Hello there,

Something™ breaks in util-linux >2.42, likely in `libmount1`.
This was caught by the `squashfs-mount` autopkgtest in Ubuntu, because we do run them with `isolation-machine` as opposed to Debian:
* https://ci.debian.net/packages/s/squashfs-mount/unstable/amd64/
* https://autopkgtest.ubuntu.com/packages/s/squashfs-mount/stonking/amd64

I didn't dig into the `util-linux` codebase, but here is a simple reproducer script:
```bash
#!/usr/bin/bash

WORK_DIR=$(mktemp -d)
cd "$WORK_DIR"

mkdir -p folder
echo "Hello there" > folder/hello.txt
mksquashfs folder image.sqfs -quiet -noappend

mkdir -p mnt
squashfs-mount image.sqfs:mnt -- cat mnt/hello.txt

cd -
rm "$WORK_DIR" -rf
```

Run that in an Ubuntu Stonking VM against 2.41.3-3ubuntu2:
```
skia@kiffex:~
❯ lxc launch -e ubuntu-daily:26.10 ubuntu-stonking --vm
Launching ubuntu-stonking

skia@kiffex:~
❯ lxc shell ubuntu-stonking
root@ubuntu-stonking:~# apt update -qq
178 packages can be upgraded. Run 'apt list --upgradable' to see them.
root@ubuntu-stonking:~# apt install -qq squashfs-mount
Installing:
   squashfs-mount
[...]
root@ubuntu-stonking:~# su ubuntu
ubuntu@ubuntu-stonking:~$ dpkg-query -W libmount1
libmount1:amd64	2.41.3-3ubuntu2
ubuntu@ubuntu-stonking:~$ ./reproducer.sh
[==================================================================================================================================================|] 1/1 100%
Hello there
/home/ubuntu
ubuntu@ubuntu-stonking:~$
```

And the same against 2.42.3-1 on Debian unstable:
```
skia@kiffex:~
❯ lxc launch -e images:debian/sid debian-sid --vm
Launching debian-sid

skia@kiffex:~
❯ lxc shell debian-sid
root@debian-sid:~# apt update && apt install squashfs-tools squashfs-mount
[...]
root@debian-sid:~# useradd debian
root@debian-sid:~# su debian
$ bash
debian@debian-sid:/root$ cd /tmp
debian@debian-sid:/tmp$ vim reproducer.sh
debian@debian-sid:/tmp$ dpkg-query -W libmount1
libmount1:amd64	2.42.3-1
debian@debian-sid:/tmp$ ./reproducer.sh
[==================================================================================================================================================|] 1/1 100%
/tmp/tmp.fgOxcel9XK/mnt: can't find in /etc/fstab
/tmp

```

I left severity "normal" because technically, the Debian QA didn't raise an eyebrow on the issue, so nothing is broken per say, but I would consider raising the priority considering this is indeed breaking other packages.

Let me know if you need anything else to help reproduce or debug.

Cheers
Skia

* Florent 'Skia' Jacquet <skia@ubuntu.com> [260909 14:05]:

util-linux got more strict in various "mount" topics. You could try
building with --disable-libmount-mountfd-support and see if that
changes anything for squashfs-mount.

I imagine other packages will need to follow the changes, whatever
they are. Obviously best done upstream.

Chris