Strange behaviour with mount --bind

[Daniel]

The following problem appears with Ubuntu wily 15.10 and xenial 16.04:

I bind-mount my rootfs to a temporary folder. I explicitly use --bind and NOT --rbind. The result is as expected: I only see the files which are in the rootfs, any other mounts are invisible in this directory. But when I mount another directory later, It automatically also gets mounted in my temporary directory.

Example (tested with the Live CD xubuntu-16.04-desktop-amd64.iso):

root@xubuntu:~# mkdir /mnt/source
root@xubuntu:~# mkdir /mnt/target
root@xubuntu:~# mount --bind / /mnt/source
root@xubuntu:~# ls /mnt/source/mnt/target/
root@xubuntu:~# mount -t tmpfs none /mnt/target
root@xubuntu:~# touch /mnt/target/testfile
root@xubuntu:~# ls /mnt/source/mnt/target/
testfile

The testfile in the last line SHOULD NOT APPEAR because /mnt/source/mnt/target/ should not be mounted.

I don't find an explanation for this behaviour.

I found this because I wanted to run:

tar cpC /mnt/source/ . | tar xpC /mnt/target/

The command began to cyclic copy /mnt/source/mnt/target/mnt/target/mnt/target/mnt/target/

This scenario always worked in the past versions of Ubuntu and Debian

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: linux-image-4.4.0-21-generic 4.4.0-21.37
ProcVersionSignature: Ubuntu 4.4.0-21.37-generic 4.4.6
Uname: Linux 4.4.0-21-generic x86_64
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: xubuntu 1766 F.... pulseaudio
CasperVersion: 1.376
Date: Mon Apr 25 08:40:52 2016
IwConfig:
 enp0s3 no wireless extensions.

 lo no wireless extensions.
LiveMediaBuild: Xubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
Lsusb:
 Bus 001 Device 002: ID 80ee:0021 VirtualBox USB Tablet
 Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
MachineType: innotek GmbH VirtualBox
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcFB:

ProcKernelCmdLine: file=/cdrom/preseed/hostname.seed boot=casper initrd=/casper/initrd.lz quiet splash --- keyboard-configuration/layoutcode=ch
PulseList: Error: command ['pacmd', 'list'] failed with exit code 1: No PulseAudio daemon running, or not running as session daemon.
RelatedPackageVersions:
 linux-restricted-modules-4.4.0-21-generic N/A
 linux-backports-modules-4.4.0-21-generic N/A
 linux-firmware 1.157
RfKill:

SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 12/01/2006
dmi.bios.vendor: innotek GmbH
dmi.bios.version: VirtualBox
dmi.board.name: VirtualBox
dmi.board.vendor: Oracle Corporation
dmi.board.version: 1.2
dmi.chassis.type: 1
dmi.chassis.vendor: Oracle Corporation
dmi.modalias: dmi:bvninnotekGmbH:bvrVirtualBox:bd12/01/2006:svninnotekGmbH:pnVirtualBox:pvr1.2:rvnOracleCorporation:rnVirtualBox:rvr1.2:cvnOracleCorporation:ct1:cvr:
dmi.product.name: VirtualBox
dmi.product.version: 1.2
dmi.sys.vendor: innotek GmbH

[Brad Figg (brad-figg)]

This change was made by a bot.

[Daniel (hackie)]

I just found this mail which describes a problem which has the same root: http://www.spinics.net/lists/linux-fsdevel/msg92569.html:

root@xubuntu:~# mkdir tmp1
root@xubuntu:~# mkdir tmp2
root@xubuntu:~# mount -o bind /dev tmp1
root@xubuntu:~# ls tmp1/pts
root@xubuntu:~# mount -o bind /dev/pts tmp1/pts
root@xubuntu:~# ls tmp1/pts
0 1 2 3 4 5 6 ptmx
root@xubuntu:~# mount -o bind /dev tmp2
root@xubuntu:~# ls tmp1/pts tmp2/pts
tmp1/pts:
0 1 2 3 4 5 6 ptmx

tmp2/pts:
root@xubuntu:~# mount -o bind /dev/pts tmp2/pts
root@xubuntu:~# ls tmp1/pts tmp2/pts
tmp1/pts:
0 1 2 3 4 5 6 ptmx

tmp2/pts:
0 1 2 3 4 5 6 ptmx
root@xubuntu:~# grep pts /proc/mounts
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /root/tmp1/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /mnt/source/root/tmp1/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /root/tmp2/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /mnt/source/root/tmp1/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /root/tmp1/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
devpts /mnt/source/root/tmp2/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0

# now you see: pts got mounted to tmp1 again and even to /mnt/source from the first test - why???

root@xubuntu:~# umount tmp1/pts

# tmp1/pts should now be empty after unmounting, but it is mounted twice so the files are still visible. on the other hand: why did tmp2/pts unmount???:

root@xubuntu:~# ls tmp1/pts tmp2/pts
tmp1/pts:
0 1 2 3 4 5 6 ptmx

tmp2/pts:
root@xubuntu:~# umount tmp1/pts

# umount was possible a second time

root@xubuntu:~# ls tmp1/pts tmp2/pts
tmp1/pts:

tmp2/pts:

root@xubuntu:~#

[Seth Forshee (sforshee)]

This has to do with the propagation behavior of the bind mount.

# mount --bind src dest
# findmnt -o+PROPAGATION | grep dest
├─.../dest shared

Shared propagation means that mounts and unmounts in any of the mirrors propagate to the others. To change this:

# mount --make-private dest
# findmnt -o+PROPAGATION | grep dest
├─.../dest private
# mount ... src/mnt
# touch src/mnt/foo
# ls src/mnt
foo lost+found
# ls dest/mnt

dest/mnt will have the same contents as before the mount in src.

I think I remember something about the default behavior changing with the switch to systemd, from private to shared, but I can't remember for sure. If you're seeing a different behavior than in older, upstart-based releases of Ubuntu that may be why.

Marking this bug invalid because the kernel's behavior is exactly what's expected.