From 787ee3a769d0bd1d85613610c92c80c3e0de067c Mon Sep 17 00:00:00 2001
From: Ekaterina Pavlova <ekt@morphbits.io>
Date: Mon, 28 Oct 2024 16:08:01 +0300
Subject: [PATCH] vm: fix MODPOW operation

Close #3612

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>
---
 pkg/compiler/syscall_test.go |  4 ++--
 pkg/vm/vm.go                 | 11 ++++++++---
 pkg/vm/vm_test.go            |  6 ++++--
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/pkg/compiler/syscall_test.go b/pkg/compiler/syscall_test.go
index 0995eb8bd7..4bc87feef4 100644
--- a/pkg/compiler/syscall_test.go
+++ b/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),
 		})
 	})
diff --git a/pkg/vm/vm.go b/pkg/vm/vm.go
index 4149f9ac64..ff0ddcaa36 100644
--- a/pkg/vm/vm.go
+++ b/pkg/vm/vm.go
@@ -1036,6 +1036,7 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		modulus := v.estack.Pop().BigInt()
 		exponent := v.estack.Pop().BigInt()
 		base := v.estack.Pop().BigInt()
+		absModulus := new(big.Int).Abs(modulus)
 		res := new(big.Int)
 		switch exponent.Cmp(bigMinusOne) {
 		case -1:
@@ -1044,17 +1045,21 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 			if base.Cmp(bigZero) <= 0 {
 				panic("invalid base")
 			}
-			if modulus.Cmp(bigTwo) < 0 {
+			if absModulus.Cmp(bigTwo) < 0 {
 				panic("invalid modulus")
 			}
-			if res.ModInverse(base, modulus) == nil {
+			if res.ModInverse(base, absModulus) == nil {
 				panic("base and modulus are not relatively prime")
 			}
 		case 1:
 			if modulus.Sign() == 0 {
 				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)
+			res.Exp(base, exponent, absModulus)
+
+			if base.Sign() < 0 && exponent.Bit(0) == 1 {
+				res.Sub(res, absModulus)
+			}
 		}
 
 		v.estack.PushItem(stackitem.NewBigInteger(res))
diff --git a/pkg/vm/vm_test.go b/pkg/vm/vm_test.go
index 38e29f1ff5..8337120438 100644
--- a/pkg/vm/vm_test.go
+++ b/pkg/vm/vm_test.go
@@ -747,9 +747,11 @@ 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, negative base", getTestFuncForVM(prog, -1, -1, 3, 3))
 	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("good, negative base, negative mod", getTestFuncForVM(prog, -2, -5, 5, -3))
 	t.Run("bad, big negative exponent", getTestFuncForVM(prog, nil, 3, -2, 5))
 	t.Run("bad, zero modulus", getTestFuncForVM(prog, nil, 3, 4, 0))