timedata.cpp

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// Copyright (c) 2014-2016 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#if defined(HAVE_CONFIG_H)
#include "config/bitcoin-config.h"
#endif
 
#include "timedata.h"
 
#include "netaddress.h"
#include "sync.h"
#include "ui_interface.h"
#include "util.h"
#include "utilstrencodings.h"
#include "warnings.h"
 
#include <boost/foreach.hpp>
 
static CCriticalSection cs_nTimeOffset;
static int64_t nTimeOffset = 0;
 
int64_t GetTimeOffset()
{
    LOCK(cs_nTimeOffset);
    return nTimeOffset;
}
 
int64_t GetAdjustedTime()
{
    return GetTime() + GetTimeOffset();
}
 
static int64_t abs64(int64_t n)
{
    return (n >= 0 ? n : -n);
}
 
#define BITCOIN_TIMEDATA_MAX_SAMPLES 200
 
void AddTimeData(const CNetAddr& ip, int64_t nOffsetSample)
{
    LOCK(cs_nTimeOffset);
    // Ignore duplicates
    static std::set<CNetAddr> setKnown;
    if (setKnown.size() == BITCOIN_TIMEDATA_MAX_SAMPLES)
        return;
    if (!setKnown.insert(ip).second)
        return;
 
    // Add data
    static CMedianFilter<int64_t> vTimeOffsets(BITCOIN_TIMEDATA_MAX_SAMPLES, 0);
    vTimeOffsets.input(nOffsetSample);
    LogPrint("net","added time data, samples %d, offset %+d (%+d minutes)\n", vTimeOffsets.size(), nOffsetSample, nOffsetSample/60);
 
    // There is a known issue here (see issue #4521):
    //
    // - The structure vTimeOffsets contains up to 200 elements, after which
    // any new element added to it will not increase its size, replacing the
    // oldest element.
    //
    // - The condition to update nTimeOffset includes checking whether the
    // number of elements in vTimeOffsets is odd, which will never happen after
    // there are 200 elements.
    //
    // But in this case the 'bug' is protective against some attacks, and may
    // actually explain why we've never seen attacks which manipulate the
    // clock offset.
    //
    // So we should hold off on fixing this and clean it up as part of
    // a timing cleanup that strengthens it in a number of other ways.
    //
    if (vTimeOffsets.size() >= 5 && vTimeOffsets.size() % 2 == 1)
    {
        int64_t nMedian = vTimeOffsets.median();
        std::vector<int64_t> vSorted = vTimeOffsets.sorted();
        // Only let other nodes change our time by so much
        if (abs64(nMedian) <= std::max<int64_t>(0, GetArg("-maxtimeadjustment", DEFAULT_MAX_TIME_ADJUSTMENT)))
        {
            nTimeOffset = nMedian;
        }
        else
        {
            nTimeOffset = 0;
 
            static bool fDone;
            if (!fDone)
            {
                // If nobody has a time different than ours but within 5 minutes of ours, give a warning
                bool fMatch = false;
                BOOST_FOREACH(int64_t nOffset, vSorted)
                    if (nOffset != 0 && abs64(nOffset) < 5 * 60)
                        fMatch = true;
 
                if (!fMatch)
                {
                    fDone = true;
                    std::string strMessage = strprintf(_("Please check that your computer's date and time are correct! If your clock is wrong, %s will not work properly."), _(PACKAGE_NAME));
                    SetMiscWarning(strMessage);
                    uiInterface.ThreadSafeMessageBox(strMessage, "", CClientUIInterface::MSG_WARNING);
                }
            }
        }
        
        BOOST_FOREACH(int64_t n, vSorted)
            LogPrint("net", "%+d  ", n);
        LogPrint("net", "|  ");
        
        LogPrint("net", "nTimeOffset = %+d  (%+d minutes)\n", nTimeOffset, nTimeOffset/60);
    }
}