WSL spawns an extra shell process

[yacoob]

Windows Version

Microsoft Windows [Version 10.0.22631.3593]

WSL Version

WSL version: 2.1.5.0

Are you using WSL 1 or WSL 2?

• WSL 2
• WSL 1

Kernel Version

5.15.146.1-microsoft-standard-WSL2

Distro Version

Debian 12 bookworm

Other Software

No response

Repro Steps

1. Install Debian via wsl.exe --install Debian, create thatuser account
2. Run wsl via wsl -d Debian -u thatuser
3. Run ps afux

Expected Behavior

There should be a single shell process that belongs to thatuser running on the system.

Actual Behavior

There are two shell processes running - one of them spawned by /init, handling the user interaction, the other spawned by /bin/login -f doing... something else.

root           2  0.0  0.0   2280  1300 ?        Sl   19:14   0:00 /init
root           8  0.0  0.0   2280     4 ?        Sl   19:14   0:00  \_ plan9 --control-socket 6 --log-level 4 --server-fd 7
root         136  0.0  0.0   2284   112 ?        Ss   19:14   0:00  \_ /init
root         137  0.0  0.0   2300   116 ?        S    19:14   0:00  |   \_ /init
thatuser     138  0.0  0.0   9732  3916 pts/0    Ss   19:14   0:00  |       \_ -bash
thatuser     172  0.0  0.0  13576  4304 pts/0    R+   19:18   0:00  |           \_ ps afux
root         139  0.0  0.0   4256  2692 pts/1    Ss   19:14   0:00  \_ /bin/login -f
thatuser     145  0.0  0.0   4188  3260 pts/1    S+   19:14   0:00      \_ -bash

Both of those shells are marked as a login shell. In another Debian 12 box I have access to, the process tree looks as expected: single login shell spawned by the login process:

root       73971  0.0  0.0   5800  3432 tty6     Ss   May22   0:00 /bin/login -p --
thatuser  211682  0.7  0.0  12564  6316 tty6     S    19:30   0:00  \_ -zsh
thatuser  211758  0.0  0.0  13568  4452 tty6     R+   19:30   0:00      \_ ps afux

Just for completness, I've tested how login -f works on a physical Debian12 box, and it did not spawn a second shell. From my cursory interrogation of /proc for both of those shells, they're both running in the same cgroup but different sessions. I have no idea how systemd perceives that due to #8879 .

Last but not least: mintty/wsltty#197 is very similar, except in my case I'm not using mintty at all :)

I'd appreciate some explanation around that second shell - why is it needed, what does it do, and - if it's a WSL thing - how can I get rid of it?

Thanks! :)

Diagnostic Logs

No response

[github-actions]

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[yacoob]

/question

[github-actions]

Diagnostic information

Found '/question', adding tag 'question'

[pmartincic]

By design. You won't get rid of it. You're using systemd with wsl I assume?

Edit: double checked, use wsl without systemd if you don't want that there. It creates a systemd user session.

[yacoob]

Thanks for the information, at least I know it's expected :) Is it documented anywhere?

[pmartincic]

It wouldn't be realistic for us to do that. So many pitfalls in doing that. We don't want people to rely on details like this. Details that are side effects of the implementation.

[yacoob]

I was thinking primarily along the lines of "there's this extra shell as a result of WSL inner workings". At least we have this bug now :)

Thanks!

[gruffdavies]

I just spent the better part of an afternoon trying to figure out wtf this extra login was (concerned about security) and eventually came across this... I second @yacoob on the documentation. if this is normal behaviour for systemd but isn't documented as such then it's suspicious...