src/timedata.cpp

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