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Add comment about never updating nTimeOffset past 199 samples

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Refer to issue #4521 for details.
This commit is contained in:
Wladimir J. van der Laan 2014-07-24 19:00:24 +02:00
parent 70d0325999
commit 93659379bd
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1 changed files with 18 additions and 0 deletions
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18
src/timedata.cpp
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@ -49,6 +49,24 @@ void AddTimeData(const CNetAddr& ip, int64_t nTime)
static CMedianFilter<int64_t> vTimeOffsets(200,0);
vTimeOffsets.input(nOffsetSample);
LogPrintf("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();