Ytdyklly

I have run the following docker command to create an instance of an Ubuntu image...

docker run -it ubuntu bash

when I issue a mount command I see the following...

root@ca13e87462b5:/# mount

overlay on / type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay2/l/O253SM3ZGX5EFP7OSBPPU5UW2C:/var/lib/docker/overlay2/l/IZEDWP7V4P4JWPLEL4EMN6GA5V:/var/lib/docker/overlay2/l/ZMFQPLBLMSGQYI7LDX5FSBHLEY:/var/lib/docker/overlay2/l/AVPGKJLG4AQJZLP3PKPGWQOMK6:/var/lib/docker/overlay2/l/6AFZ5ARHQ3PKYRWFQPUULZIMHB,upperdir=/var/lib/docker/overlay2/5a86c70a335f15d9c93bfb440b63b64aaef2b03b35ea42e3cef3609f96075514/diff,workdir=/var/lib/docker/overlay2/5a86c70a335f15d9c93bfb440b63b64aaef2b03b35ea42e3cef3609f96075514/work)

proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)

tmpfs on /dev type tmpfs (rw,nosuid,size=65536k,mode=755)

devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666)

sysfs on /sys type sysfs (ro,nosuid,nodev,noexec,relatime)

tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,relatime,mode=755)

cgroup on /sys/fs/cgroup/systemd type cgroup (ro,nosuid,nodev,noexec,relatime,xattr,name=systemd)

cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (ro,nosuid,nodev,noexec,relatime,net_cls,net_prio)

cgroup on /sys/fs/cgroup/perf_event type cgroup (ro,nosuid,nodev,noexec,relatime,perf_event)

cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (ro,nosuid,nodev,noexec,relatime,cpu,cpuacct)

cgroup on /sys/fs/cgroup/pids type cgroup (ro,nosuid,nodev,noexec,relatime,pids)

cgroup on /sys/fs/cgroup/rdma type cgroup (ro,nosuid,nodev,noexec,relatime,rdma)

cgroup on /sys/fs/cgroup/blkio type cgroup (ro,nosuid,nodev,noexec,relatime,blkio)

cgroup on /sys/fs/cgroup/cpuset type cgroup (ro,nosuid,nodev,noexec,relatime,cpuset)

cgroup on /sys/fs/cgroup/freezer type cgroup (ro,nosuid,nodev,noexec,relatime,freezer)

cgroup on /sys/fs/cgroup/devices type cgroup (ro,nosuid,nodev,noexec,relatime,devices)

cgroup on /sys/fs/cgroup/memory type cgroup (ro,nosuid,nodev,noexec,relatime,memory)

cgroup on /sys/fs/cgroup/hugetlb type cgroup (ro,nosuid,nodev,noexec,relatime,hugetlb)

mqueue on /dev/mqueue type mqueue (rw,nosuid,nodev,noexec,relatime)

/dev/sdb2 on /etc/resolv.conf type ext4 (rw,relatime,errors=remount-ro,data=ordered)

/dev/sdb2 on /etc/hostname type ext4 (rw,relatime,errors=remount-ro,data=ordered)

/dev/sdb2 on /etc/hosts type ext4 (rw,relatime,errors=remount-ro,data=ordered)

shm on /dev/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=65536k)

devpts on /dev/console type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=666)

proc on /proc/bus type proc (ro,relatime)

proc on /proc/fs type proc (ro,relatime)

proc on /proc/irq type proc (ro,relatime)

proc on /proc/sys type proc (ro,relatime)

proc on /proc/sysrq-trigger type proc (ro,relatime)

tmpfs on /proc/asound type tmpfs (ro,relatime)

tmpfs on /proc/acpi type tmpfs (ro,relatime)

tmpfs on /proc/kcore type tmpfs (rw,nosuid,size=65536k,mode=755)

tmpfs on /proc/keys type tmpfs (rw,nosuid,size=65536k,mode=755)

tmpfs on /proc/timer_list type tmpfs (rw,nosuid,size=65536k,mode=755)

tmpfs on /proc/sched_debug type tmpfs (rw,nosuid,size=65536k,mode=755)

tmpfs on /proc/scsi type tmpfs (ro,relatime)

tmpfs on /sys/firmware type tmpfs (ro,relatime)

So I issue a disk free command...

root@ca13e87462b5:/# df -h

Filesystem      Size  Used Avail Use% Mounted on

overlay         219G  162G   46G  79% /

tmpfs            64M     0   64M   0% /dev

tmpfs           7.9G     0  7.9G   0% /sys/fs/cgroup

/dev/sdb2       219G  162G   46G  79% /etc/hosts

shm              64M     0   64M   0% /dev/shm

tmpfs           7.9G     0  7.9G   0% /proc/asound

tmpfs           7.9G     0  7.9G   0% /proc/acpi

tmpfs           7.9G     0  7.9G   0% /proc/scsi

tmpfs           7.9G     0  7.9G   0% /sys/firmware

It seems to me the mount point for device /dev/sdb2 is a file - /etc/hosts.

I wasn't aware that I could mount a device to a file. I am misunderstanding what is happening here?

If I perform an ls on /etc I see hosts is a file...

root@ca13e87462b5:/# ls -al /etc

total 268

drwxr-xr-x 1 root root    4096 Feb 24 17:40 .

drwxr-xr-x 1 root root    4096 Feb 24 17:40 ..

-rw------- 1 root root       0 Feb  4 21:03 .pwd.lock

-rw-r--r-- 1 root root    3028 Feb  4 21:03 adduser.conf

drwxr-xr-x 2 root root    4096 Feb  4 21:04 alternatives

drwxr-xr-x 1 root root    4096 Feb  4 21:03 apt

-rw-r--r-- 1 root root    2319 Apr  4  2018 bash.bashrc

-rw-r--r-- 1 root root     367 Jan 27  2016 bindresvport.blacklist

drwxr-xr-x 2 root root    4096 Feb  4 21:04 cron.daily

-rw-r--r-- 1 root root    2969 Feb 28  2018 debconf.conf

-rw-r--r-- 1 root root      11 Jun 25  2017 debian_version

drwxr-xr-x 2 root root    4096 Feb  4 21:04 default

-rw-r--r-- 1 root root     604 Aug 13  2017 deluser.conf

drwxr-xr-x 1 root root    4096 Feb  4 21:04 dpkg

-rw-r--r-- 1 root root      96 Feb  4 21:03 environment

-rw-r--r-- 1 root root      37 Feb  4 21:02 fstab

-rw-r--r-- 1 root root    2584 Feb  1  2018 gai.conf

-rw-r--r-- 1 root root     446 Feb  4 21:03 group

-rw-r----- 1 root shadow   374 Feb  4 21:03 gshadow

-rw-r--r-- 1 root root      92 Apr  9  2018 host.conf

-rw-r--r-- 1 root root      13 Feb 24 17:40 hostname

-rw-r--r-- 1 root root     174 Feb 24 17:40 hosts

drwxr-xr-x 2 root root    4096 Feb  4 21:04 init.d

-rw-r--r-- 1 root root      26 Aug 19  2018 issue

-rw-r--r-- 1 root root      19 Aug 19  2018 issue.net

drwxr-xr-x 3 root root    4096 Feb  4 21:03 kernel

-rw-r--r-- 1 root root    7274 Feb  4 21:04 ld.so.cache

-rw-r--r-- 1 root root      34 Jan 27  2016 ld.so.conf

drwxr-xr-x 2 root root    4096 Feb  4 21:04 ld.so.conf.d

-rw-r--r-- 1 root root     267 Apr  9  2018 legal

-rw-r--r-- 1 root root     191 Feb  7  2018 libaudit.conf

-rw-r--r-- 1 root root   10550 Jan 25  2018 login.defs

drwxr-xr-x 2 root root    4096 Feb  4 21:04 logrotate.d

-rw-r--r-- 1 root root     105 Aug 19  2018 lsb-release

-rw-r--r-- 1 root root       0 Feb  4 21:04 machine-id

-rw-r--r-- 1 root root     812 Mar 24  2018 mke2fs.conf

lrwxrwxrwx 1 root root      12 Feb 24 17:40 mtab -> /proc/mounts

-rw-r--r-- 1 root root      91 Apr  9  2018 networks

-rw-r--r-- 1 root root     497 Oct  5  2016 nsswitch.conf

drwxr-xr-x 2 root root    4096 Feb  4 21:02 opt

lrwxrwxrwx 1 root root      21 Aug 19  2018 os-release -> ../usr/lib/os-release

-rw-r--r-- 1 root root     552 Apr  4  2018 pam.conf

drwxr-xr-x 2 root root    4096 Feb  4 21:04 pam.d

-rw-r--r-- 1 root root     926 Feb  4 21:03 passwd

-rw-r--r-- 1 root root     581 Apr  9  2018 profile

drwxr-xr-x 2 root root    4096 Feb  4 21:04 profile.d

drwxr-xr-x 2 root root    4096 Oct 25  2017 rc0.d

drwxr-xr-x 2 root root    4096 Oct 25  2017 rc1.d

drwxr-xr-x 2 root root    4096 Oct 25  2017 rc2.d

drwxr-xr-x 2 root root    4096 Oct 25  2017 rc3.d

drwxr-xr-x 2 root root    4096 Oct 25  2017 rc4.d

drwxr-xr-x 2 root root    4096 Oct 25  2017 rc5.d

drwxr-xr-x 2 root root    4096 Oct 25  2017 rc6.d

drwxr-xr-x 2 root root    4096 Feb  4 21:03 rcS.d

-rw-r--r-- 1 root root     628 Feb 24 17:40 resolv.conf

-rwxr-xr-x 1 root root     268 Jul 21  2017 rmt

-rw-r--r-- 1 root root    4141 Jan 25  2018 securetty

drwxr-xr-x 4 root root    4096 Feb  4 21:03 security

drwxr-xr-x 2 root root    4096 Feb  4 21:04 selinux

-rw-r----- 1 root shadow   501 Feb  4 21:03 shadow

-rw-r--r-- 1 root root      73 Feb  4 21:03 shells

drwxr-xr-x 2 root root    4096 Feb  4 21:03 skel

-rw-r--r-- 1 root root       0 Feb  4 21:03 subgid

-rw-r--r-- 1 root root       0 Feb  4 21:03 subuid

-rw-r--r-- 1 root root    2683 Jan 17  2018 sysctl.conf

drwxr-xr-x 2 root root    4096 Feb  4 21:04 sysctl.d

drwxr-xr-x 3 root root    4096 Oct 25  2017 systemd

drwxr-xr-x 2 root root    4096 Feb  4 21:04 terminfo

drwxr-xr-x 2 root root    4096 Feb  4 21:04 update-motd.d

No. Docker is bind mounting a file on a file.

From man 8 mount:

Bind mounts

Remount part of the file hierarchy somewhere else. The call is:

  mount --bind olddir newdir

...

One can also remount a single file (on a single file)

The reason why it looks like Docker is mounting a device on a file is that the mount -l command's output is incomplete: It does not show the source-path of the bind mount.

Again from man 8 mount:

Listing the mounts

The listing mode is maintained for backward compatibility only.

For more robust and customizable output use findmnt(8)

Run findmnt and you'll see the source of the bind mounts (in the form of device[source-path]).