doc: Add instructions on how to fuzz the P2P layer using Honggfuzz NetDriver

doc/fuzzing.md

@@ -157,3 +157,77 @@ $ honggfuzz/honggfuzz -i inputs/ -- src/test/fuzz/process_message
 ```
 
 Read the [Honggfuzz documentation](https://github.com/google/honggfuzz/blob/master/docs/USAGE.md) for more information.
+
+## Fuzzing the Bitcoin Core P2P layer using Honggfuzz NetDriver
+
+Honggfuzz NetDriver allows for very easy fuzzing of TCP servers such as Bitcoin
+Core without having to write any custom fuzzing harness. The `bitcoind` server
+process is largely fuzzed without modification.
+
+This makes the fuzzing highly realistic: a bug reachable by the fuzzer is likely
+also remotely triggerable by an untrusted peer.
+
+To quickly get started fuzzing the P2P layer using Honggfuzz NetDriver:
+
+```sh
+$ mkdir bitcoin-honggfuzz-p2p/
+$ cd bitcoin-honggfuzz-p2p/
+$ git clone https://github.com/bitcoin/bitcoin
+$ cd bitcoin/
+$ ./autogen.sh
+$ git clone https://github.com/google/honggfuzz
+$ cd honggfuzz/
+$ make
+$ cd ..
+$ CC=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang \
+      CXX=$(pwd)/honggfuzz/hfuzz_cc/hfuzz-clang++ \
+      ./configure --disable-wallet --with-gui=no \
+                  --with-sanitizers=address,undefined
+$ git apply << "EOF"
+diff --git a/src/bitcoind.cpp b/src/bitcoind.cpp
+index 455a82e39..2faa3f80f 100644
+--- a/src/bitcoind.cpp
++++ b/src/bitcoind.cpp
+@@ -158,7 +158,11 @@ static bool AppInit(int argc, char* argv[])
+     return fRet;
+ }
+
++#ifdef HFND_FUZZING_ENTRY_FUNCTION_CXX
++HFND_FUZZING_ENTRY_FUNCTION_CXX(int argc, char* argv[])
++#else
+ int main(int argc, char* argv[])
++#endif
+ {
+ #ifdef WIN32
+     util::WinCmdLineArgs winArgs;
+diff --git a/src/net.cpp b/src/net.cpp
+index cf987b699..636a4176a 100644
+--- a/src/net.cpp
++++ b/src/net.cpp
+@@ -709,7 +709,7 @@ int V1TransportDeserializer::readHeader(const char *pch, unsigned int nBytes)
+     }
+
+     // Check start string, network magic
+-    if (memcmp(hdr.pchMessageStart, m_chain_params.MessageStart(), CMessageHeader::MESSAGE_START_SIZE) != 0) {
++    if (false && memcmp(hdr.pchMessageStart, m_chain_params.MessageStart(), CMessageHeader::MESSAGE_START_SIZE) != 0) { // skip network magic checking
+         LogPrint(BCLog::NET, "HEADER ERROR - MESSAGESTART (%s, %u bytes), received %s, peer=%d\n", hdr.GetCommand(), hdr.nMessageSize, HexStr(hdr.pchMessageStart), m_node_id);
+         return -1;
+     }
+@@ -768,7 +768,7 @@ Optional<CNetMessage> V1TransportDeserializer::GetMessage(const std::chrono::mic
+     RandAddEvent(ReadLE32(hash.begin()));
+
+     // Check checksum and header command string
+-    if (memcmp(hash.begin(), hdr.pchChecksum, CMessageHeader::CHECKSUM_SIZE) != 0) {
++    if (false && memcmp(hash.begin(), hdr.pchChecksum, CMessageHeader::CHECKSUM_SIZE) != 0) { // skip checksum checking
+         LogPrint(BCLog::NET, "CHECKSUM ERROR (%s, %u bytes), expected %s was %s, peer=%d\n",
+                  SanitizeString(msg->m_command), msg->m_message_size,
+                  HexStr(Span<uint8_t>(hash.begin(), hash.begin() + CMessageHeader::CHECKSUM_SIZE)),
+EOF
+$ make -C src/ bitcoind
+$ mkdir -p inputs/
+$ honggfuzz/honggfuzz --exit_upon_crash --quiet --timeout 4 -n 1 -Q \
+      -E HFND_TCP_PORT=18444 -f inputs/ -- \
+          src/bitcoind -regtest -discover=0 -dns=0 -dnsseed=0 -listenonion=0 \
+                       -nodebuglogfile -bind=127.0.0.1:18444 -logthreadnames \
+                       -debug
+```