Sunday, September 12, 2010

Using Downloaded IronPython Modules

One of Internet Explorer’s many “helpful” features is one that will “taint” any downloaded files as so that the system knows they are from the internet. Honestly, I can’t see what value this feature adds other than breaking CHM files, and preventing IronPython from using downloaded modules.
This was brought to my attention by Shay Friedman, who was trying to use IronPython.Zlib but couldn’t get it to work. In particular, the error message was misleading:
IronPython 2.6.1 (2.6.10920.0) on .NET 4.0.30319.1
Type "help", "copyright", "credits" or "license" for more information.
>>> import clr
>>> clr.AddReferenceToFileAndPath('C:\Users\Jeff\Downloads\IronPython.Zlib-2.6-clr4\IronPython.Zlib.dll')
Traceback (most recent call last):
  File "", line 1, in 
IOError: System.IO.IOException: file does not exist: C:\Users\Jeff\Downloads\IronPython.Zlib-2.6-clr4\IronPython.Zlib.dll
   at Microsoft.Scripting.Actions.Calls.MethodCandidate.Caller.Call(Object[] args, Boolean& shouldOptimize)
The file, of course, does exist, so why can’t IronPython find it?
There are actually a few things that interplay here: first, it must be downloaded with a  browser that taints the file (which I believe are just IE and Chrome), and second, it must be unzipped with Windows’ built in unzipping tools. The built in tools have the interesting property that when unzipping a tainted zip file will also taint all of the unzipped files. Finally, the punchline: .NET will not load an assembly that is tainted.
So how do we get around this? Well, you can:
  • use a different browser
  • use a different unzipping tool (I highly recommend 7-zip)
  • unblock the zip file prior to unzipping
To unblock the file, just right click on the zip file, click “Properties”, and click “Unblock”:
If you’ve already unzipped the file, you can just unblock the DLL. Depending on where you unzipped the file to, you my need to use an elevated Explorer window. You can also unblock multiple files from the command line.
This may well affect applications other than IronPython, so it’s just one more thing to watch for.