NEO Vm原理及其实现
简介及与evm主要区别
neo vm和evm类似。底层都实现了一套opcode以及对应的执行器，opcode设计差距蛮大的，总体上来说evm的更加简洁，neo vm的功能更加丰富强大。
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neo底层原生支持类型系统，所有栈上的内容都是有类型的，而在evm中所有的栈上内容都是无类型的，依赖于运行时转换。实质上在于空间和时间的取舍。
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neo在opcode级别支持类型及其操作，在内部函数调用方面evm通过jumpdest解决，neo vm的jump类指令则仅用于循环，函数调用则通过call和systemcall解决（这样很好的支持了调用函数栈）。总的来说neo vm一些高级操作可以下沉到code级别执行，方便了代码的转换。而evm则更加考验编译器的能力。
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隔离执行器和外部对象的具体实现，vm提供了一些外部接口用于执行一些和外部对象和存储相关的操作。eth的外部对象和指令揉合在一起。
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理论上来说eth也可以支持多种语言，实际上来说solidity一家独大，其中未尝没有eth的opcode转换难度大的原因，而neo vm的opcode则相对友好，支持多种语言开发。当然evm也并非没有好处，solidity支持内联编程，如果熟悉evm的opcode能写出效率非常高的合约。neo在这方面尚未提供支持。
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neo vm支持debug，支持step into，step over，break point operation。
如需了解evm的详细实现可以参考另一篇博文 https://my.oschina.net/hunjixin/blog/1805306
代码结构
. ├── ExecutionContext.cs //执行上下文 引擎，代码，断点 ├── ExecutionEngine.cs //执行引擎 ├── Helper.cs //编码及long型 ├── ICrypto.cs //加密解密 ├── IInteropInterface.cs //外部对象相关 ├── InteropService.cs //存储相关 ├── IScriptContainer.cs //脚本 ├── IScriptTable.cs //脚本读取 ├── neo-vm.csproj ├── OpCode.cs //操作码 ├── RandomAccessStack.cs //快速访问栈 ├── ScriptBuilder.cs //脚本构建 ├── StackItem.cs //栈 ├── Types //类型 │ ├── Array.cs //数组 │ ├── Boolean.cs //bool │ ├── ByteArray.cs //比特数组 │ ├── Integer.cs //整数 │ ├── InteropInterface.cs //操作对象接口 │ ├── Map.cs //映射 │ └── Struct.cs //结构体 └── VMState.cs //执行状态
类型系统
vm支持七种类型，分别是数组，bool，byte数组，整数，外部对象，映射，结构体。所有的结构体都是继承子StackItem，同时具有自己的数据字段。
stackItem
public abstract class StackItem : IEquatable<StackItem> { public abstract bool Equals(StackItem other); public sealed override bool Equals(object obj) { if (obj == null) return false; if (obj == this) return true; if (obj is StackItem other) return Equals(other); return false; } public static StackItem FromInterface(IInteropInterface value) { return new InteropInterface(value); } public virtual BigInteger GetBigInteger() { return new BigInteger(GetByteArray()); } public virtual bool GetBoolean() { return GetByteArray().Any(p => p != 0); } public abstract byte[] GetByteArray(); public override int GetHashCode() { unchecked { int hash = 17; foreach (byte element in GetByteArray()) hash = hash * 31 + element; return hash; } } public virtual string GetString() { return Encoding.UTF8.GetString(GetByteArray()); } public static implicit operator StackItem(int value) { return (BigInteger)value; } public static implicit operator StackItem(uint value) { return (BigInteger)value; } public static implicit operator StackItem(long value) { return (BigInteger)value; } public static implicit operator StackItem(ulong value) { return (BigInteger)value; } public static implicit operator StackItem(BigInteger value) { return new Integer(value); } public static implicit operator StackItem(bool value) { return new Boolean(value); } public static implicit operator StackItem(byte[] value) { return new ByteArray(value); } public static implicit operator StackItem(StackItem[] value) { return new Array(value); } public static implicit operator StackItem(List<StackItem> value) { return new Array(value); } }
bool 为例子
public class Boolean : StackItem { private static readonly byte[] TRUE = { 1 }; private static readonly byte[] FALSE = new byte[0]; private bool value; public Boolean(bool value) { this.value = value; } public override bool Equals(StackItem other) { if (ReferenceEquals(this, other)) return true; if (ReferenceEquals(null, other)) return false; if (other is Boolean b) return value == b.value; byte[] bytes_other; try { bytes_other = other.GetByteArray(); } catch (NotSupportedException) { return false; } return GetByteArray().SequenceEqual(bytes_other); } public override BigInteger GetBigInteger() { return value ? BigInteger.One : BigInteger.Zero; } public override bool GetBoolean() { return value; } public override byte[] GetByteArray() { return value ? TRUE : FALSE; } }
opecode
贴一些上来感受下
数值常量
PUSH2 = 0x52, // The number 2 is pushed onto the stack. PUSH3 = 0x53, // The number 3 is pushed onto the stack. PUSH4 = 0x54, // The number 4 is pushed onto the stack. PUSH5 = 0x55, // The number 5 is pushed onto the stack.
跳转
JMP = 0x62, JMPIF = 0x63, JMPIFNOT = 0x64,
调用
CALL = 0x65, RET = 0x66, APPCALL = 0x67, SYSCALL = 0x68, TAILCALL = 0x69,
栈操作，neo再着一块的相对比较丰富（这里并不是全部）
DROP = 0x75, // Removes the top stack item. DUP = 0x76, // Duplicates the top stack item. PICK = 0x79, // The item n back in the stack is copied to the top. ROLL = 0x7A, // The item n back in the stack is moved to the top. SWAP = 0x7C, // The top two items on the stack are swapped.
运算，仅贴了些代表性的上来
INC = 0x8B, // 1 is added to the input. SIGN = 0x8D, ABS = 0x90, // The input is made positive. NZ = 0x92, // Returns 0 if the input is 0. 1 otherwise. DIV = 0x96, // a is divided by b. MOD = 0x97, // Returns the remainder after dividing a by b. SHR = 0x99, // Shifts a right b bits, preserving sign. BOOLAND = 0x9A, // If both a and b are not 0, the output is 1. Otherwise 0. GTE = 0xA2, // Returns 1 if a is greater than or equal to b, 0 otherwise. MAX = 0xA4, // Returns the larger of a and b. WITHIN = 0xA5, // Returns 1 if x is within the specified range (left-inclusive), 0 otherwise.
加密验证
SHA1 = 0xA7, // The input is hashed using SHA-1. SHA256 = 0xA8, // The input is hashed using SHA-256. HASH160 = 0xA9, HASH256 = 0xAA, CHECKSIG = 0xAC, VERIFY = 0xAD, CHECKMULTISIG = 0xAE,
数组，结构体及相关操作
ARRAYSIZE = 0xC0, PACK = 0xC1, UNPACK = 0xC2, PICKITEM = 0xC3, SETITEM = 0xC4, NEWARRAY = 0xC5, //用作引用類型 NEWSTRUCT = 0xC6, //用作值類型 NEWMAP = 0xC7, APPEND = 0xC8, REVERSE = 0xC9, REMOVE = 0xCA, HASKEY = 0xCB, KEYS = 0xCC, VALUES = 0xCD,
异常
THROW = 0xF0, THROWIFNOT = 0xF1
外部接口
脚本容器，保存当前执行脚本
public interface IScriptContainer : IInteropInterface { byte[] GetMessage(); }
合约脚本查找
public interface IScriptTable { byte[] GetScript(byte[] script_hash); }
加密
public interface ICrypto { byte[] Hash160(byte[] message); byte[] Hash256(byte[] message); bool VerifySignature(byte[] message, byte[] signature, byte[] pubkey); }
外部服务调用接口
public class InteropService { private Dictionary<string, Func<ExecutionEngine, bool>> dictionary = new Dictionary<string, Func<ExecutionEngine, bool>>(); public InteropService() { Register("System.ExecutionEngine.GetScriptContainer", GetScriptContainer); Register("System.ExecutionEngine.GetExecutingScriptHash", GetExecutingScriptHash); Register("System.ExecutionEngine.GetCallingScriptHash", GetCallingScriptHash); Register("System.ExecutionEngine.GetEntryScriptHash", GetEntryScriptHash); } protected void Register(string method, Func<ExecutionEngine, bool> handler) { dictionary[method] = handler; } internal bool Invoke(string method, ExecutionEngine engine) { if (!dictionary.TryGetValue(method, out Func<ExecutionEngine, bool> func)) return false; return func(engine); } private static bool GetScriptContainer(ExecutionEngine engine) { engine.EvaluationStack.Push(StackItem.FromInterface(engine.ScriptContainer)); return true; } private static bool GetExecutingScriptHash(ExecutionEngine engine) { engine.EvaluationStack.Push(engine.CurrentContext.ScriptHash); return true; } private static bool GetCallingScriptHash(ExecutionEngine engine) { engine.EvaluationStack.Push(engine.CallingContext.ScriptHash); return true; } private static bool GetEntryScriptHash(ExecutionEngine engine) { engine.EvaluationStack.Push(engine.EntryContext.ScriptHash); return true; } }
外部对象接口
public interface IInteropInterface { }
执行器
public class ExecutionEngine : IDisposable { //调用栈 public RandomAccessStack<ExecutionContext> InvocationStack { get; } = new RandomAccessStack<ExecutionContext>(); //执行栈 public RandomAccessStack<StackItem> EvaluationStack { get; } = new RandomAccessStack<StackItem>(); //参数栈 public RandomAccessStack<StackItem> AltStack { get; } = new RandomAccessStack<StackItem>(); public ExecutionContext CurrentContext => InvocationStack.Peek(); public ExecutionContext CallingContext => InvocationStack.Count > 1 ? InvocationStack.Peek(1) : null; public ExecutionContext EntryContext => InvocationStack.Peek(InvocationStack.Count - 1); //执行状态 public VMState State { get; protected set; } = VMState.BREAK; //载入执行脚本 void LoadScript(byte[] script, bool push_only = false){} //添加断点 void AddBreakPoint(uint position){} //删除断点 bool RemoveBreakPoint(uint position){} //执行脚本 void Execute(){} //执行opcode void ExecuteOp(OpCode opcode, ExecutionContext context){} //执行下一步 void StepInto(){} //当前call执行完成 void StepOut(){} //全部执行 void StepOver(){} }
操作符号执行过程
private void ExecuteOp(OpCode opcode, ExecutionContext context) { if (opcode > OpCode.PUSH16 && opcode != OpCode.RET && context.PushOnly) { State |= VMState.FAULT; return; } if (opcode >= OpCode.PUSHBYTES1 && opcode <= OpCode.PUSHBYTES75) EvaluationStack.Push(context.OpReader.ReadBytes((byte)opcode)); else switch (opcode) { //常量push case OpCode.PUSH1: case OpCode.PUSH16: EvaluationStack.Push((int)opcode - (int)OpCode.PUSH1 + 1); break; case OpCode.JMP: //跳转 { int offset = context.OpReader.ReadInt16(); offset = context.InstructionPointer + offset - 3; if (offset < 0 || offset > context.Script.Length) { State |= VMState.FAULT; return; } bool fValue = true; if (opcode > OpCode.JMP) { fValue = EvaluationStack.Pop().GetBoolean(); if (opcode == OpCode.JMPIFNOT) fValue = !fValue; } if (fValue) context.InstructionPointer = offset; } break; case OpCode.CALL: //和systemcall差不多， 区别在于system是系统预先注册的函数 call调用的是用户自己写的函数 InvocationStack.Push(context.Clone()); context.InstructionPointer += 2; ExecuteOp(OpCode.JMP, CurrentContext); break; case OpCode.RET: //退出当前函数栈 InvocationStack.Pop().Dispose(); if (InvocationStack.Count == 0) State |= VMState.HALT; break; case OpCode.APPCALL: //调用外部合约 case OpCode.TAILCALL: { if (table == null) { State |= VMState.FAULT; return; } byte[] script_hash = context.OpReader.ReadBytes(20); if (script_hash.All(p => p == 0)) { script_hash = EvaluationStack.Pop().GetByteArray(); } byte[] script = table.GetScript(script_hash); if (script == null) { State |= VMState.FAULT; return; } if (opcode == OpCode.TAILCALL) InvocationStack.Pop().Dispose(); LoadScript(script); } break; case OpCode.SYSCALL: //内部合约函数调用 if (!service.Invoke(Encoding.ASCII.GetString(context.OpReader.ReadVarBytes(252)), this)) State |= VMState.FAULT; break; case OpCode.DROP: //移除栈顶 EvaluationStack.Pop(); break; case OpCode.DUP: //赋值栈顶 有对应按位置复制的指令 EvaluationStack.Push(EvaluationStack.Peek()); break; case OpCode.EQUAL: //判等 { StackItem x2 = EvaluationStack.Pop(); StackItem x1 = EvaluationStack.Pop(); EvaluationStack.Push(x1.Equals(x2)); } break; // Numeric case OpCode.ABS: //运算 加减乘除 最大值最小值等等 { BigInteger x = EvaluationStack.Pop().GetBigInteger(); EvaluationStack.Push(BigInteger.Abs(x)); } break; // Crypto case OpCode.SHA256: //加密 using (SHA256 sha = SHA256.Create()) { byte[] x = EvaluationStack.Pop().GetByteArray(); EvaluationStack.Push(sha.ComputeHash(x)); } break; case OpCode.CHECKSIG: //验证 { byte[] pubkey = EvaluationStack.Pop().GetByteArray(); byte[] signature = EvaluationStack.Pop().GetByteArray(); try { EvaluationStack.Push(Crypto.VerifySignature(ScriptContainer.GetMessage(), signature, pubkey)); } catch (ArgumentException) { EvaluationStack.Push(false); } } break; // Array case OpCode.PICKITEM: //数组映射取值 { StackItem key = EvaluationStack.Pop(); if (key is ICollection) { State |= VMState.FAULT; return; } switch (EvaluationStack.Pop()) { case VMArray array: int index = (int)key.GetBigInteger(); if (index < 0 || index >= array.Count) { State |= VMState.FAULT; return; } EvaluationStack.Push(array[index]); break; case Map map: if (map.TryGetValue(key, out StackItem value)) { EvaluationStack.Push(value); } else { State |= VMState.FAULT; return; } break; default: State |= VMState.FAULT; return; } } break; case OpCode.SETITEM: //数组 映射赋值 { StackItem value = EvaluationStack.Pop(); if (value is Struct s) value = s.Clone(); StackItem key = EvaluationStack.Pop(); if (key is ICollection) { State |= VMState.FAULT; return; } switch (EvaluationStack.Pop()) { case VMArray array: int index = (int)key.GetBigInteger(); if (index < 0 || index >= array.Count) { State |= VMState.FAULT; return; } array[index] = value; break; case Map map: map[key] = value; break; default: State |= VMState.FAULT; return; } } break; case OpCode.NEWARRAY: //创建数组 { int count = (int)EvaluationStack.Pop().GetBigInteger(); List<StackItem> items = new List<StackItem>(count); for (var i = 0; i < count; i++) { items.Add(false); } EvaluationStack.Push(new Types.Array(items)); } break; case OpCode.NEWSTRUCT: //创建结构体 { int count = (int)EvaluationStack.Pop().GetBigInteger(); List<StackItem> items = new List<StackItem>(count); for (var i = 0; i < count; i++) { items.Add(false); } EvaluationStack.Push(new VM.Types.Struct(items)); } break; case OpCode.NEWMAP: //创建映射 EvaluationStack.Push(new Map()); break; case OpCode.APPEND: //追加元素 { StackItem newItem = EvaluationStack.Pop(); if (newItem is Types.Struct s) { newItem = s.Clone(); } StackItem arrItem = EvaluationStack.Pop(); if (arrItem is VMArray array) { array.Add(newItem); } else { State |= VMState.FAULT; return; } } break; case OpCode.REMOVE: //移除元素 { StackItem key = EvaluationStack.Pop(); if (key is ICollection) { State |= VMState.FAULT; return; } switch (EvaluationStack.Pop()) { case VMArray array: int index = (int)key.GetBigInteger(); if (index < 0 || index >= array.Count) { State |= VMState.FAULT; return; } array.RemoveAt(index); break; case Map map: map.Remove(key); break; default: State |= VMState.FAULT; return; } } break; case OpCode.KEYS: //获取映射键集合，对应的还有获取值集合 haskey switch (EvaluationStack.Pop()) { case Map map: EvaluationStack.Push(new VMArray(map.Keys)); break; default: State |= VMState.FAULT; return; } break; // Exceptions case OpCode.THROW: //异常中止 State |= VMState.FAULT; return; case OpCode.THROWIFNOT: if (!EvaluationStack.Pop().GetBoolean()) { State |= VMState.FAULT; return; } break; default: State |= VMState.FAULT; return; } if (!State.HasFlag(VMState.FAULT) && InvocationStack.Count > 0) { //断点起效的位置 if (CurrentContext.BreakPoints.Contains((uint)CurrentContext.InstructionPointer)) State |= VMState.BREAK; } }
转载自 https://my.oschina.net/hunjixin/blog/1812516