Mono Breaks WSL Interop

[reynoldsbd]

Version

Microsoft Windows [Version 10.0.22000.739]

WSL Version

• WSL 2
• WSL 1

Kernel Version

5.10.102.1-microsoft-standard-WSL2

Distro Version

Ubuntu 20.04

Other Software

mono-runtime, version 6.12.0.182-0xamarin1+ubuntu2004b1

Installed from APT source provided by upstream Mono project as documented here.

Reason I need latest Mono is so I can use NuGet.exe on Linux. This is the officially documented workflow for using NuGet on macOS/Linux.

Repro Steps

1. Within WSL, install mono-complete package according to the Mono project's instructions (linked above)
2. Within WSL, run the command notepad.exe

Expected Behavior

I expect to se the Notepad application start.

Actual Behavior

Notepad does not start, and the following is printed in my terminal:

user@host:~$ notepad.exe
Cannot open assembly '/mnt/c/windows/system32/notepad.exe': File does not contain a valid CIL image.
user@host:~$

I believe this is because the mono-runtime package is installing a new binary format (named cli) which matches any PE executable file or DLL.

I can work around by running the following command:

user@host:~$ sudo update-binfmts --disable cli

Diagnostic Logs

No response

[reynoldsbd]

Credit for workaround comes from here. I am opening this as a new issue because that issue was closed and appears to be ignored.

cc @benhillis who participated in the initial conversation.

[elsaco]

When installing mono on Linux it registers a new interpreter with binfmt_misc:

tux@RIPPER:~$ update-binfmts --display cli
cli (enabled):
     package = mono-runtime
        type = magic
      offset = 0
       magic = MZ
        mask =
 interpreter = /usr/bin/cli
    detector = /usr/lib/cli/binfmt-detector-cli

Notice the magic = MZ, notepad.exe has the same file signature 0x4D5A:

tux@RIPPER: xxd /mnt/c/Windows/system32/notepad.exe
00000000: 4d5a 9000 0300 0000 0400 0000 ffff 0000  MZ..............

so mono is trying to run it, however notepad is a win32 not .net application, thus the error.

As mention above and other posts cli can be unregistered with update-binfmts --disable cli

wsl --shutdown works also.

[benhillis]

This is by design. Mono is overwriting the WSL binfmt interpreter, there is nothing we can do about that.

[Biswa96]

Can init detect/distinguish native vs. managed PE?

[benhillis]

The problem is with mono registering a binfmt with magic = MZ, WSL will never get invoked. The kernel will use whatever interpreter was registered last.

[viceice]

update-binfmts --disable cli doesn't work on wsl with ubuntu 22.04 (upgraded from 20.04). I needed to remove mono completely (installed by nuget) and restart wsl.