Why mounts are shared on Ubuntu 15.04, and how to turn it off?

Question

This breaks unshare(CLONE_NEWNS), and feels like leftover from some testings.

Ubuntu 15.04:

nir@reed:~$ grep '/ / ' /proc/self/mountinfo 
21 0 8:5 / / rw,relatime shared:1 - ext4 /dev/disk/by-uuid/70f9850c-8ebd-4881-a504-e14ec8d37f66 rw,discard,errors=remount-ro,data=ordered

Ubuntu 14.10:

nir@pilgrim:~$ grep '/ / ' /proc/self/mountinfo 
21 0 8:1 / / rw,relatime - ext4 /dev/disk/by-uuid/1d8ff8a8-6026-48dd-b6ce-e52b46c1f33d rw,errors=remount-ro,data=ordered

Answer 1

This may "feel" like "leftover from some testings", but it is not. It's a deliberate design choice by Lennart Poettering, made nearly three years ago in order to make systemd's containerization work. The intent was that if one wanted a private / mount one used a (future) /etc/fstab setting for it.

Note that those fstab options came about in those intervening years and now exist. One could also, alternatively, use them in an explicit /etc/systemd/system/-.mount unit.

See Poettering's own explanation.

Further reading

• Karel Zak. mount. 8. Ubuntu version 14 manual pages.
• Lennart Poettering. systemd.mount. systemd manual pages. freedesktop.org.
• Lennart Poettering. systemd-remount-fs.service. systemd manual pages. freedesktop.org.