From 91f8626c4286b38717c43dbc53fe87939edaaf39 Mon Sep 17 00:00:00 2001
From: Roman Khimov <roman@nspcc.ru>
Date: Sun, 19 Apr 2020 19:15:00 +0300
Subject: [PATCH] vm: disallow negative shifts for SHL/SHR

Follow neo-project/neo-vm#257 change for Neo 3.
---
 pkg/vm/vm.go      | 16 +++-------------
 pkg/vm/vm_test.go |  8 ++------
 2 files changed, 5 insertions(+), 19 deletions(-)

diff --git a/pkg/vm/vm.go b/pkg/vm/vm.go
index e36ef521a..55133c3e2 100644
--- a/pkg/vm/vm.go
+++ b/pkg/vm/vm.go
@@ -53,7 +53,6 @@ const (
 	MaxStackSize = 2 * 1024
 
 	maxSHLArg = MaxBigIntegerSizeBits
-	minSHLArg = -MaxBigIntegerSizeBits
 )
 
 // VM represents the virtual machine.
@@ -797,23 +796,14 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		b := v.estack.Pop().BigInt().Int64()
 		if b == 0 {
 			return
-		} else if b < minSHLArg || b > maxSHLArg {
-			panic(fmt.Sprintf("operand must be between %d and %d", minSHLArg, maxSHLArg))
+		} else if b < 0 || b > maxSHLArg {
+			panic(fmt.Sprintf("operand must be between %d and %d", 0, maxSHLArg))
 		}
 		a := v.estack.Pop().BigInt()
 		v.checkBigIntSize(a)
 
-		newOp := op
-		if b < 0 {
-			b = -b
-			if op == opcode.SHR {
-				newOp = opcode.SHL
-			} else {
-				newOp = opcode.SHR
-			}
-		}
 		var item big.Int
-		if newOp == opcode.SHL {
+		if op == opcode.SHL {
 			item.Lsh(a, uint(b))
 		} else {
 			item.Rsh(a, uint(b))
diff --git a/pkg/vm/vm_test.go b/pkg/vm/vm_test.go
index dc847bef5..24e77eea0 100644
--- a/pkg/vm/vm_test.go
+++ b/pkg/vm/vm_test.go
@@ -884,16 +884,12 @@ func TestArithNegativeArguments(t *testing.T) {
 
 	t.Run("SHR", func(t *testing.T) {
 		t.Run("positive/positive", runCase(opcode.SHR, 5, 2, 1))
-		t.Run("positive/negative", runCase(opcode.SHR, 5, -2, 20))
 		t.Run("negative/positive", runCase(opcode.SHR, -5, 2, -2))
-		t.Run("negative/negative", runCase(opcode.SHR, -5, -2, -20))
 	})
 
 	t.Run("SHL", func(t *testing.T) {
 		t.Run("positive/positive", runCase(opcode.SHL, 5, 2, 20))
-		t.Run("positive/negative", runCase(opcode.SHL, 5, -2, 1))
 		t.Run("negative/positive", runCase(opcode.SHL, -5, 2, -20))
-		t.Run("negative/negative", runCase(opcode.SHL, -5, -2, -2))
 	})
 }
 
@@ -934,11 +930,11 @@ func TestSHRZero(t *testing.T) {
 	assert.Equal(t, makeStackItem([]byte{0, 1}), vm.estack.Pop().value)
 }
 
-func TestSHRSmallValue(t *testing.T) {
+func TestSHRNegative(t *testing.T) {
 	prog := makeProgram(opcode.SHR)
 	vm := load(prog)
 	vm.estack.PushVal(5)
-	vm.estack.PushVal(minSHLArg - 1)
+	vm.estack.PushVal(-1)
 	checkVMFailed(t, vm)
 }