[FS#3798] flashing the install image leaves overlay filesystem in undefined state

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FS#3798 - flashing the install image leaves overlay filesystem in undefined state
User who did this - Shivaram Lingamneni (slingamn)

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Thanks! I was able to reproduce this with the squashfs factory image:

* https://downloads.openwrt.org/releases/21.02.0-rc1/targets/bcm27xx/bcm2709/openwrt-21.02.0-rc1-bcm27xx-bcm2709-rpi-2-squashfs-factory.img.gz 
* SHA256SUM f07d683424fa1e7d642cd0a6a3136d0e56119dc44f8c1c83826961731064723f

using the following steps:

1. Download and flash the image to a SD card
2. Boot the device from the SD card, set a root password, ssh in
3. Create files on the overlayfs, e.g.

root at OpenWrt:~# df -Th
Filesystem           Type            Size      Used Available Use% Mounted on
/dev/root            squashfs        5.3M      5.3M         0 100% /rom
tmpfs                tmpfs         431.4M    212.0K    431.2M   0% /tmp
/dev/loop0           ext4           91.7M      1.5M     83.2M   2% /overlay
overlayfs:/overlay   overlay        91.7M      1.5M     83.2M   2% /
/dev/mmcblk0p1       vfat           63.9M     26.1M     37.7M  41% /boot
tmpfs                tmpfs         512.0K         0    512.0K   0% /dev
root at OpenWrt:~# touch /etc/openwrt_issue_fs_3798
root at OpenWrt:~# touch /usr/bin/openwrt_issue_fs_3798
root at OpenWrt:~# find /overlay/upper/ | grep 3798
/overlay/upper/usr/bin/openwrt_issue_fs_3798
/overlay/upper/etc/openwrt_issue_fs_3798
root at OpenWrt:~# find /overlay/upper/ | grep 3798 | xargs ls -lai
     49 -rw-r--r--    1 root     root             0 Apr 18 10:14 /overlay/upper/etc/openwrt_issue_fs_3798
     52 -rw-r--r--    1 root     root             0 Apr 18 10:14 /overlay/upper/usr/bin/openwrt_issue_fs_3798

4. `poweroff` the device
5. Reflash the SD card with the factory image, reinsert, boot up again
6. Root password remains set; files remain in place:

root at OpenWrt:~# find /overlay/upper/ | grep 3798 | xargs ls -lai
     49 -rw-r--r--    1 root     root             0 Apr 18 10:14 /overlay/upper/etc/openwrt_issue_fs_3798
     52 -rw-r--r--    1 root     root             0 Apr 18 10:14 /overlay/upper/usr/bin/openwrt_issue_fs_3798

What I said about the superblock is at best informed speculation :-) What I'm seeing is that the squashfs image (decompressed) is only 80881906 bytes long, but the total length of the partition table is 360447 sectors, i.e., 184548864 bytes. So there's data referenced in the partitions that is not overwritten by a factory flash. My belief is that the ext4 superblock of /overlay/upper is in this area.
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More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=3798#comment9636

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