vm: isolate stack in LoadScript

When calling contract it must be provided with a new stack containing
only it's arguments. The result is then copied back on RET.

Fix #1220.

pkg/core/interop_system_test.go

@@ -329,6 +329,7 @@ func getTestContractState() *state.Contract {
 		byte(opcode.ABORT), // abort if no offset was provided
 		byte(opcode.ADD), byte(opcode.RET),
 		byte(opcode.PUSH7), byte(opcode.RET),
+		byte(opcode.DROP), byte(opcode.RET),
 	}
 	h := hash.Hash160(script)
 	m := manifest.NewManifest(h)
@@ -348,6 +349,11 @@ func getTestContractState() *state.Contract {
 			Parameters: []manifest.Parameter{},
 			ReturnType: smartcontract.IntegerType,
 		},
+		{
+			Name:       "drop",
+			Offset:     5,
+			ReturnType: smartcontract.VoidType,
+		},
 	}
 	return &state.Contract{
 		Script:   script,
@@ -375,6 +381,7 @@ func TestContractCall(t *testing.T) {
 	m := manifest.NewManifest(hash.Hash160(currScript))
 	perm := manifest.NewPermission(manifest.PermissionHash, h)
 	perm.Methods.Add("add")
+	perm.Methods.Add("drop")
 	m.Permissions = append(m.Permissions, *perm)
 
 	require.NoError(t, ic.DAO.PutContractState(&state.Contract{
@@ -425,6 +432,15 @@ func TestContractCall(t *testing.T) {
 		t.Run("NotEnoughArguments", runInvalid(
 			stackitem.NewArray([]stackitem.Item{stackitem.Make(1)}), "add", h.BytesBE()))
 	})
+
+	t.Run("IsolatedStack", func(t *testing.T) {
+		initVM(v)
+		v.Estack().PushVal(stackitem.NewArray(nil))
+		v.Estack().PushVal("drop")
+		v.Estack().PushVal(h.BytesBE())
+		require.NoError(t, contractCall(ic, v))
+		require.Error(t, v.Run())
+	})
 }
 
 func TestContractCreate(t *testing.T) {

pkg/vm/vm.go

@@ -279,6 +279,7 @@ func (v *VM) LoadScript(b []byte) {
 // LoadScriptWithFlags loads script and sets call flag to f.
 func (v *VM) LoadScriptWithFlags(b []byte, f smartcontract.CallFlag) {
 	ctx := NewContext(b)
+	v.estack = v.newItemStack("estack")
 	ctx.estack = v.estack
 	ctx.tryStack = NewStack("exception")
 	ctx.callFlag = f