btcsuite / btcd
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LAST BUILD ON BRANCH fix-codesep-unexecuted-branch
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Roasbeef 
txscript: reject OP_CODESEPARATOR in unexecuted branches for non-segwit

In this commit, we fix a policy-level divergence with Bitcoin Core when
handling OP_CODESEPARATOR inside unexecuted OP_IF branches in non-segwit
scripts with the ScriptVerifyConstScriptCode flag.

Bitcoin Core's EvalScript (interpreter.cpp:474-476) places the
SCRIPT_VERIFY_CONST_SCRIPTCODE check for OP_CODESEPARATOR before the
fExec branch-execution gate, causing it to fire unconditionally on every
OP_CODESEPARATOR encountered during script iteration -- even inside
OP_FALSE OP_IF ... OP_ENDIF envelopes.

Previously, btcd's equivalent check lived inside the opcodeCodeSeparator
handler, which was never reached for opcodes in unexecuted branches due
to the early return in executeOpcode that skips non-conditional opcodes
when isBranchExecuting() is false. This meant a script like:

  OP_FALSE OP_IF OP_CODESEPARATOR OP_ENDIF <validation>

would be rejected by Bitcoin Core's mempool but accepted by btcd's.

The fix moves the check before the branch-execution gate in
executeOpcode, matching Bitcoin Core's structure. This follows the
existing pattern in btcd where isOpcodeDisabled and isOpcodeAlwaysIllegal
checks already fire regardless of branch execution state.

Note: SCRIPT_VERIFY_CONST_SCRIPTCODE is purely a policy flag (included
in STANDARD_SCRIPT_VERIFY_FLAGS but not MANDATORY_SCRIPT_VERIFY_FLAGS),
so this was not a consensus divergence. Both implementations would accept
such transactions if mined in a block.

Found via differential fuzzing by Bruno from bitcoinfuzz.

7 of 7 new or added lines in 1 file covered. (100.0%)

6 existing lines in 2 files now uncovered.

31203 of 56722 relevant lines covered (55.01%)

445113.36 hits per line

Relevant lines Covered
56722 RELEVANT LINES 31203 COVERED LINES
445113.36 HITS PER LINE
Source Files on fix-codesep-unexecuted-branch

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Builds Branch Commit Type Ran Committer Via Coverage
23467752405 fix-codesep-unexecuted-branch txscript: reject OP_CODESEPARATOR in unexecuted branches for non-segwit In this commit, we fix a policy-level divergence with Bitcoin Core when handling OP_CODESEPARATOR inside unexecuted OP_IF branches in non-segwit scripts with the ScriptVerify... push Roasbeef github
55.01
22081381425 fix-codesep-unexecuted-branch Merge 3e17ea43c into 3eacced04 Pull #2485 web-flow github
54.91
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