src/timedata.cpp

   1 // Copyright (c) 2014-2016 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 #include "warnings.h"
  17
  18 #include <boost/foreach.hpp>
  19
  20 static CCriticalSection cs_nTimeOffset;
  21 static int64_t nTimeOffset = 0;
  22
  23 /**
  24  * "Never go to sea with two chronometers; take one or three."
  25  * Our three time sources are:
  26  *  - System clock
  27  *  - Median of other nodes clocks
  28  *  - The user (asking the user to fix the system clock if the first two disagree)
  29  */
  30 int64_t GetTimeOffset()
  31 {
  32     LOCK(cs_nTimeOffset);
  33     return nTimeOffset;
  34 }
  35
  36 int64_t GetAdjustedTime()
  37 {
  38     return GetTime() + GetTimeOffset();
  39 }
  40
  41 static int64_t abs64(int64_t n)
  42 {
  43     return (n >= 0 ? n : -n);
  44 }
  45
  46 #define BITCOIN_TIMEDATA_MAX_SAMPLES 200
  47
  48 void AddTimeData(const CNetAddr& ip, int64_t nOffsetSample)
  49 {
  50     LOCK(cs_nTimeOffset);
  51     // Ignore duplicates
  52     static std::set<CNetAddr> setKnown;
  53     if (setKnown.size() == BITCOIN_TIMEDATA_MAX_SAMPLES)
  54         return;
  55     if (!setKnown.insert(ip).second)
  56         return;
  57
  58     // Add data
  59     static CMedianFilter<int64_t> vTimeOffsets(BITCOIN_TIMEDATA_MAX_SAMPLES, 0);
  60     vTimeOffsets.input(nOffsetSample);
  61     LogPrint("net","added time data, samples %d, offset %+d (%+d minutes)\n", vTimeOffsets.size(), nOffsetSample, nOffsetSample/60);
  62
  63     // There is a known issue here (see issue #4521):
  64     //
  65     // - The structure vTimeOffsets contains up to 200 elements, after which
  66     // any new element added to it will not increase its size, replacing the
  67     // oldest element.
  68     //
  69     // - The condition to update nTimeOffset includes checking whether the
  70     // number of elements in vTimeOffsets is odd, which will never happen after
  71     // there are 200 elements.
  72     //
  73     // But in this case the 'bug' is protective against some attacks, and may
  74     // actually explain why we've never seen attacks which manipulate the
  75     // clock offset.
  76     //
  77     // So we should hold off on fixing this and clean it up as part of
  78     // a timing cleanup that strengthens it in a number of other ways.
  79     //
  80     if (vTimeOffsets.size() >= 5 && vTimeOffsets.size() % 2 == 1)
  81     {
  82         int64_t nMedian = vTimeOffsets.median();
  83         std::vector<int64_t> vSorted = vTimeOffsets.sorted();
  84         // Only let other nodes change our time by so much
  85         if (abs64(nMedian) <= std::max<int64_t>(0, GetArg("-maxtimeadjustment", DEFAULT_MAX_TIME_ADJUSTMENT)))
  86         {
  87             nTimeOffset = nMedian;
  88         }
  89         else
  90         {
  91             nTimeOffset = 0;
  92
  93             static bool fDone;
  94             if (!fDone)
  95             {
  96                 // If nobody has a time different than ours but within 5 minutes of ours, give a warning
  97                 bool fMatch = false;
  98                 BOOST_FOREACH(int64_t nOffset, vSorted)
  99                     if (nOffset != 0 && abs64(nOffset) < 5 * 60)
 100                         fMatch = true;
 101
 102                 if (!fMatch)
 103                 {
 104                     fDone = true;
 105                     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));
 106                     SetMiscWarning(strMessage);
 107                     uiInterface.ThreadSafeMessageBox(strMessage, "", CClientUIInterface::MSG_WARNING);
 108                 }
 109             }
 110         }
 111
 112         BOOST_FOREACH(int64_t n, vSorted)
 113             LogPrint("net", "%+d  ", n);
 114         LogPrint("net", "|  ");
 115
 116         LogPrint("net", "nTimeOffset = %+d  (%+d minutes)\n", nTimeOffset, nTimeOffset/60);
 117     }
 118 }