tests: Add fuzzing harness for versionbits

Github-Pull: #21380
Rebased-From: 1639c3b76c3f2b74606f62ecd3ca725154e27f1b

2 files changed, 353 insertions, 1 deletions

diff --git a/src/Makefile.test.include b/src/Makefile.test.include
index 9cc383c240..45f2a04cde 100644
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -157,7 +157,8 @@ FUZZ_TARGETS = \
   test/fuzz/txrequest \
   test/fuzz/txundo_deserialize \
   test/fuzz/uint160_deserialize \
-  test/fuzz/uint256_deserialize
+  test/fuzz/uint256_deserialize \
+  test/fuzz/versionbits
 
 if ENABLE_FUZZ
 noinst_PROGRAMS += $(FUZZ_TARGETS:=)
@@ -1260,6 +1261,12 @@ test_fuzz_uint256_deserialize_LDADD = $(FUZZ_SUITE_LD_COMMON)
 test_fuzz_uint256_deserialize_LDFLAGS = $(FUZZ_SUITE_LDFLAGS_COMMON)
 test_fuzz_uint256_deserialize_SOURCES = test/fuzz/deserialize.cpp
 
+test_fuzz_versionbits_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES)
+test_fuzz_versionbits_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
+test_fuzz_versionbits_LDADD = $(FUZZ_SUITE_LD_COMMON)
+test_fuzz_versionbits_LDFLAGS = $(FUZZ_SUITE_LDFLAGS_COMMON)
+test_fuzz_versionbits_SOURCES = test/fuzz/versionbits.cpp
+
 endif # ENABLE_FUZZ
 
 nodist_test_test_bitcoin_SOURCES = $(GENERATED_TEST_FILES)
diff --git a/src/test/fuzz/versionbits.cpp b/src/test/fuzz/versionbits.cpp
new file mode 100644
index 0000000000..992a5c1321
--- /dev/null
+++ b/src/test/fuzz/versionbits.cpp
@@ -0,0 +1,345 @@
+// Copyright (c) 2020-2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <chain.h>
+#include <chainparams.h>
+#include <consensus/params.h>
+#include <primitives/block.h>
+#include <versionbits.h>
+
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+
+#include <cstdint>
+#include <limits>
+#include <memory>
+#include <vector>
+
+namespace {
+class TestConditionChecker : public AbstractThresholdConditionChecker
+{
+private:
+    mutable ThresholdConditionCache m_cache;
+    const Consensus::Params dummy_params{};
+
+public:
+    const int64_t m_begin = 0;
+    const int64_t m_end = 0;
+    const int m_period = 0;
+    const int m_threshold = 0;
+    const int m_bit = 0;
+
+    TestConditionChecker(int64_t begin, int64_t end, int period, int threshold, int bit)
+        : m_begin{begin}, m_end{end}, m_period{period}, m_threshold{threshold}, m_bit{bit}
+    {
+        assert(m_period > 0);
+        assert(0 <= m_threshold && m_threshold <= m_period);
+        assert(0 <= m_bit && m_bit <= 32 && m_bit < VERSIONBITS_NUM_BITS);
+    }
+
+    bool Condition(const CBlockIndex* pindex, const Consensus::Params& params) const override { return Condition(pindex->nVersion); }
+    int64_t BeginTime(const Consensus::Params& params) const override { return m_begin; }
+    int64_t EndTime(const Consensus::Params& params) const override { return m_end; }
+    int Period(const Consensus::Params& params) const override { return m_period; }
+    int Threshold(const Consensus::Params& params) const override { return m_threshold; }
+
+    ThresholdState GetStateFor(const CBlockIndex* pindexPrev) const { return AbstractThresholdConditionChecker::GetStateFor(pindexPrev, dummy_params, m_cache); }
+    int GetStateSinceHeightFor(const CBlockIndex* pindexPrev) const { return AbstractThresholdConditionChecker::GetStateSinceHeightFor(pindexPrev, dummy_params, m_cache); }
+    BIP9Stats GetStateStatisticsFor(const CBlockIndex* pindexPrev) const { return AbstractThresholdConditionChecker::GetStateStatisticsFor(pindexPrev, dummy_params); }
+
+    bool Condition(int64_t version) const
+    {
+        return ((version >> m_bit) & 1) != 0 && (version & VERSIONBITS_TOP_MASK) == VERSIONBITS_TOP_BITS;
+    }
+
+    bool Condition(const CBlockIndex* pindex) const { return Condition(pindex->nVersion); }
+};
+
+/** Track blocks mined for test */
+class Blocks
+{
+private:
+    std::vector<std::unique_ptr<CBlockIndex>> m_blocks;
+    const uint32_t m_start_time;
+    const uint32_t m_interval;
+    const int32_t m_signal;
+    const int32_t m_no_signal;
+
+public:
+    Blocks(uint32_t start_time, uint32_t interval, int32_t signal, int32_t no_signal)
+        : m_start_time{start_time}, m_interval{interval}, m_signal{signal}, m_no_signal{no_signal} {}
+
+    size_t size() const { return m_blocks.size(); }
+
+    CBlockIndex* tip() const
+    {
+        return m_blocks.empty() ? nullptr : m_blocks.back().get();
+    }
+
+    CBlockIndex* mine_block(bool signal)
+    {
+        CBlockHeader header;
+        header.nVersion = signal ? m_signal : m_no_signal;
+        header.nTime = m_start_time + m_blocks.size() * m_interval;
+        header.nBits = 0x1d00ffff;
+
+        auto current_block = std::make_unique<CBlockIndex>(header);
+        current_block->pprev = tip();
+        current_block->nHeight = m_blocks.size();
+        current_block->BuildSkip();
+
+        return m_blocks.emplace_back(std::move(current_block)).get();
+    }
+};
+} // namespace
+
+void initialize()
+{
+    SelectParams(CBaseChainParams::MAIN);
+}
+
+constexpr uint32_t MAX_TIME = 4102444800; // 2100-01-01
+
+void test_one_input(const std::vector<uint8_t>& buffer)
+{
+    const CChainParams& params = Params();
+
+    const int64_t interval = params.GetConsensus().nPowTargetSpacing;
+    assert(interval > 1); // need to be able to halve it
+    assert(interval < std::numeric_limits<int32_t>::max());
+
+    FuzzedDataProvider fuzzed_data_provider(buffer.data(), buffer.size());
+
+    // making period/max_periods larger slows these tests down significantly
+    const int period = 32;
+    const size_t max_periods = 16;
+    const size_t max_blocks = 2 * period * max_periods;
+
+    const int threshold = fuzzed_data_provider.ConsumeIntegralInRange(1, period);
+    assert(0 < threshold && threshold <= period); // must be able to both pass and fail threshold!
+
+    // too many blocks at 10min each might cause uint32_t time to overflow if
+    // block_start_time is at the end of the range above
+    assert(std::numeric_limits<uint32_t>::max() - MAX_TIME > interval * max_blocks);
+
+    const int64_t block_start_time = fuzzed_data_provider.ConsumeIntegralInRange<uint32_t>(params.GenesisBlock().nTime, MAX_TIME);
+
+    // what values for version will we use to signal / not signal?
+    const int32_t ver_signal = fuzzed_data_provider.ConsumeIntegral<int32_t>();
+    const int32_t ver_nosignal = fuzzed_data_provider.ConsumeIntegral<int32_t>();
+
+    // select deployment parameters: bit, start time, timeout
+    const int bit = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, VERSIONBITS_NUM_BITS - 1);
+
+    bool always_active_test = false;
+    bool never_active_test = false;
+    int64_t start_time;
+    int64_t timeout;
+    if (fuzzed_data_provider.ConsumeBool()) {
+        // pick the timestamp to switch based on a block
+        // note states will change *after* these blocks because mediantime lags
+        int start_block = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, period * (max_periods - 3));
+        int end_block = fuzzed_data_provider.ConsumeIntegralInRange<int>(start_block, period * (max_periods - 3));
+
+        start_time = block_start_time + start_block * interval;
+        timeout = block_start_time + end_block * interval;
+
+        assert(start_time <= timeout);
+
+        // allow for times to not exactly match a block
+        if (fuzzed_data_provider.ConsumeBool()) start_time += interval / 2;
+        if (fuzzed_data_provider.ConsumeBool()) timeout += interval / 2;
+
+        // this may make timeout too early; if so, don't run the test
+        if (start_time > timeout) return;
+    } else {
+        if (fuzzed_data_provider.ConsumeBool()) {
+            start_time = Consensus::BIP9Deployment::ALWAYS_ACTIVE;
+            timeout = Consensus::BIP9Deployment::NO_TIMEOUT;
+            always_active_test = true;
+        } else {
+            start_time = 1199145601; // January 1, 2008
+            timeout = 1230767999;    // December 31, 2008
+            never_active_test = true;
+        }
+    }
+
+    TestConditionChecker checker(start_time, timeout, period, threshold, bit);
+
+    // Early exit if the versions don't signal sensibly for the deployment
+    if (!checker.Condition(ver_signal)) return;
+    if (checker.Condition(ver_nosignal)) return;
+    if (ver_nosignal < 0) return;
+
+    // TOP_BITS should ensure version will be positive
+    assert(ver_signal > 0);
+
+    // Now that we have chosen time and versions, setup to mine blocks
+    Blocks blocks(block_start_time, interval, ver_signal, ver_nosignal);
+
+    /* Strategy:
+     *  * we will mine a final period worth of blocks, with
+     *    randomised signalling according to a mask
+     *  * but before we mine those blocks, we will mine some
+     *    randomised number of prior periods; with either all
+     *    or no blocks in the period signalling
+     *
+     * We establish the mask first, then consume "bools" until
+     * we run out of fuzz data to work out how many prior periods
+     * there are and which ones will signal.
+     */
+
+    // establish the mask
+    const uint32_t signalling_mask = fuzzed_data_provider.ConsumeIntegral<uint32_t>();
+
+    // mine prior periods
+    while (fuzzed_data_provider.remaining_bytes() > 0) {
+        // all blocks in these periods either do or don't signal
+        bool signal = fuzzed_data_provider.ConsumeBool();
+        for (int b = 0; b < period; ++b) {
+            blocks.mine_block(signal);
+        }
+
+        // don't risk exceeding max_blocks or times may wrap around
+        if (blocks.size() + period*2 > max_blocks) break;
+    }
+    // NOTE: fuzzed_data_provider may be fully consumed at this point and should not be used further
+
+    // now we mine the final period and check that everything looks sane
+
+    // count the number of signalling blocks
+    int blocks_sig = 0;
+
+    // get the info for the first block of the period
+    CBlockIndex* prev = blocks.tip();
+    const int exp_since = checker.GetStateSinceHeightFor(prev);
+    const ThresholdState exp_state = checker.GetStateFor(prev);
+    BIP9Stats last_stats = checker.GetStateStatisticsFor(prev);
+
+    int prev_next_height = (prev == nullptr ? 0 : prev->nHeight + 1);
+    assert(exp_since <= prev_next_height);
+
+    // mine (period-1) blocks and check state
+    for (int b = 1; b < period; ++b) {
+        const bool signal = (signalling_mask >> (b % 32)) & 1;
+        if (signal) ++blocks_sig;
+
+        CBlockIndex* current_block = blocks.mine_block(signal);
+
+        // verify that signalling attempt was interpreted correctly
+        assert(checker.Condition(current_block) == signal);
+
+        // state and since don't change within the period
+        const ThresholdState state = checker.GetStateFor(current_block);
+        const int since = checker.GetStateSinceHeightFor(current_block);
+        assert(state == exp_state);
+        assert(since == exp_since);
+
+        // GetStateStatistics may crash when state is not STARTED
+        if (state != ThresholdState::STARTED) continue;
+
+        // check that after mining this block stats change as expected
+        const BIP9Stats stats = checker.GetStateStatisticsFor(current_block);
+        assert(stats.period == period);
+        assert(stats.threshold == threshold);
+        assert(stats.elapsed == b);
+        assert(stats.count == last_stats.count + (signal ? 1 : 0));
+        assert(stats.possible == (stats.count + period >= stats.elapsed + threshold));
+        last_stats = stats;
+    }
+
+    if (exp_state == ThresholdState::STARTED) {
+        // double check that stats.possible is sane
+        if (blocks_sig >= threshold - 1) assert(last_stats.possible);
+    }
+
+    // mine the final block
+    bool signal = (signalling_mask >> (period % 32)) & 1;
+    if (signal) ++blocks_sig;
+    CBlockIndex* current_block = blocks.mine_block(signal);
+    assert(checker.Condition(current_block) == signal);
+
+    // GetStateStatistics is safe on a period boundary
+    // and has progressed to a new period
+    const BIP9Stats stats = checker.GetStateStatisticsFor(current_block);
+    assert(stats.period == period);
+    assert(stats.threshold == threshold);
+    assert(stats.elapsed == 0);
+    assert(stats.count == 0);
+    assert(stats.possible == true);
+
+    // More interesting is whether the state changed.
+    const ThresholdState state = checker.GetStateFor(current_block);
+    const int since = checker.GetStateSinceHeightFor(current_block);
+
+    // since is straightforward:
+    assert(since % period == 0);
+    assert(0 <= since && since <= current_block->nHeight + 1);
+    if (state == exp_state) {
+        assert(since == exp_since);
+    } else {
+        assert(since == current_block->nHeight + 1);
+    }
+
+    // state is where everything interesting is
+    switch (state) {
+    case ThresholdState::DEFINED:
+        assert(since == 0);
+        assert(exp_state == ThresholdState::DEFINED);
+        assert(current_block->GetMedianTimePast() < checker.m_begin);
+        assert(current_block->GetMedianTimePast() < checker.m_end);
+        break;
+    case ThresholdState::STARTED:
+        assert(current_block->GetMedianTimePast() >= checker.m_begin);
+        assert(current_block->GetMedianTimePast() < checker.m_end);
+        if (exp_state == ThresholdState::STARTED) {
+            assert(blocks_sig < threshold);
+        } else {
+            assert(exp_state == ThresholdState::DEFINED);
+        }
+        break;
+    case ThresholdState::LOCKED_IN:
+        assert(exp_state == ThresholdState::STARTED);
+        assert(current_block->GetMedianTimePast() < checker.m_end);
+        assert(blocks_sig >= threshold);
+        break;
+    case ThresholdState::ACTIVE:
+        assert(exp_state == ThresholdState::ACTIVE || exp_state == ThresholdState::LOCKED_IN);
+        break;
+    case ThresholdState::FAILED:
+        assert(current_block->GetMedianTimePast() >= checker.m_end);
+        assert(exp_state != ThresholdState::LOCKED_IN && exp_state != ThresholdState::ACTIVE);
+        break;
+    default:
+        assert(false);
+    }
+
+    if (blocks.size() >= max_periods * period) {
+        // we chose the timeout (and block times) so that by the time we have this many blocks it's all over
+        assert(state == ThresholdState::ACTIVE || state == ThresholdState::FAILED);
+    }
+
+    // "always active" has additional restrictions
+    if (always_active_test) {
+        assert(state == ThresholdState::ACTIVE);
+        assert(exp_state == ThresholdState::ACTIVE);
+        assert(since == 0);
+    } else {
+        // except for always active, the initial state is always DEFINED
+        assert(since > 0 || state == ThresholdState::DEFINED);
+        assert(exp_since > 0 || exp_state == ThresholdState::DEFINED);
+    }
+
+    // "never active" does too
+    if (never_active_test) {
+        assert(state == ThresholdState::FAILED);
+        assert(since == period);
+        if (exp_since == 0) {
+            assert(exp_state == ThresholdState::DEFINED);
+        } else {
+            assert(exp_state == ThresholdState::FAILED);
+        }
+    }
+}