abi: fix wrong unpack value for nagative int256

accounts/abi/unpack.go

@@ -26,7 +26,15 @@ import (
 )
 
 // reads the integer based on its kind
-func readInteger(kind reflect.Kind, b []byte) interface{} {
+func readInteger(typ byte, kind reflect.Kind, b []byte) interface{} {
+	var maxUint256 = big.NewInt(0).Add(
+		big.NewInt(0).Exp(big.NewInt(2), big.NewInt(256), nil),
+		big.NewInt(-1))
+
+	var maxInt256 = big.NewInt(0).Add(
+		big.NewInt(0).Exp(big.NewInt(2), big.NewInt(255), nil),
+		big.NewInt(-1))
+
 	switch kind {
 	case reflect.Uint8:
 		return b[len(b)-1]
@@ -45,7 +53,20 @@ func readInteger(kind reflect.Kind, b []byte) interface{} {
 	case reflect.Int64:
 		return int64(binary.BigEndian.Uint64(b[len(b)-8:]))
 	default:
-		return new(big.Int).SetBytes(b)
+		// the only case lefts for integer is int256/uint256.
+		// big.SetBytes can't tell if a number is negative, positive on itself.
+		// On EVM, if the returned number > max int256, it is negative.
+		ret := new(big.Int).SetBytes(b)
+		if typ == UintTy {
+			return ret
+		}
+
+		if ret.Cmp(maxInt256) > 0 {
+			ret.Add(maxUint256, big.NewInt(0).Neg(ret))
+			ret.Add(ret, big.NewInt(1))
+			ret.Neg(ret)
+		}
+		return ret
 	}
 }
 
@@ -179,7 +200,7 @@ func toGoType(index int, t Type, output []byte) (interface{}, error) {
 	case StringTy: // variable arrays are written at the end of the return bytes
 		return string(output[begin : begin+end]), nil
 	case IntTy, UintTy:
-		return readInteger(t.Kind, returnOutput), nil
+		return readInteger(t.T, t.Kind, returnOutput), nil
 	case BoolTy:
 		return readBool(returnOutput)
 	case AddressTy:

accounts/abi/unpack_test.go

@@ -117,6 +117,11 @@ var unpackTests = []unpackTest{
 		enc:  "0000000000000000000000000000000000000000000000000000000000000001",
 		want: big.NewInt(1),
 	},
+	{
+		def:  `[{"type": "int256"}]`,
+		enc:  "ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
+		want: big.NewInt(-1),
+	},
 	{
 		def:  `[{"type": "address"}]`,
 		enc:  "0000000000000000000000000100000000000000000000000000000000000000",