Line data Source code
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 <headerssync.h>
6 : #include <logging.h>
7 : #include <pow.h>
8 : #include <timedata.h>
9 : #include <util/check.h>
10 : #include <util/vector.h>
11 :
12 : // The two constants below are computed using the simulation script on
13 : // https://gist.github.com/sipa/016ae445c132cdf65a2791534dfb7ae1
14 :
15 : //! Store a commitment to a header every HEADER_COMMITMENT_PERIOD blocks.
16 : constexpr size_t HEADER_COMMITMENT_PERIOD{584};
17 2 :
18 2 : //! Only feed headers to validation once this many headers on top have been
19 : //! received and validated against commitments.
20 : constexpr size_t REDOWNLOAD_BUFFER_SIZE{13959}; // 13959/584 = ~23.9 commitments
21 :
22 : // Our memory analysis assumes 48 bytes for a CompressedHeader (so we should
23 : // re-calculate parameters if we compress further)
24 : static_assert(sizeof(CompressedHeader) == 48);
25 :
26 0 : HeadersSyncState::HeadersSyncState(NodeId id, const Consensus::Params& consensus_params,
27 : const CBlockIndex* chain_start, const arith_uint256& minimum_required_work) :
28 0 : m_commit_offset(GetRand<unsigned>(HEADER_COMMITMENT_PERIOD)),
29 0 : m_id(id), m_consensus_params(consensus_params),
30 0 : m_chain_start(chain_start),
31 0 : m_minimum_required_work(minimum_required_work),
32 0 : m_current_chain_work(chain_start->nChainWork),
33 0 : m_last_header_received(m_chain_start->GetBlockHeader()),
34 0 : m_current_height(chain_start->nHeight)
35 : {
36 : // Estimate the number of blocks that could possibly exist on the peer's
37 : // chain *right now* using 6 blocks/second (fastest blockrate given the MTP
38 : // rule) times the number of seconds from the last allowed block until
39 : // today. This serves as a memory bound on how many commitments we might
40 : // store from this peer, and we can safely give up syncing if the peer
41 : // exceeds this bound, because it's not possible for a consensus-valid
42 : // chain to be longer than this (at the current time -- in the future we
43 : // could try again, if necessary, to sync a longer chain).
44 0 : m_max_commitments = 6*(Ticks<std::chrono::seconds>(GetAdjustedTime() - NodeSeconds{std::chrono::seconds{chain_start->GetMedianTimePast()}}) + MAX_FUTURE_BLOCK_TIME) / HEADER_COMMITMENT_PERIOD;
45 :
46 0 : LogPrint(BCLog::NET, "Initial headers sync started with peer=%d: height=%i, max_commitments=%i, min_work=%s\n", m_id, m_current_height, m_max_commitments, m_minimum_required_work.ToString());
47 0 : }
48 :
49 : /** Free any memory in use, and mark this object as no longer usable. This is
50 : * required to guarantee that we won't reuse this object with the same
51 : * SaltedTxidHasher for another sync. */
52 0 : void HeadersSyncState::Finalize()
53 : {
54 0 : Assume(m_download_state != State::FINAL);
55 0 : ClearShrink(m_header_commitments);
56 0 : m_last_header_received.SetNull();
57 0 : ClearShrink(m_redownloaded_headers);
58 0 : m_redownload_buffer_last_hash.SetNull();
59 0 : m_redownload_buffer_first_prev_hash.SetNull();
60 0 : m_process_all_remaining_headers = false;
61 0 : m_current_height = 0;
62 :
63 0 : m_download_state = State::FINAL;
64 0 : }
65 :
66 : /** Process the next batch of headers received from our peer.
67 : * Validate and store commitments, and compare total chainwork to our target to
68 : * see if we can switch to REDOWNLOAD mode. */
69 0 : HeadersSyncState::ProcessingResult HeadersSyncState::ProcessNextHeaders(const
70 : std::vector<CBlockHeader>& received_headers, const bool full_headers_message)
71 : {
72 0 : ProcessingResult ret;
73 :
74 2 : Assume(!received_headers.empty());
75 0 : if (received_headers.empty()) return ret;
76 :
77 0 : Assume(m_download_state != State::FINAL);
78 0 : if (m_download_state == State::FINAL) return ret;
79 :
80 0 : if (m_download_state == State::PRESYNC) {
81 : // During PRESYNC, we minimally validate block headers and
82 : // occasionally add commitments to them, until we reach our work
83 2 : // threshold (at which point m_download_state is updated to REDOWNLOAD).
84 0 : ret.success = ValidateAndStoreHeadersCommitments(received_headers);
85 0 : if (ret.success) {
86 0 : if (full_headers_message || m_download_state == State::REDOWNLOAD) {
87 : // A full headers message means the peer may have more to give us;
88 : // also if we just switched to REDOWNLOAD then we need to re-request
89 : // headers from the beginning.
90 0 : ret.request_more = true;
91 0 : } else {
92 0 : Assume(m_download_state == State::PRESYNC);
93 : // If we're in PRESYNC and we get a non-full headers
94 : // message, then the peer's chain has ended and definitely doesn't
95 : // have enough work, so we can stop our sync.
96 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (presync phase)\n", m_id, m_current_height);
97 : }
98 0 : }
99 0 : } else if (m_download_state == State::REDOWNLOAD) {
100 : // During REDOWNLOAD, we compare our stored commitments to what we
101 : // receive, and add headers to our redownload buffer. When the buffer
102 : // gets big enough (meaning that we've checked enough commitments),
103 : // we'll return a batch of headers to the caller for processing.
104 0 : ret.success = true;
105 0 : for (const auto& hdr : received_headers) {
106 0 : if (!ValidateAndStoreRedownloadedHeader(hdr)) {
107 : // Something went wrong -- the peer gave us an unexpected chain.
108 : // We could consider looking at the reason for failure and
109 : // punishing the peer, but for now just give up on sync.
110 0 : ret.success = false;
111 0 : break;
112 : }
113 : }
114 :
115 0 : if (ret.success) {
116 : // Return any headers that are ready for acceptance.
117 0 : ret.pow_validated_headers = PopHeadersReadyForAcceptance();
118 :
119 : // If we hit our target blockhash, then all remaining headers will be
120 : // returned and we can clear any leftover internal state.
121 0 : if (m_redownloaded_headers.empty() && m_process_all_remaining_headers) {
122 0 : LogPrint(BCLog::NET, "Initial headers sync complete with peer=%d: releasing all at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height);
123 0 : } else if (full_headers_message) {
124 : // If the headers message is full, we need to request more.
125 0 : ret.request_more = true;
126 0 : } else {
127 : // For some reason our peer gave us a high-work chain, but is now
128 : // declining to serve us that full chain again. Give up.
129 : // Note that there's no more processing to be done with these
130 : // headers, so we can still return success.
131 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: incomplete headers message at height=%i (redownload phase)\n", m_id, m_redownload_buffer_last_height);
132 : }
133 0 : }
134 0 : }
135 :
136 0 : if (!(ret.success && ret.request_more)) Finalize();
137 0 : return ret;
138 0 : }
139 :
140 0 : bool HeadersSyncState::ValidateAndStoreHeadersCommitments(const std::vector<CBlockHeader>& headers)
141 : {
142 : // The caller should not give us an empty set of headers.
143 0 : Assume(headers.size() > 0);
144 0 : if (headers.size() == 0) return true;
145 :
146 0 : Assume(m_download_state == State::PRESYNC);
147 0 : if (m_download_state != State::PRESYNC) return false;
148 :
149 0 : if (headers[0].hashPrevBlock != m_last_header_received.GetHash()) {
150 : // Somehow our peer gave us a header that doesn't connect.
151 : // This might be benign -- perhaps our peer reorged away from the chain
152 : // they were on. Give up on this sync for now (likely we will start a
153 : // new sync with a new starting point).
154 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (presync phase)\n", m_id, m_current_height);
155 0 : return false;
156 : }
157 :
158 : // If it does connect, (minimally) validate and occasionally store
159 : // commitments.
160 0 : for (const auto& hdr : headers) {
161 0 : if (!ValidateAndProcessSingleHeader(hdr)) {
162 0 : return false;
163 : }
164 : }
165 :
166 0 : if (m_current_chain_work >= m_minimum_required_work) {
167 0 : m_redownloaded_headers.clear();
168 0 : m_redownload_buffer_last_height = m_chain_start->nHeight;
169 0 : m_redownload_buffer_first_prev_hash = m_chain_start->GetBlockHash();
170 0 : m_redownload_buffer_last_hash = m_chain_start->GetBlockHash();
171 0 : m_redownload_chain_work = m_chain_start->nChainWork;
172 0 : m_download_state = State::REDOWNLOAD;
173 0 : LogPrint(BCLog::NET, "Initial headers sync transition with peer=%d: reached sufficient work at height=%i, redownloading from height=%i\n", m_id, m_current_height, m_redownload_buffer_last_height);
174 0 : }
175 0 : return true;
176 0 : }
177 :
178 0 : bool HeadersSyncState::ValidateAndProcessSingleHeader(const CBlockHeader& current)
179 : {
180 0 : Assume(m_download_state == State::PRESYNC);
181 0 : if (m_download_state != State::PRESYNC) return false;
182 :
183 0 : int next_height = m_current_height + 1;
184 :
185 : // Verify that the difficulty isn't growing too fast; an adversary with
186 : // limited hashing capability has a greater chance of producing a high
187 : // work chain if they compress the work into as few blocks as possible,
188 : // so don't let anyone give a chain that would violate the difficulty
189 : // adjustment maximum.
190 0 : if (!PermittedDifficultyTransition(m_consensus_params, next_height,
191 0 : m_last_header_received.nBits, current.nBits)) {
192 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (presync phase)\n", m_id, next_height);
193 0 : return false;
194 : }
195 :
196 0 : if (next_height % HEADER_COMMITMENT_PERIOD == m_commit_offset) {
197 : // Add a commitment.
198 0 : m_header_commitments.push_back(m_hasher(current.GetHash()) & 1);
199 0 : if (m_header_commitments.size() > m_max_commitments) {
200 : // The peer's chain is too long; give up.
201 : // It's possible the chain grew since we started the sync; so
202 : // potentially we could succeed in syncing the peer's chain if we
203 : // try again later.
204 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: exceeded max commitments at height=%i (presync phase)\n", m_id, next_height);
205 0 : return false;
206 : }
207 0 : }
208 :
209 0 : m_current_chain_work += GetBlockProof(CBlockIndex(current));
210 0 : m_last_header_received = current;
211 0 : m_current_height = next_height;
212 :
213 0 : return true;
214 0 : }
215 :
216 0 : bool HeadersSyncState::ValidateAndStoreRedownloadedHeader(const CBlockHeader& header)
217 : {
218 0 : Assume(m_download_state == State::REDOWNLOAD);
219 0 : if (m_download_state != State::REDOWNLOAD) return false;
220 :
221 0 : int64_t next_height = m_redownload_buffer_last_height + 1;
222 :
223 : // Ensure that we're working on a header that connects to the chain we're
224 : // downloading.
225 0 : if (header.hashPrevBlock != m_redownload_buffer_last_hash) {
226 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: non-continuous headers at height=%i (redownload phase)\n", m_id, next_height);
227 0 : return false;
228 : }
229 :
230 : // Check that the difficulty adjustments are within our tolerance:
231 0 : uint32_t previous_nBits{0};
232 0 : if (!m_redownloaded_headers.empty()) {
233 0 : previous_nBits = m_redownloaded_headers.back().nBits;
234 0 : } else {
235 0 : previous_nBits = m_chain_start->nBits;
236 : }
237 0 :
238 0 : if (!PermittedDifficultyTransition(m_consensus_params, next_height,
239 0 : previous_nBits, header.nBits)) {
240 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: invalid difficulty transition at height=%i (redownload phase)\n", m_id, next_height);
241 0 : return false;
242 : }
243 :
244 : // Track work on the redownloaded chain
245 0 : m_redownload_chain_work += GetBlockProof(CBlockIndex(header));
246 :
247 0 : if (m_redownload_chain_work >= m_minimum_required_work) {
248 0 : m_process_all_remaining_headers = true;
249 0 : }
250 :
251 0 : // If we're at a header for which we previously stored a commitment, verify
252 : // it is correct. Failure will result in aborting download.
253 : // Also, don't check commitments once we've gotten to our target blockhash;
254 : // it's possible our peer has extended its chain between our first sync and
255 : // our second, and we don't want to return failure after we've seen our
256 : // target blockhash just because we ran out of commitments.
257 0 : if (!m_process_all_remaining_headers && next_height % HEADER_COMMITMENT_PERIOD == m_commit_offset) {
258 0 : if (m_header_commitments.size() == 0) {
259 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: commitment overrun at height=%i (redownload phase)\n", m_id, next_height);
260 : // Somehow our peer managed to feed us a different chain and
261 : // we've run out of commitments.
262 0 : return false;
263 : }
264 0 : bool commitment = m_hasher(header.GetHash()) & 1;
265 0 : bool expected_commitment = m_header_commitments.front();
266 0 : m_header_commitments.pop_front();
267 0 : if (commitment != expected_commitment) {
268 0 : LogPrint(BCLog::NET, "Initial headers sync aborted with peer=%d: commitment mismatch at height=%i (redownload phase)\n", m_id, next_height);
269 0 : return false;
270 : }
271 0 : }
272 0 :
273 : // Store this header for later processing.
274 0 : m_redownloaded_headers.push_back(header);
275 0 : m_redownload_buffer_last_height = next_height;
276 0 : m_redownload_buffer_last_hash = header.GetHash();
277 :
278 0 : return true;
279 0 : }
280 :
281 0 : std::vector<CBlockHeader> HeadersSyncState::PopHeadersReadyForAcceptance()
282 : {
283 0 : std::vector<CBlockHeader> ret;
284 :
285 0 : Assume(m_download_state == State::REDOWNLOAD);
286 0 : if (m_download_state != State::REDOWNLOAD) return ret;
287 :
288 0 : while (m_redownloaded_headers.size() > REDOWNLOAD_BUFFER_SIZE ||
289 0 : (m_redownloaded_headers.size() > 0 && m_process_all_remaining_headers)) {
290 0 : ret.emplace_back(m_redownloaded_headers.front().GetFullHeader(m_redownload_buffer_first_prev_hash));
291 0 : m_redownloaded_headers.pop_front();
292 0 : m_redownload_buffer_first_prev_hash = ret.back().GetHash();
293 : }
294 0 : return ret;
295 0 : }
296 :
297 0 : CBlockLocator HeadersSyncState::NextHeadersRequestLocator() const
298 : {
299 0 : Assume(m_download_state != State::FINAL);
300 0 : if (m_download_state == State::FINAL) return {};
301 :
302 0 : auto chain_start_locator = LocatorEntries(m_chain_start);
303 0 : std::vector<uint256> locator;
304 :
305 0 : if (m_download_state == State::PRESYNC) {
306 : // During pre-synchronization, we continue from the last header received.
307 0 : locator.push_back(m_last_header_received.GetHash());
308 0 : }
309 :
310 0 : if (m_download_state == State::REDOWNLOAD) {
311 : // During redownload, we will download from the last received header that we stored.
312 0 : locator.push_back(m_redownload_buffer_last_hash);
313 0 : }
314 :
315 0 : locator.insert(locator.end(), chain_start_locator.begin(), chain_start_locator.end());
316 :
317 0 : return CBlockLocator{std::move(locator)};
318 0 : }
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