Order of mounting of entries in a mount namespace

Question

I'm aware of unshare -m creates a new mount namespace moving the process executing it into the new mount namespace being created.

The latter gets a copy of parent's mount namespace. Indeed look at the following

root@ubuntu:~# cat /proc/self/mountinfo 
25 31 0:23 / /sys rw,nosuid,nodev,noexec,relatime shared:7 - sysfs sysfs rw
26 31 0:24 / /proc rw,nosuid,nodev,noexec,relatime shared:12 - proc proc rw
27 31 0:5 / /dev rw,nosuid,relatime shared:2 - devtmpfs udev rw,size=4008708k,nr_inodes=1002177,mode=755,inode64
28 27 0:25 / /dev/pts rw,nosuid,noexec,relatime shared:3 - devpts devpts rw,gid=5,mode=620,ptmxmode=000
29 31 0:26 / /run rw,nosuid,nodev,noexec,relatime shared:5 - tmpfs tmpfs rw,size=812844k,mode=755,inode64
31 1 8:2 / / rw,relatime shared:1 - ext4 /dev/sda2 rw
32 25 0:6 / /sys/kernel/security rw,nosuid,nodev,noexec,relatime shared:8 - securityfs securityfs rw
33 27 0:28 / /dev/shm rw,nosuid,nodev shared:4 - tmpfs tmpfs rw,inode64
34 29 0:29 / /run/lock rw,nosuid,nodev,noexec,relatime shared:6 - tmpfs tmpfs rw,size=5120k,inode64
35 25 0:30 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime shared:9 - cgroup2 cgroup2 rw,nsdelegate,memory_recursiveprot
36 25 0:31 / /sys/fs/pstore rw,nosuid,nodev,noexec,relatime shared:10 - pstore pstore rw
37 25 0:32 / /sys/fs/bpf rw,nosuid,nodev,noexec,relatime shared:11 - bpf bpf rw,mode=700
38 26 0:33 / /proc/sys/fs/binfmt_misc rw,relatime shared:13 - autofs systemd-1 rw,fd=29,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=17383
39 27 0:20 / /dev/mqueue rw,nosuid,nodev,noexec,relatime shared:14 - mqueue mqueue rw
40 27 0:34 / /dev/hugepages rw,relatime shared:15 - hugetlbfs hugetlbfs rw,pagesize=2M
41 25 0:7 / /sys/kernel/debug rw,nosuid,nodev,noexec,relatime shared:16 - debugfs debugfs rw
42 25 0:12 / /sys/kernel/tracing rw,nosuid,nodev,noexec,relatime shared:17 - tracefs tracefs rw
---------------------------- output omitted ------------------------------------------
root@ubuntu:~#

root@ubuntu:~# unshare -m /bin/bash
root@ubuntu:~# 
root@ubuntu:~# cat /proc/self/mountinfo 
714 713 8:2 / / rw,relatime - ext4 /dev/sda2 rw
715 714 0:5 / /dev rw,nosuid,relatime - devtmpfs udev rw,size=4008708k,nr_inodes=1002177,mode=755,inode64
716 715 0:25 / /dev/pts rw,nosuid,noexec,relatime - devpts devpts rw,gid=5,mode=620,ptmxmode=000
719 715 0:28 / /dev/shm rw,nosuid,nodev - tmpfs tmpfs rw,inode64
720 715 0:20 / /dev/mqueue rw,nosuid,nodev,noexec,relatime - mqueue mqueue rw
725 715 0:34 / /dev/hugepages rw,relatime - hugetlbfs hugetlbfs rw,pagesize=2M
726 714 0:26 / /run rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,size=812844k,mode=755,inode64
739 726 0:29 / /run/lock rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,size=5120k,inode64
740 726 0:36 / /run/credentials/systemd-sysusers.service ro,nosuid,nodev,noexec,relatime - ramfs none rw,mode=700
741 726 0:26 /snapd/ns /run/snapd/ns rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,size=812844k,mode=755,inode64
742 726 0:26 /netns /run/netns rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,size=812844k,mode=755,inode64
743 726 0:46 / /run/user/1000 rw,nosuid,nodev,relatime - tmpfs tmpfs rw,size=812840k,nr_inodes=203210,mode=700,uid=1000,gid=1000,inode64
744 714 0:23 / /sys rw,nosuid,nodev,noexec,relatime - sysfs sysfs rw
745 744 0:6 / /sys/kernel/security rw,nosuid,nodev,noexec,relatime - securityfs securityfs rw
746 744 0:30 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - cgroup2 cgroup2 rw,nsdelegate,memory_recursiveprot
747 744 0:31 / /sys/fs/pstore rw,nosuid,nodev,noexec,relatime - pstore pstore rw
748 744 0:32 / /sys/fs/bpf rw,nosuid,nodev,noexec,relatime - bpf bpf rw,mode=700
814 744 0:7 / /sys/kernel/debug rw,nosuid,nodev,noexec,relatime - debugfs debugfs rw
815 744 0:12 / /sys/kernel/tracing rw,nosuid,nodev,noexec,relatime - tracefs tracefs rw
---------------------------- output omitted ------------------------------------------
root@ubuntu:~#

The outputs are slightly different, even though the mountpoint's parent-child dependencies are the same (different Ids in the first two columns are expected since mount namespaces are not the same).

Now the question is: is there a reason behind the different order in which the mountpoints are actually shown ?

Answer 1

The kernel documentation makes absolutely no guarantees on the order of the entries in /proc/*/mountinfo. (It happens such that they are ordered by ID, but the ID itself is just that – a unique ID, not anything meaningful.)

So, no, this order has no meaning. Neither in the parent nor in the child namespace. Thus, the change of ordering can have no meaning, either.