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Documented bug in sign-extension behavior of opcodes OP_AND, OP_OR, and OP_XOR.

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Due to a bug in the implementation of MakeSameSize(), using OP_AND, OP_OR, or OP_XOR with signed values of unequal size will result in the sign-value becoming part of the smaller integer, with nonsensical results. This patch documents the unexpected behavior and provides the basis of a solution should decision be made to fix the bug in the future.
This commit is contained in:
Mark Friedenbach 2012-09-25 16:40:34 -07:00
parent 1381ad2b21
commit 95d7f00295
1 changed files with 23 additions and 1 deletions
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24
src/script.cpp
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@ -54,12 +54,29 @@ bool CastToBool(const valtype& vch)
return false; return false;
} }
//
// WARNING: This does not work as expected for signed integers; the sign-bit
// is left in place as the integer is zero-extended. The correct behavior
// would be to move the most significant bit of the last byte during the
// resize process. MakeSameSize() is currently only used by the disabled
// opcodes OP_AND, OP_OR, and OP_XOR.
//
void MakeSameSize(valtype& vch1, valtype& vch2) void MakeSameSize(valtype& vch1, valtype& vch2)
{ {
// Lengthen the shorter one // Lengthen the shorter one
if (vch1.size() < vch2.size()) if (vch1.size() < vch2.size())
// PATCH:
// +unsigned char msb = vch1[vch1.size()-1];
// +vch1[vch1.size()-1] &= 0x7f;
// vch1.resize(vch2.size(), 0);
// +vch1[vch1.size()-1] = msb;
vch1.resize(vch2.size(), 0); vch1.resize(vch2.size(), 0);
if (vch2.size() < vch1.size()) if (vch2.size() < vch1.size())
// PATCH:
// +unsigned char msb = vch2[vch2.size()-1];
// +vch2[vch2.size()-1] &= 0x7f;
// vch2.resize(vch1.size(), 0);
// +vch2[vch2.size()-1] = msb;
vch2.resize(vch1.size(), 0); vch2.resize(vch1.size(), 0);
} }
@ -663,6 +680,11 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
} }
break; break;
//
// WARNING: These disabled opcodes exhibit unexpected behavior
// when used on signed integers due to a bug in MakeSameSize()
// [see definition of MakeSameSize() above].
//
case OP_AND: case OP_AND:
case OP_OR: case OP_OR:
case OP_XOR: case OP_XOR:
@ -672,7 +694,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, co
return false; return false;
valtype& vch1 = stacktop(-2); valtype& vch1 = stacktop(-2);
valtype& vch2 = stacktop(-1); valtype& vch2 = stacktop(-1);
MakeSameSize(vch1, vch2); MakeSameSize(vch1, vch2); // <-- NOT SAFE FOR SIGNED VALUES
if (opcode == OP_AND) if (opcode == OP_AND)
{ {
for (unsigned int i = 0; i < vch1.size(); i++) for (unsigned int i = 0; i < vch1.size(); i++)