WSL kernel 6.6 disables `CONFIG_MODIFY_LDT_SYSCALL`

[JoshSchreuder]

Windows Version

Microsoft Windows [Version 10.0.26100.4061]

WSL Version

2.5.7.0

Are you using WSL 1 or WSL 2?

• [x] WSL 2
• [ ] WSL 1

Kernel Version

6.6.87.1-1

Distro Version

Ubuntu 24.04

Other Software

wibo, version 0.6.16

Repro Steps

• Update to WSL 2.5.7.0
• Run wibo

Expected Behavior

• wibo should run wrapped executable

Actual Behavior

• wibo exits with error

This was working previously with kernel 5.5, and also works when custom compiling 6.x kernel with CONFIG_MODIFY_LDT_SYSCALL re-enabled (was disabled in this commit)

With that said, this is a breaking change between kernel versions that was unexpected and not trivial to diagnose. Additionally resolving it requires custom compiling either a 5.x kernel which works by default, or a 6.x kernel with this flag enabled (since MS does not appear to have pre-built kernel bzImages available for older versions as far as I could tell)

Diagnostic Logs

WslLogs-2025-05-28_20-38-42.zip

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Diagnostic information

Issue was edited and new log file was found: https://github.com/user-attachments/files/20481117/WslLogs-2025-05-28_20-38-42.zip
.wslconfig found
Detected appx version: 2.5.7.0

[benhillis]

@chessturo - looks like CONFIG_MODIFY_LDT_SYSCALL was dropped from the 6.6 kernel.

[evmar]

FWIW, this also impacts https://github.com/evmar/retrowin32

$ zgrep MODIFY_LDT /proc/config.gz
# CONFIG_MODIFY_LDT_SYSCALL is not set

$ ...retrowin32 run
thread 'main' panicked at win32/src/ldt.rs:68:17:
syscall(SYS_modify_ldt): Function not implemented (os error 38)

[chessturo]

We're planning to fix this issue in the next kernel release. In the meantime, you can build a custom kernel with the config enabled as a workaround.