core/vm: review changes (gas, comments)

* Changed the gas for RETURNDATASIZE
* Added comments for read only check
* Fixed opReturnDataCopy

core/vm/instructions.go

@@ -30,7 +30,8 @@ import (
 
 var (
 	bigZero            = new(big.Int)
-	errWriteProtection = errors.New("evm write protection")
+	errWriteProtection = errors.New("evm: write protection")
+	errReadOutOfBound  = errors.New("evm: read out of bound")
 )
 
 func opAdd(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
@@ -778,9 +779,14 @@ func opReturnDataCopy(pc *uint64, evm *EVM, contract *Contract, memory *Memory,
 		cOff = stack.pop()
 		l    = stack.pop()
 	)
-	memory.Set(mOff.Uint64(), l.Uint64(), getData(evm.interpreter.returnData, cOff, l))
+	defer evm.interpreter.intPool.put(mOff, cOff, l)
+
+	cEnd := new(big.Int).Add(cOff, l)
+	if cEnd.BitLen() > 64 || uint64(len(evm.interpreter.returnData)) < cEnd.Uint64() {
+		return nil, errReadOutOfBound
+	}
+	memory.Set(mOff.Uint64(), l.Uint64(), evm.interpreter.returnData[cOff.Uint64():cEnd.Uint64()])
 
-	evm.interpreter.intPool.put(mOff, cOff, l)
 	return nil, nil
 }
 

core/vm/interpreter.go

@@ -93,7 +93,10 @@ func (in *Interpreter) enforceRestrictions(op OpCode, operation operation, stack
 	if in.evm.chainRules.IsMetropolis {
 		if in.readonly {
 			// if the interpreter is operating in readonly mode, make sure no
-			// state-modifying operation is performed.
+			// state-modifying operation is performed. The 4th stack item
+			// for a call operation is the value. Transfering value from one
+			// account to the others means the state is modified and should also
+			// return with an error.
 			if operation.writes ||
 				((op == CALL || op == CALLCODE) && stack.Back(3).BitLen() > 0) {
 				return errWriteProtection

core/vm/jump_table.go

@@ -84,7 +84,7 @@ func NewMetropolisInstructionSet() [256]operation {
 	}
 	instructionSet[RETURNDATASIZE] = operation{
 		execute:       opReturnDataSize,
-		gasCost:       constGasFunc(0), // TODO
+		gasCost:       constGasFunc(GasQuickStep),
 		validateStack: makeStackFunc(0, 1),
 		valid:         true,
 	}