Mode to disable backward jumps on Neo AppEngine 3

neo/SmartContract/ApplicationEngine.cs

@@ -15,6 +15,8 @@ public partial class ApplicationEngine : ExecutionEngine
         public const long GasFree = 0;
         private readonly long gas_amount;
         private readonly bool testMode;
+        // limited mode (disallow backward jumps and recursive calls)
+        private readonly bool isLimited;
         private readonly RandomAccessStack<UInt160> hashes = new RandomAccessStack<UInt160>();
         private readonly List<NotifyEventArgs> notifications = new List<NotifyEventArgs>();
         private readonly List<IDisposable> disposables = new List<IDisposable>();
@@ -29,10 +31,11 @@ public partial class ApplicationEngine : ExecutionEngine
         public IReadOnlyList<NotifyEventArgs> Notifications => notifications;
         internal Dictionary<UInt160, int> InvocationCounter { get; } = new Dictionary<UInt160, int>();
 
-        public ApplicationEngine(TriggerType trigger, IVerifiable container, Snapshot snapshot, long gas, bool testMode = false)
+        public ApplicationEngine(TriggerType trigger, IVerifiable container, Snapshot snapshot, long gas, bool testMode = false, bool isLimited = false)
         {
             this.gas_amount = GasFree + gas;
             this.testMode = testMode;
+            this.isLimited = isLimited;
             this.Trigger = trigger;
             this.ScriptContainer = container;
             this.Snapshot = snapshot;
@@ -81,6 +84,27 @@ protected override bool PreExecuteInstruction()
         {
             if (CurrentContext.InstructionPointer >= CurrentContext.Script.Length)
                 return true;
+
+            Instruction instruction = CurrentContext.CurrentInstruction;
+            switch(instruction.OpCode)
+            {
+                case OpCode.JMP:
+                case OpCode.JMPIF:
+                case OpCode.JMPIFNOT:
+                case OpCode.CALL:
+                {
+                    if(isLimited)
+                    {
+                        // no backwards jump in limited mode
+                        if(instruction.TokenI16 <= 0)
+                        {
+                            return false;
+                        }
+                    }
+                    break;
+                }
+            }
+
             return AddGas(OpCodePrices[CurrentContext.CurrentInstruction.OpCode]);
         }