Prevent binfmt_misc unmount propagation across distros

[benhillis]

Summary

Minimal cherry-pick of #40612 for the 2.7 release branch.

Fixes an Exec format error bug where Windows interop breaks in all running distros when a peer distro with systemd shuts down.

Root Cause

/proc is mounted with MS_SHARED, so binfmt_misc inherits shared mount propagation. When a peer distro's systemd shuts down and unmounts binfmt_misc in its namespace, the unmount propagates back through the shared peer group to init's namespace, wiping ALL registrations globally.

Fix

One additional mount() call with MS_PRIVATE immediately after mounting binfmt_misc. This is the minimal version — the master branch PR also extends UtilMount to handle propagation flags generically.

PR Checklist

• Builds cleanly
• Includes regression test (InteropSurvivesPeerDistroShutdown)
• Passes clang-format

Fixes #13885

[Copilot]

Pull request overview

This PR is a minimal backport for the 2.7 release branch to prevent binfmt_misc unmount propagation across distros, which can otherwise clear global binfmt registrations and break Windows interop (Exec format error) when a peer systemd-enabled distro shuts down.

Changes:

• Mark the binfmt_misc mount as MS_PRIVATE after mounting to isolate it from peer namespace unmount propagation.
• Add a WSL2 regression test that imports a peer distro, enables systemd, terminates it, and validates interop/binfmt state survives.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/linux/init/main.cpp	Marks the binfmt_misc mount private to prevent shutdown unmount propagation from affecting other distros.
test/windows/UnitTests.cpp	Adds a regression test intended to validate interop survives a peer systemd distro termination.

[benhillis]

Closing in favor of #14443, which is the correct fix and has CI passing.

Our MS_PRIVATE approach was wrong: binfmt_misc entries are scoped per user-namespace (or effectively global), not per mount-namespace. WSL peer distros share the root user-ns (no CLONE_NEWUSER), so peer's systemd-binfmt --unregister -> write -1 to /proc/sys/fs/binfmt_misc/status clears the global entry table regardless of mount propagation.

Once #14443 merges to master, we'll do a clean cherry-pick backport to release/2.7.