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net: Consistent checksum handling #8822

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Sep 30, 2016
Merged

net: Consistent checksum handling #8822

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41e58fa
net: Consistent checksum handling
laanwj Sep 27, 2016
305087b
net: Hardcode protocol sizes and use fixed-size types
laanwj Sep 28, 2016
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net: Consistent checksum handling 
In principle, the checksums of P2P packets are simply 4-byte blobs which
are the first four bytes of SHA256(SHA256(payload)).

Currently they are handled as little-endian 32-bit integers half of the
time, as blobs the other half, sometimes copying the one to the other,
resulting in somewhat confused code.

This PR changes the handling to be consistent both at packet creation
and receiving, making it (I think) easier to understand.
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@laanwj
laanwj committed Sep 28, 2016
commit 41e58faf043864a64a6db08a8df527fa5fd1ec5b
9 changes: 5 additions & 4 deletions src/main.cpp
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Expand Up @@ -6253,11 +6253,12 @@ bool ProcessMessages(CNode* pfrom, CConnman& connman)
// Checksum
CDataStream& vRecv = msg.vRecv;
uint256 hash = Hash(vRecv.begin(), vRecv.begin() + nMessageSize);
unsigned int nChecksum = ReadLE32((unsigned char*)&hash);
if (nChecksum != hdr.nChecksum)
if (memcmp(hash.begin(), hdr.pchChecksum, CMessageHeader::CHECKSUM_SIZE) != 0)
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A size check of hash.size() <= CMessageHeader::CHECKSUM_SIZE is probably a total overkill...

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A compile-time assertion could be added... But I don't think there is any risk of someone increasing CHECKSUM_SIZE larger than 32 in practice.

{
LogPrintf("%s(%s, %u bytes): CHECKSUM ERROR nChecksum=%08x hdr.nChecksum=%08x\n", __func__,
SanitizeString(strCommand), nMessageSize, nChecksum, hdr.nChecksum);
LogPrintf("%s(%s, %u bytes): CHECKSUM ERROR expected %s was %s\n", __func__,
SanitizeString(strCommand), nMessageSize,
HexStr(hash.begin(), hash.begin()+CMessageHeader::CHECKSUM_SIZE),
HexStr(hdr.pchChecksum, hdr.pchChecksum+CMessageHeader::CHECKSUM_SIZE));
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are these the wrong way around? i.e. the expected checksum is supposed to be displayed before the "was"?

continue;
}

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6 changes: 2 additions & 4 deletions src/net.cpp
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Expand Up @@ -2661,10 +2661,8 @@ void CNode::EndMessage(const char* pszCommand) UNLOCK_FUNCTION(cs_vSend)

// Set the checksum
uint256 hash = Hash(ssSend.begin() + CMessageHeader::HEADER_SIZE, ssSend.end());
unsigned int nChecksum = 0;
memcpy(&nChecksum, &hash, sizeof(nChecksum));
assert(ssSend.size () >= CMessageHeader::CHECKSUM_OFFSET + sizeof(nChecksum));
memcpy((char*)&ssSend[CMessageHeader::CHECKSUM_OFFSET], &nChecksum, sizeof(nChecksum));
assert(ssSend.size () >= CMessageHeader::CHECKSUM_OFFSET + CMessageHeader::CHECKSUM_SIZE);
memcpy((char*)&ssSend[CMessageHeader::CHECKSUM_OFFSET], hash.begin(), CMessageHeader::CHECKSUM_SIZE);

LogPrint("net", "(%d bytes) peer=%d\n", nSize, id);

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4 changes: 2 additions & 2 deletions src/protocol.cpp
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Expand Up @@ -79,7 +79,7 @@ CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn)
memcpy(pchMessageStart, pchMessageStartIn, MESSAGE_START_SIZE);
memset(pchCommand, 0, sizeof(pchCommand));
nMessageSize = -1;
nChecksum = 0;
memset(pchChecksum, 0, CHECKSUM_SIZE);
}

CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn, const char* pszCommand, unsigned int nMessageSizeIn)
Expand All @@ -88,7 +88,7 @@ CMessageHeader::CMessageHeader(const MessageStartChars& pchMessageStartIn, const
memset(pchCommand, 0, sizeof(pchCommand));
strncpy(pchCommand, pszCommand, COMMAND_SIZE);
nMessageSize = nMessageSizeIn;
nChecksum = 0;
memset(pchChecksum, 0, CHECKSUM_SIZE);
}

std::string CMessageHeader::GetCommand() const
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4 changes: 2 additions & 2 deletions src/protocol.h
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Expand Up @@ -45,7 +45,7 @@ class CMessageHeader
READWRITE(FLATDATA(pchMessageStart));
READWRITE(FLATDATA(pchCommand));
READWRITE(nMessageSize);
READWRITE(nChecksum);
READWRITE(FLATDATA(pchChecksum));
}

// TODO: make private (improves encapsulation)
Expand All @@ -62,7 +62,7 @@ class CMessageHeader
char pchMessageStart[MESSAGE_START_SIZE];
char pchCommand[COMMAND_SIZE];
unsigned int nMessageSize;
unsigned int nChecksum;
uint8_t pchChecksum[CHECKSUM_SIZE];
};

/**
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