Node.js v8.9.0 文档


vm (虚拟机)#

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稳定性: 2 - 稳定的

vm 模块提供了一系列 API 用于在 V8 虚拟机环境中编译和运行代码。 它可以通过以下方式使用:

const vm = require('vm');

JavaScript 代码可以被编译并立即运行,或编译、保存然后再运行。

注意: vm模块并不是实现代码安全性的一套机制。 绝不要试图用其运行未经信任的代码.

Class: vm.Script#

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vm.Script类型的实例包含若干预编译的脚本,这些脚本能够在特定的沙箱(或者上下文)中被运行。

new vm.Script(code, options)#

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  • code <string> 需要被解析的JavaScript代码
  • options
    • filename <string> 定义供脚本生成的堆栈跟踪信息所使用的文件名
    • lineOffset <number> 定义脚本生成的堆栈跟踪信息所显示的行号偏移
    • columnOffset <number> 定义脚本生成的堆栈跟踪信息所显示的列号偏移
    • displayErrors <boolean> 当值为真的时候,假如在解析代码的时候发生错误Error,引起错误的行将会被加入堆栈跟踪信息
    • timeout <number> 定义在被终止执行之前此code被允许执行的最大毫秒数。假如执行被终止,将会抛出一个错误[Error][]。
    • cachedData <Buffer> 为源码提供一个可选的存有v8代码缓存数据的Buffer。一旦提供了此Buffer,取决于v8引擎对Buffer中数据的接受状况,cachedDataRejected值将会被设为要么 真要么为假。
    • produceCachedData <boolean> 当值为真且cachedData不存在的时候,v8将会试图为code生成代码缓存数据。一旦成功,一个有V8代码缓存数据的Buffer将会被生成和储存在vm.Script返回的实例的cachedData属性里。 取决于代码缓存数据是否被成功生成,cachedDataProduced的值会被设置为true或者false。

创建一个新的vm.Script对象只编译代码但不会执行它。编译过的vm.Script此后可以被多次执行。值得注意的是,code是不绑定于任何全局对象的,相反,它仅仅绑定于每次执行它的对象。

script.runInContext(contextifiedSandbox[, options])#

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  • contextifiedSandbox <Object>vm.createContext()返回的[contextified][]对象
  • options <Object>
    • filename <string> 定义供脚本生成的堆栈跟踪信息所使用的文件名
    • lineOffset <number> 定义脚本生成的堆栈跟踪信息所显示的行号偏移
    • columnOffset <number> 定义脚本生成的堆栈跟踪信息所显示的列号偏移
    • displayErrors <boolean> 当值为真的时候,假如在解析代码的时候发生错误Error,引起错误的行将会被加入堆栈跟踪信息
    • timeout <number> 定义在被终止执行之前此code被允许执行的最大毫秒数。假如执行被终止,将会抛出一个错误Error
    • breakOnSigint: 若值为真,当收到SIGINT (Ctrl+C)事件时,代码会被终止执行。此外,通过process.on("SIGINT")方法所设置的消息响应机制在代码被执行时会被屏蔽,但代码被终止后会被恢复。如果执行被终止,一个错误Error会被抛出。

在指定的contextifiedSandbox中执行vm.Script对象中被编译后的代码并返回其结果。被执行的代码无法获取本地作用域。

以下的例子会编译一段代码,该代码会递增一个全局变量,给另外一个全局变量赋值。同时该代码被编译后会被多次执行。全局变量会被置于sandbox对象内。

const util = require('util');
const vm = require('vm');

const sandbox = {
  animal: 'cat',
  count: 2
};

const script = new vm.Script('count += 1; name = "kitty";');

const context = vm.createContext(sandbox);
for (let i = 0; i < 10; ++i) {
  script.runInContext(context);
}

console.log(util.inspect(sandbox));

// { animal: 'cat', count: 12, name: 'kitty' }

注意: 使用timeout或者breakOnSigint选项会导致若干新的事件循环以及对应的线程,这有一个非零的性能消耗。

script.runInNewContext([sandbox[, options]])#

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  • sandbox <Object> An object that will be contextified. If undefined, a new object will be created. 一个将被[contextified][]的对象。如果是undefined, 会生成一个新的对象
  • options <Object>
    • filename <string> 定义供脚本生成的堆栈跟踪信息所使用的文件名
    • lineOffset <number> 定义脚本生成的堆栈跟踪信息所显示的行号偏移
    • columnOffset <number> 定义脚本生成的堆栈跟踪信息所显示的列号偏移
    • displayErrors <boolean> 当值为真的时候,假如在解析代码的时候发生错误Error,引起错误的行将会被加入堆栈跟踪信息
    • timeout <number> 定义在被终止执行之前此code被允许执行的最大毫秒数。假如执行被终止,将会抛出一个错误Error

首先给指定的sandbox提供一个隔离的上下文, 再在此上下文中执行vm.Script中被编译的代码,最后返回结果。运行中的代码无法获取本地作用域。

以下的例子会编译一段代码,该代码会递增一个全局变量,给另外一个全局变量赋值。同时该代码被编译后会被多次执行。全局变量会被置于各个独立的sandbox对象内。

const util = require('util');
const vm = require('vm');

const script = new vm.Script('globalVar = "set"');

const sandboxes = [{}, {}, {}];
sandboxes.forEach((sandbox) => {
  script.runInNewContext(sandbox);
});

console.log(util.inspect(sandboxes));

// [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

script.runInThisContext([options])#

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  • options <Object>
    • filename <string> 定义供脚本生成的堆栈跟踪信息所使用的文件名
    • lineOffset <number> 定义脚本生成的堆栈跟踪信息所显示的行号偏移
    • columnOffset <number> 定义脚本生成的堆栈跟踪信息所显示的列号偏移
    • displayErrors <boolean> 当值为真的时候,假如在解析代码的时候发生错误Error,引起错误的行将会被加入堆栈跟踪信息
    • timeout <number> 定义在被终止执行之前此code被允许执行的最大毫秒数。假如执行被终止,将会抛出一个错误Error

在指定的global对象的上下文中执行vm.Script对象里被编译的代码并返回其结果。被执行的代码虽然无法获取本地作用域,但是能获取global对象。

以下的例子会编译一段代码,该代码会递增一个global变量。同时该代码被编译后会被多次执行。

const vm = require('vm');

global.globalVar = 0;

const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });

for (let i = 0; i < 1000; ++i) {
  script.runInThisContext();
}

console.log(globalVar);

// 1000

vm.createContext([sandbox])#

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If given a sandbox object, the vm.createContext() method will prepare that sandbox so that it can be used in calls to vm.runInContext() or script.runInContext(). Inside such scripts, the sandbox object will be the global object, retaining all of its existing properties but also having the built-in objects and functions any standard global object has. Outside of scripts run by the vm module, global variables will remain unchanged.

const util = require('util');
const vm = require('vm');

global.globalVar = 3;

const sandbox = { globalVar: 1 };
vm.createContext(sandbox);

vm.runInContext('globalVar *= 2;', sandbox);

console.log(util.inspect(sandbox)); // { globalVar: 2 }

console.log(util.inspect(globalVar)); // 3

If sandbox is omitted (or passed explicitly as undefined), a new, empty contextified sandbox object will be returned.

The vm.createContext() method is primarily useful for creating a single sandbox that can be used to run multiple scripts. For instance, if emulating a web browser, the method can be used to create a single sandbox representing a window's global object, then run all <script> tags together within the context of that sandbox.

vm.isContext(sandbox)#

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Returns true if the given sandbox object has been contextified using vm.createContext().

vm.runInContext(code, contextifiedSandbox[, options])#

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  • code <string> The JavaScript code to compile and run.
  • contextifiedSandbox <Object> The contextified object that will be used as the global when the code is compiled and run.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

The vm.runInContext() method compiles code, runs it within the context of the contextifiedSandbox, then returns the result. Running code does not have access to the local scope. The contextifiedSandbox object must have been previously contextified using the vm.createContext() method.

The following example compiles and executes different scripts using a single contextified object:

const util = require('util');
const vm = require('vm');

const sandbox = { globalVar: 1 };
vm.createContext(sandbox);

for (let i = 0; i < 10; ++i) {
  vm.runInContext('globalVar *= 2;', sandbox);
}
console.log(util.inspect(sandbox));

// { globalVar: 1024 }

vm.runInDebugContext(code)#

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Stability: 0 - Deprecated. An alternative is in development.
  • code <string> The JavaScript code to compile and run.

The vm.runInDebugContext() method compiles and executes code inside the V8 debug context. The primary use case is to gain access to the V8 Debug object:

const vm = require('vm');
const Debug = vm.runInDebugContext('Debug');
console.log(Debug.findScript(process.emit).name);  // 'events.js'
console.log(Debug.findScript(process.exit).name);  // 'internal/process.js'

Note: The debug context and object are intrinsically tied to V8's debugger implementation and may change (or even be removed) without prior warning.

The Debug object can also be made available using the V8-specific --expose_debug_as= command line option.

vm.runInNewContext(code[, sandbox][, options])#

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  • code <string> The JavaScript code to compile and run.
  • sandbox <Object> An object that will be contextified. If undefined, a new object will be created.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

The vm.runInNewContext() first contextifies the given sandbox object (or creates a new sandbox if passed as undefined), compiles the code, runs it within the context of the created context, then returns the result. Running code does not have access to the local scope.

The following example compiles and executes code that increments a global variable and sets a new one. These globals are contained in the sandbox.

const util = require('util');
const vm = require('vm');

const sandbox = {
  animal: 'cat',
  count: 2
};

vm.runInNewContext('count += 1; name = "kitty"', sandbox);
console.log(util.inspect(sandbox));

// { animal: 'cat', count: 3, name: 'kitty' }

vm.runInThisContext(code[, options])#

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  • code <string> The JavaScript code to compile and run.
  • options
    • filename <string> Specifies the filename used in stack traces produced by this script.
    • lineOffset <number> Specifies the line number offset that is displayed in stack traces produced by this script.
    • columnOffset <number> Specifies the column number offset that is displayed in stack traces produced by this script.
    • displayErrors <boolean> When true, if an Error error occurs while compiling the code, the line of code causing the error is attached to the stack trace.
    • timeout <number> Specifies the number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

vm.runInThisContext() compiles code, runs it within the context of the current global and returns the result. Running code does not have access to local scope, but does have access to the current global object.

The following example illustrates using both vm.runInThisContext() and the JavaScript eval() function to run the same code:

const vm = require('vm');
let localVar = 'initial value';

const vmResult = vm.runInThisContext('localVar = "vm";');
console.log('vmResult:', vmResult);
console.log('localVar:', localVar);

const evalResult = eval('localVar = "eval";');
console.log('evalResult:', evalResult);
console.log('localVar:', localVar);

// vmResult: 'vm', localVar: 'initial value'
// evalResult: 'eval', localVar: 'eval'

Because vm.runInThisContext() does not have access to the local scope, localVar is unchanged. In contrast, eval() does have access to the local scope, so the value localVar is changed. In this way vm.runInThisContext() is much like an indirect eval() call, e.g. (0,eval)('code').

Example: Running an HTTP Server within a VM#

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When using either script.runInThisContext() or vm.runInThisContext(), the code is executed within the current V8 global context. The code passed to this VM context will have its own isolated scope.

In order to run a simple web server using the http module the code passed to the context must either call require('http') on its own, or have a reference to the http module passed to it. For instance:

'use strict';
const vm = require('vm');

const code = `
(function(require) {
  const http = require('http');

  http.createServer((request, response) => {
    response.writeHead(200, { 'Content-Type': 'text/plain' });
    response.end('Hello World\\n');
  }).listen(8124);

  console.log('Server running at http://127.0.0.1:8124/');
})`;

vm.runInThisContext(code)(require);

Note: The require() in the above case shares the state with the context it is passed from. This may introduce risks when untrusted code is executed, e.g. altering objects in the context in unwanted ways.

What does it mean to "contextify" an object?#

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All JavaScript executed within Node.js runs within the scope of a "context". According to the V8 Embedder's Guide:

In V8, a context is an execution environment that allows separate, unrelated, JavaScript applications to run in a single instance of V8. You must explicitly specify the context in which you want any JavaScript code to be run.

When the method vm.createContext() is called, the sandbox object that is passed in (or a newly created object if sandbox is undefined) is associated internally with a new instance of a V8 Context. This V8 Context provides the code run using the vm module's methods with an isolated global environment within which it can operate. The process of creating the V8 Context and associating it with the sandbox object is what this document refers to as "contextifying" the sandbox.