How does CorFlags.exe /32BIT+ work?

Question

I guess my question is about the CLR Loader. I want to understand the mechanics behind CorFlags.exe /32BIT+ functionality.

We know that when one starts an assembly compiled with the Any CPU flag set on 64-bit Windows, it starts as a 64-bit process. If one run CorFlags /32BIT+ on that assembly, it will start as a 32-bit process. I think this is a fascinating feature.

I have so many questions about it:

1. How is it implemented?
2. Does the OS Loader get involved?
3. Is possible to build a custom application (I guess an unmanaged one) that loads 32-bit or 64-bit CLR at a wish?

Is there an article, book, blog, etc that explains the inner workings of this feature?

Answer 1

This isn't well documented in any place I know of, I can only point you to a relevant MSDN article. Yes, your assumption is correct, the loader in Windows XP and up has awareness of managed executables. It automatically loads the .NET loader shim (c:\windows\system32\mscoree.dll), the relevant entrypoint is _CorValidateImage(). The Remarks section in the linked MSDN article describes the mechanism that turns a 32-bit .exe file into a 64-bit process:

In Windows XP and later versions, the operating system loader checks for managed modules by examining the COM Descriptor Directory bit in the common object file format (COFF) header. A set bit indicates a managed module. If the loader detects a managed module, it loads MsCorEE.dll and calls _CorValidateImage, which performs the following actions:

• Confirms that the image is a valid managed module.
• Changes the entry point in the image to an entry point in the common language runtime (CLR).
• For 64-bit versions of Windows, modifies the image that is in memory by transforming it from PE32 to PE32+ format.
• Returns to the loader when the managed module images are loaded.

For executable images, the operating system loader then calls the _CorExeMain function, regardless of the entry point specified in the executable. For DLL assembly images, the loader calls the _CorDllMain function.

_CorExeMain or _CorDllMain performs the following actions:

• Initializes the CLR.
• Locates the managed entry point from the assembly's CLR header.
• Begins execution.

The loader calls the _CorImageUnloading function when managed module images are unloaded. However, this function does not perform any action; it just returns.