interop hangs inside sshd session initiated by Win32-OpenSSH client

[therealkenc]

• Your Windows build number: (Type ver at a Windows Command Prompt)

19569 WSL2

• What you're doing and what's happening

Start sshd in Ubuntu 18.04

$ sudo apt install openssh-server
$ sudo service ssh start

From a Windows CMD prompt, ssh into WSL and run a win32 command via WSL interop:

• What's wrong / what should be happening instead:

The cmd.exe /c echo hangs and does not respond to Ctrl-C on the pty. wsl.exe --shutdown or SIGINT/SIGTERM on the PID recovers.

• Strace of the failing command

Here is the gist.

[edit: more plausible strace interpretation]
The command loops on a poll() of (looks like) parent fd 0,7,8,9 and AF_VSOCK fd 4, maybe (?) doing some sort of keepalive exchange.

193   execve("/mnt/c/Windows/System32/cmd.exe", ["/mnt/c/Windows/System32/cmd.exe", "/c", "echo", "test"], 0x7ffe028f2430 /* 19 vars */) = 0
...
193   socket(AF_VSOCK, SOCK_STREAM|SOCK_CLOEXEC, 0) = 4
193   bind(4, {sa_family=AF_VSOCK, sa_data="\0\0\377\377\377\377\377\377\377\377\0\0\0\0"}, 16) = 0
...
193   poll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=9, events=POLLIN}, {fd=4, events=POLLIN}], 5, -1) = 1 ([{fd=0, revents=POLLIN}])
193   poll([{fd=0, events=POLLIN}], 1, -1) = 1 ([{fd=0, revents=POLLIN}])
193   read(0, "\3", 4096)               = 1
193   write(6, "\3", 1)                 = 1
193   poll([{fd=0, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=9, events=POLLIN}, {fd=4, events=POLLIN}], 5, -1

I suspect but can't prove the problem is related to Win32-OpenSSH aka OpenSSH in Windows and ConPTY. The problem does not manifest with a ssh localhost inside WSL, orssh from an external Linux box (say from a VM). This is blocking a generic resolution of question #4911.

[DHowett-MSFT]

This is probably resolved with the 8.x builds of the Win32-OpenSSH client. On startup, in “inherit cursor” mode, ConPTY blocks until it receives a cursor location. Old versions of W32-OpenSSH implemented their own VT input stack that couldn’t send the cursor position escapes.

[DHowett-MSFT]

This does mean that any SSH client running in a terminal that doesn’t support the DEC cursor position report request will experience a similar failure.

[therealkenc]

Yep. Thanks for the quick answer Dustin.