src/timedata.cpp

   1 // Copyright (c) 2014-2015 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 #if defined(HAVE_CONFIG_H)
   6 #include "config/bitcoin-config.h"
   7 #endif
   8
   9 #include "timedata.h"
  10
  11 #include "netaddress.h"
  12 #include "sync.h"
  13 #include "ui_interface.h"
  14 #include "util.h"
  15 #include "utilstrencodings.h"
  16
  17 #include <boost/foreach.hpp>
  18
  19 using namespace std;
  20
  21 static CCriticalSection cs_nTimeOffset;
  22 static int64_t nTimeOffset = 0;
  23
  24 /**
  25  * "Never go to sea with two chronometers; take one or three."
  26  * Our three time sources are:
  27  *  - System clock
  28  *  - Median of other nodes clocks
  29  *  - The user (asking the user to fix the system clock if the first two disagree)
  30  */
  31 int64_t GetTimeOffset()
  32 {
  33     LOCK(cs_nTimeOffset);
  34     return nTimeOffset;
  35 }
  36
  37 int64_t GetAdjustedTime()
  38 {
  39     return GetTime() + GetTimeOffset();
  40 }
  41
  42 static int64_t abs64(int64_t n)
  43 {
  44     return (n >= 0 ? n : -n);
  45 }
  46
  47 #define BITCOIN_TIMEDATA_MAX_SAMPLES 200
  48
  49 void AddTimeData(const CNetAddr& ip, int64_t nOffsetSample)
  50 {
  51     LOCK(cs_nTimeOffset);
  52     // Ignore duplicates
  53     static set<CNetAddr> setKnown;
  54     if (setKnown.size() == BITCOIN_TIMEDATA_MAX_SAMPLES)
  55         return;
  56     if (!setKnown.insert(ip).second)
  57         return;
  58
  59     // Add data
  60     static CMedianFilter<int64_t> vTimeOffsets(BITCOIN_TIMEDATA_MAX_SAMPLES, 0);
  61     vTimeOffsets.input(nOffsetSample);
  62     LogPrint("net","added time data, samples %d, offset %+d (%+d minutes)\n", vTimeOffsets.size(), nOffsetSample, nOffsetSample/60);
  63
  64     // There is a known issue here (see issue #4521):
  65     //
  66     // - The structure vTimeOffsets contains up to 200 elements, after which
  67     // any new element added to it will not increase its size, replacing the
  68     // oldest element.
  69     //
  70     // - The condition to update nTimeOffset includes checking whether the
  71     // number of elements in vTimeOffsets is odd, which will never happen after
  72     // there are 200 elements.
  73     //
  74     // But in this case the 'bug' is protective against some attacks, and may
  75     // actually explain why we've never seen attacks which manipulate the
  76     // clock offset.
  77     //
  78     // So we should hold off on fixing this and clean it up as part of
  79     // a timing cleanup that strengthens it in a number of other ways.
  80     //
  81     if (vTimeOffsets.size() >= 5 && vTimeOffsets.size() % 2 == 1)
  82     {
  83         int64_t nMedian = vTimeOffsets.median();
  84         std::vector<int64_t> vSorted = vTimeOffsets.sorted();
  85         // Only let other nodes change our time by so much
  86         if (abs64(nMedian) <= std::max<int64_t>(0, GetArg("-maxtimeadjustment", DEFAULT_MAX_TIME_ADJUSTMENT)))
  87         {
  88             nTimeOffset = nMedian;
  89         }
  90         else
  91         {
  92             nTimeOffset = 0;
  93
  94             static bool fDone;
  95             if (!fDone)
  96             {
  97                 // If nobody has a time different than ours but within 5 minutes of ours, give a warning
  98                 bool fMatch = false;
  99                 BOOST_FOREACH(int64_t nOffset, vSorted)
 100                     if (nOffset != 0 && abs64(nOffset) < 5 * 60)
 101                         fMatch = true;
 102
 103                 if (!fMatch)
 104                 {
 105                     fDone = true;
 106                     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));
 107                     strMiscWarning = strMessage;
 108                     uiInterface.ThreadSafeMessageBox(strMessage, "", CClientUIInterface::MSG_WARNING);
 109                 }
 110             }
 111         }
 112
 113         BOOST_FOREACH(int64_t n, vSorted)
 114             LogPrint("net", "%+d  ", n);
 115         LogPrint("net", "|  ");
 116
 117         LogPrint("net", "nTimeOffset = %+d  (%+d minutes)\n", nTimeOffset, nTimeOffset/60);
 118     }
 119 }