How is that possible : the same partition mounted twice, and each mount shows a different content?

Question

Same partition mounted twice, showing a different content on each mountpoint. How is that possible ?

# mkdir /mnt/tmp
# mount -t ext4 -o noatime /dev/block/sda1 /mnt/tmp/

# mount | grep sda1
/dev/block/sda1 on /data type ext4 (rw,seclabel,noatime)
/dev/block/sda1 on /mnt/tmp type ext4 (rw,seclabel,noatime)

# ll /data
drwx------  2 root     root       4096 2021-08-19 18:34 adb
drwxrwxr-x  2 system   system     4096 2021-08-19 18:34 anr
drwxrwx--x  8 system   system     4096 2021-08-21 22:42 app
(...)
drwx--x--x  2 system   system     4096 2021-08-19 18:34 user
drwx--x--x  3 system   system     4096 2021-08-19 18:34 user_de
drwxrwx--x  3 root     root       4096 2021-08-19 18:34 vendor

# ll /mnt/tmp/                                                    
drwxr-xr-x 3 root root 4096 2021-08-19 18:33 android-8.1-r6
drwxr-xr-x 2 root root 4096 2021-08-19 18:33 grub
drwx------ 2 root root 4096 1970-01-01 01:00 lost+found

The same partition, /dev/block/sda1, is mounted twice, and each mountpoint (/data and /mnt/tmp) shows a different content.

Background : I wanted to modify GRUB's menu.lst file, and in android-x86 it appears that file is only available from withing GRUB's debug mode, or from android itself by mounting /dev/block/sda1. I did that, and indeed I could then access menu.lst (/mnt/tmp/grub/menu.lst), but how this works, and why /dev/block/sda1 shows a different content depending on the mountpoint is beyond me !

[ EDIT : answer given by user488112 below ]

It appears /data is in fact mounted on /android-8.1-r6/data, a sub-directory of /dev/block/sda1, and not on the "root" of dev/block/sda1. # mount is incomplete (it only shows the device block, not the "root" of the mount) :

# egrep '/data|/mnt/tmp' /proc/self/mountinfo                         
18 15 8:1 /android-8.1-r6/data /data rw,noatime shared:3 - ext4 /dev/block/sda1 rw,seclabel
237 28 8:1 / /mnt/tmp rw,noatime shared:23 - ext4 /dev/block/sda1 rw,seclabel

Answer 1

Here's how:

# cd /tmp
# mkdir foo bar
# mount -B /usr/bin foo
# mount -B /var/log bar
# mount | egrep 'foo|bar'
/dev/sda1 on /tmp/foo type ext4 (rw,relatime,errors=remount-ro)
/dev/sda1 on /tmp/bar type ext4 (rw,relatime,errors=remount-ro)

tl;dr; the output of the mount command is not reliable on modern versions of Linux. Use cat /proc/self/mountinfo instead (the format is documented in man 5 mountinfo), which is also showing the "root" of the mount, not just its "device":

# egrep 'foo|bar' /proc/self/mountinfo 
380 26 8:1 /usr/bin /tmp/foo rw,relatime shared:1 - ext4 /dev/sda1 rw,errors=remount-ro
457 26 8:1 /var/log /tmp/bar rw,relatime shared:1 - ext4 /dev/sda1 rw,errors=remount-ro