGroupNodes[naMostConnections]);
237 : :
238 : : // Disconnect from the network group with the most connections
239 : 0 : return vEvictionCandidates.front().id;
240 : 0 : }
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