Merge pull request #3649 from nspcc-dev/vm-modpow

vm: fix MODPOW operation

Author: AnnaShaleva

pkg/compiler/syscall_test.go

@@ -375,9 +375,9 @@ func TestOpcode(t *testing.T) {
 		}`
 		eval(t, src, []stackitem.Item{
 			stackitem.Make(1),
-			stackitem.Make(2),
+			stackitem.Make(-3),
 			stackitem.Make(1),
-			stackitem.Make(2),
+			stackitem.Make(-3),
 			stackitem.Make(52),
 		})
 	})

pkg/vm/vm.go

@@ -1055,6 +1055,12 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 				panic("zero modulus") // https://docs.microsoft.com/en-us/dotnet/api/system.numerics.biginteger.modpow?view=net-6.0#exceptions
 			}
 			res.Exp(base, exponent, modulus)
+
+			// https://github.com/nspcc-dev/neo-go/issues/3612
+			if base.Sign() < 0 && exponent.Bit(0) == 1 && res.Sign() != 0 {
+				absModulus := new(big.Int).Abs(modulus)
+				res.Sub(res, absModulus)
+			}
 		}
 
 		v.estack.PushItem(stackitem.NewBigInteger(res))

pkg/vm/vm_test.go

@@ -747,11 +747,12 @@ func TestMODMUL(t *testing.T) {
 func TestMODPOW(t *testing.T) {
 	prog := makeProgram(opcode.MODPOW)
 	t.Run("good, positive base", getTestFuncForVM(prog, 1, 3, 4, 5))
-	t.Run("good, negative base", getTestFuncForVM(prog, 2, -3, 5, 5))
+	t.Run("good, negative base", getTestFuncForVM(prog, -3, -3, 5, 5))
 	t.Run("good, positive base, negative mod", getTestFuncForVM(prog, 1, 3, 4, -5))
-	t.Run("good, negative base, negative mod", getTestFuncForVM(prog, 2, -3, 5, -5))
+	t.Run("good, negative base, negative mod", getTestFuncForVM(prog, -3, -3, 5, -5))
 	t.Run("bad, big negative exponent", getTestFuncForVM(prog, nil, 3, -2, 5))
 	t.Run("bad, zero modulus", getTestFuncForVM(prog, nil, 3, 4, 0))
+	t.Run("zero result, negative base, even exponent", getTestFuncForVM(prog, 0, -2, 3, 8))
 
 	t.Run("inverse compatibility", func(t *testing.T) { // Tests are taken from C# node.
 		t.Run("bad mod", getTestFuncForVM(prog, nil, 1, -1, 0))
@@ -760,6 +761,7 @@ func TestMODPOW(t *testing.T) {
 		t.Run("bad base", getTestFuncForVM(prog, nil, 0, -1, 1))
 		t.Run("no inverse exists", getTestFuncForVM(prog, nil, math.MaxUint16, -1, math.MaxUint8))
 		t.Run("good", getTestFuncForVM(prog, 52, 19, -1, 141))
+		t.Run("good", getTestFuncForVM(prog, 1, 5, -1, 4))
 	})
 }