Repository: bitcoin/bitcoin
File: src/test/fuzz/util/net.cpp (C++)

Code before:
        if ((flags & MSG_PEEK) == 0) {
            m_peek_data.reset();
        }
        pad_to_len_bytes = false;
    } else if ((flags & MSG_PEEK) != 0) {
        // New call with `MSG_PEEK`.
        random_bytes = m_fuzzed_data_provider.ConsumeBytes<uint8_t>(1);
        if (!random_bytes.empty()) {
            m_peek_data = random_bytes[0];
            pad_to_len_bytes = false;
        }
    } else {
        random_bytes = m_fuzzed_data_provider.ConsumeBytes<uint8_t>(
            m_fuzzed_data_provider.ConsumeIntegralInRange<size_t>(0, len));
    }
    if (random_bytes.empty()) {
        const ssize_t r = m_fuzzed_data_provider.ConsumeBool() ? 0 : -1;
        if (r == -1) {
            SetFuzzedErrNo(m_fuzzed_data_provider, recv_errnos);
        }
        return r;
    }
    std::memcpy(buf, random_bytes.data(), random_bytes.size());
    if (pad_to_len_bytes) {
        if (len > random_bytes.size()) {
            std::memset((char*)buf + random_bytes.size(), 0, len - random_bytes.size());
        }
        return len;
    }
    if (m_fuzzed_data_provider.ConsumeBool() && std::getenv("FUZZED_SOCKET_FAKE_LATENCY") != nullptr) {
        std::this_thread::sleep_for(std::chrono::milliseconds{2});
    }
    return random_bytes.size();
}

int FuzzedSock::Connect(const sockaddr*, socklen_t) const
{
    // Have a permanent error at connect_errnos[0] because when the fuzzed data is exhausted
    // SetFuzzedErrNo() will always return the first element and we want to avoid Connect()
    // returning -1 and setting errno to EAGAIN repeatedly.
    constexpr std::array connect_errnos{
        ECONNREFUSED,
        EAGAIN,
        ECONNRESET,
        EHOSTUNREACH,
        EINPROGRESS,
        EINTR,
        ENETUNREACH,
        ETIMEDOUT,
    };
    if (m_fuzzed_data_provider.ConsumeBool()) {

Review comment (bug):
Now it will not overflow `buf` but it will lose the trailing `N - M` bytes from the peek.

To fix this, when assigning `random_bytes` earlier, it should only take/pop the first `len` bytes from `m_peek_data`.

I think all this may be unnecessary if the app has to work with peek only returning 1 byte, as discussed in https://github.com/bitcoin/bitcoin/pull/30211#discussion_r1623330207

Code after:
        if ((flags & MSG_PEEK) == 0) {
            m_peek_data.reset();
        }
        pad_to_len_bytes = false;
    } else if ((flags & MSG_PEEK) != 0) {
        // New call with `MSG_PEEK`.
        random_bytes = ConsumeRandomLengthByteVector(m_fuzzed_data_provider, len);
        if (!random_bytes.empty()) {
            m_peek_data = random_bytes;
            pad_to_len_bytes = false;
        }
    } else {
        random_bytes = ConsumeRandomLengthByteVector(m_fuzzed_data_provider, len);
    }
    if (random_bytes.empty()) {
        const ssize_t r = m_fuzzed_data_provider.ConsumeBool() ? 0 : -1;
        if (r == -1) {
            SetFuzzedErrNo(m_fuzzed_data_provider, recv_errnos);
        }
        return r;
    }
    // `random_bytes` might exceed the size of `buf` if e.g. Recv is called with
    // len=N and MSG_PEEK first and afterwards called with len=M (M < N) and
    // without MSG_PEEK.
    size_t recv_len{std::min(random_bytes.size(), len)};
    std::memcpy(buf, random_bytes.data(), recv_len);
    if (pad_to_len_bytes) {
        if (len > random_bytes.size()) {
            std::memset((char*)buf + random_bytes.size(), 0, len - random_bytes.size());
        }
        return len;
    }
    if (m_fuzzed_data_provider.ConsumeBool() && std::getenv("FUZZED_SOCKET_FAKE_LATENCY") != nullptr) {
        std::this_thread::sleep_for(std::chrono::milliseconds{2});
    }
    return recv_len;
}

int FuzzedSock::Connect(const sockaddr*, socklen_t) const
{
    // Have a permanent error at connect_errnos[0] because when the fuzzed data is exhausted
    // SetFuzzedErrNo() will always return the first element and we want to avoid Connect()
    // returning -1 and setting errno to EAGAIN repeatedly.
    constexpr std::array connect_errnos{
        ECONNREFUSED,
        EAGAIN,
        ECONNRESET,
        EHOSTUNREACH,
        EINPROGRESS,
        EINTR,
        ENETUNREACH,