Plugin Engine

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Revision as of 13:35, 26 May 2005 by Mashiharu (Talk | contribs)
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A C#/VB/JScript.NET Plugin Engine has been added to World Wind (still unreleased, only available from CVS).

Contents

Features

  • Dynamic compilation and execution of script code written in the following languages:
    • C#
    • VB.NET
    • JSCript.net
  • Load plugins from precompiled .dll files
  • Full access to World Wind internals (same as internal code)
  • Runs at full speed (compilation on load)

Engine logic

When World Wind starts the following steps are performed:

  • The ScriptCompiler scans bin\Plugins directory and the 1st level subdirectories of the plugin directory for files ending in any of the supported file extensions.
  • When it finds one it tries to compile the source (to memory) with references to the same assemblies as the app has referenced.
  • It then searches the compiled assembly for an implementation of Plugin.

Benefits

  • Since the compiler for C#,VB and JSCript comes with the .Net Framework no additional third party library is required.
    • For other languages (like J#) an install will be needed, and unfortunately I haven't found an easy way of enumerating all available compilers (before .Net 2.0)
  • Very fast on-the-fly compilation of the scripts.
  • Scripts executes with same speed as a precompiled assembly.
  • Debuggable inside World Wind project (World Wind in debug build loads internal plugins)
  • Custom dlls may be dynamically loaded

Drawbacks

  • Security: The script could wipe out the hard drive or worse. But with a script at least the user can verify the intentions by looking at the source. Even current add-ons (layers) are often .exe installers giving the user no way of identifying evil code.
  • To unload a script's assembly from memory completely, WW needs a restarted (there are workarounds (appdomains))
  • With no nailed down limited interface into WW the script could stop working on a new version of WW.

Security Lockdown Fix

Following this example we should be able to lockdown FileIO to the AppDir

public static string ReadFile(string filename)
{
  string appDir = Directory.GetCurrentDirectory();
  FileIOPermission f = new FileIOPermission(PermissionState.None);
  f.SetPathList(FileIOPermissionAccess.Read, appDir);
  f.SetPathList(FileIOPermissionAccess.PathDiscovery, appDir);
  f.PermitOnly(); 

  // Use Path.GetFilePath() to canonicalize the file name
  // Use FileStream.OpenRead to open the file
  // Use FileStream.Read to access and return the data
}

From Code Access Security in Practice - MSDN

Code Translator

C#<->VB [1] So no one can complain about languages. :)

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