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Ruby用户指南 3、开始 4、简单的例子 5、字符串 6、正则表达式 7、数组 8、回到那些简单的例子 9、流程控制 10、迭代器 11、面向对象思维 12、方法 13、类 14、继承 15、重载方法 16、访问控制 17、单态方法 18、模块 19、过程对象 20、变量 21、全局变量 22、实变量 23、局部变量 24、类常量 25、异常处理:rescue 26、异常处理:ensure 27、存取器 28、对象的初始化 29、杂项 RGSS入门教程 1、什么是RGSS 2、开始：最简单的脚本 3、数据类型：数字 4、数据类型：常量与变量 5、数据类型：字符串 6、控制语句：条件分歧语句 7、控制语句：循环 8、函数 9、对象与类 10、显示图片 11、数组 12、哈希表（关联数组） 13、类 14、数据库 15、游戏对象 16、精灵的管理 17、窗口的管理 18、活动指令 19、场景类 Programming Ruby的翻译 Programming Ruby: The Pragmatic Programmer's Guide 前言序 Roadmap Ruby.new 类，对象和变量容器Containers，块Blocks和迭代Iterators 标准类型深入方法表达式Expressions 异常，捕捉和抛出（已经开始，by jellen）模块基本输入输出线程和进程当遭遇挫折 Ruby和它的世界 Ruby和Web开发 Ruby Tk Ruby 和微软的 Windows 扩展Ruby Ruby语言（by jellen）类和对象（by jellen） Ruby安全反射Reflection 内建类和方法标准库 OO设计网络和Web库 Windows支持内嵌文档交互式Ruby Shell 支持 Ruby参考手册 Ruby首页卷首语 Ruby的启动环境变量对象类执行结束时的相关处理线程安全模型正则表达式字句构造程序变量和常数字面值操作符表达式控制结构方法调用类／方法的定义内部函数内部变量内部常数内部类／模块／异常类附加库 Ruby变更记录 ruby 1.6 特性 ruby 1.7 特性 Ruby术语集 Ruby的运行平台 pack模板字符串 sprintf格式 Marshal格式 Ruby FAQ Ruby的陷阱

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Locking Ruby in the Safe

Walter Webcoder has a great idea for a portal site: The Web Arithmetic Page. Surrounded by all sorts of cool mathematical links and banner ads that will make him rich is a simple central frame, containing a text field and a button. Users type an arithmetic expression into the field, press the button, and the answer is displayed. All the world's calculators become obsolete overnight, and Walter cashes in and retires to devote his life to his collection of car license plate numbers.

Implementing the calculator is easy, thinks Walter. He accesses the contents of the form field using Ruby's CGI library, and uses the eval method to evaluate the string as an expression.

require 'cgi'

cgi = CGI::new("html4")

# Fetch the value of the form field "expression"
expr = cgi["expression"].to_s

begin
  result = eval(expr)
rescue Exception => detail
  # handle bad expressions
end

# display result back to user...

Roughly seven seconds after Walter puts the application online, a twelve-year-old from Waxahachie with glandular problems and no real life types ``system("rm *")'' into the form and, like his application, Walter's dreams come tumbling down.

Walter learned an important lesson: All external data is dangerous. Don't let it close to interfaces that can modify your system. In this case, the content of the form field was the external data, and the call to eval was the security breach.

Fortunately, Ruby provides support for reducing this risk. All information from the outside world can be marked as tainted. When running in a safe mode, potentially dangerous methods will raise a SecurityError if passed a tainted object.

Safe Levels

The variable $SAFE determines Ruby's level of paranoia. Table 20.1 on page 257 gives details of the checks performed at each safe level.

$SAFE	Constraints
0	No checking of the use of externally supplied (tainted) data is performed. This is Ruby's default mode.
>= 1	Ruby disallows the use of tainted data by potentially dangerous operations.
>= 2	Ruby prohibits the loading of program files from globally writable locations.
>= 3	All newly created objects are considered tainted.
>= 4	Ruby effectively partitions the running program in two. Nontainted objects may not be modified. Typically, this will be used to create a sandbox: the program sets up an environment using a lower `$SAFE` level, then resets `$SAFE` to 4 to prevent subsequent changes to that environment.

The default value of $SAFE is zero under most circumstances. However, if a Ruby script is run setuid or setgid,[A Unix script may be flagged to be run under a different user or group id than the person running it. This allows the script to have privileges that the user does not have; the script can access resources that the user would otherwise be prohibited from using. These scripts are called setuid or setgid.] its safe level is automatically set to 1. The safe level may also be set using the -T command-line option, and by assigning to $SAFE within the program. It is not possible to lower the value of $SAFE by assignment.

The current value of $SAFE is inherited when new threads are created. However, within each thread, the value of $SAFE may be changed without affecting the value in other threads. This facility may be used to implement secure ``sandboxes,'' areas where external code may run safely without risking the rest of your application or system. Do this by wrapping code that you load from a file in its own, anonymous module. This will protect your program's namespace from any unintended alteration.

f=open(fileName,"w")
f.print ...   # write untrusted program into file.
f.close
Thread.start {
  $SAFE = 4
  load(fileName, true)
}

With a $SAFE level of 4, you can load only wrapped files. See Kernel::load on page 418 for details.

Tainted Objects

Any Ruby object derived from some external source (for example, a string read from a file, or an environment variable) is automatically marked as being tainted. If your program uses a tainted object to derive a new object, then that new object will also be tainted, as shown in the code below. Any object with external data somewhere in its past will be tainted. This tainting process is performed regardless of the current safe level. You can inspect the tainted status of an object using Object#tainted? .

`# internal data`
`# =============`
`x1 = "a string"`
`x1.tainted?`