NativeModuleWriter - Could not create header

[acizmarik]

Currently native writer fails for some assemblies.
This is caused by an unimplemented feature at the end of CreateHeaderSection in class NativeModuleWriter:

    //TODO: Support this too
    throw new ModuleWriterException("Could not create header");

Reproduce issue

1. Create clean console application using dotnet new console
2. Publish with ubuntu RID, for example, dotnet publish --runtime ubuntu.18.04-x64
3. Try to load published System.Private.CoreLib.dll and write it back to disk

    var module = AssemblyDef.Load("System.Private.CoreLib.dll");
    ((ModuleDefMD)module.ManifestModule).NativeWrite("System.Private.CorLib.dll-2");

Exprected result: Assembly is stored to disk
Actual result: An exception is thrown

dnlib.DotNet.Writer.ModuleWriterException: Could not create header
   at dnlib.DotNet.Writer.NativeModuleWriter.CreateHeaderSection(IChunk& extraHeaderData)
   at dnlib.DotNet.Writer.NativeModuleWriter.WriteFile()
   at dnlib.DotNet.Writer.NativeModuleWriter.WriteImpl()
   at dnlib.DotNet.Writer.ModuleWriterBase.Write(Stream dest)
   at dnlib.DotNet.Writer.ModuleWriterBase.Write(String fileName)

Is there any chance that this will be implemented?
Also let me know if you need additional info.

[0xd4d]

Yes, this is an old todo.

That file is probably an R2R assembly (created by the crossgen tool), and you should probably just save it as a normal IL assembly, and optionally run crossgen on it again.

[0xd4d]

If you save it as an IL assembly, you may need to clear the IL Library bit too.

[acizmarik]

I tried setting IsILOnly to true and saving it as normal IL library. It no longer crashes on assembly write. Also I noticed that the new assembly is significantly smaller (~14MB -> ~2,5MB).

However, now it crashes when I try to use it. Whenever I try to load an assembly which uses new System.Private.CoreLib it crashes with following message: Failed to initialize CoreCLR, HRESULT: 0x80004005. The result is same when trying both on Windows and on Linux.

When I try to run crossgen on new System.Private.CoreLib it throws this:

Could not load file or assembly '<PATH>\System.Private.CoreLib.dll'. Unspecified error (Exception from HRESULT: 0x80004005 (E_FAIL))
Error: compilation failed for ".\System.Private.CoreLib.dll" (0x80004005)

I forgot to mention that my original approach (using native write) worked on Windows and I have been actually using it for the past 3 or 4 months. I started to experience issues once I tried to get it working on Linux.

[acizmarik]

When testing new / instrumented assemblies I am doing it with following settings:
COMPlus_ReadyToRun = 0 - disable R2R
COMPlus_JITMinOpts = 1 - disable JIT optimizations

[0xd4d]

It's the IL Library bit, but there could be more bits. I suggest you compare the bits with a normal DLL and make sure they're identical. Then save it.

The file is smaller because it doesn't contain native code. System.Private.CoreLib.dll is crossgen'd.

[0xd4d]

Also compare other stuff, eg. the PE stuff such as the Machine property (should be eg. I386 or AMD64 etc)

[acizmarik]

You were right with your first comment. I did not realize that there is actually an ILLibrary flag in Cor20 header. Once the flag is cleared it works.

Just one more questions regarding the fix:

• Can I break something by "stripping" native code from assemblies? I am just curious if I should expect any problems or whether this is a completely valid operation.

Anyway thank you for your help with this issue. This is truly a great library.

[0xd4d]

I doubt you'll break anything, but you can always re-run crossgen to get the native code back and probably faster startup (make sure you use the correct version).

[acizmarik]

Ok. Thanks for you help!