This page https://source.android.com/docs/core/ota/apex

apex_payload.img is an ext4 file system image backed by dm-verity. The image is mounted at runtime via a loopback device.

I have installed a debug image on my android device and have used adb root;adb shell to get root access on the android device. When I look at the mounts I see a lot of /dev/block/loop* entries:

/dev/block/loop20 on /apex/com.android.apex.cts.shim@1 type ext4 (ro,dirsync,seclabel,nodev,noatime)
/dev/block/loop21 on /apex/com.android.vndk.v34@1 type ext4 (ro,dirsync,seclabel,nodev,noatime)

If I look at

# cat /sys/devices/virtual/block/loop20/loop/backing_file
/system/apex/com.android.apex.cts.shim.apex

If I look that apex file seems to be a zip file as the document above mentions:

# file /system/apex/com.android.apex.cts.shim.apex
/system/apex/com.android.apex.cts.shim.apex: Zip archive data, requires at least v1.0 to extract
# unzip -l /system/apex/com.android.apex.cts.shim.apex
Archive:  /system/apex/com.android.apex.cts.shim.apex
  Length      Date    Time    Name
---------  ---------- -----   ----
      644  2009-01-01 00:00   META-INF/MANIFEST.MF
     1124  2009-01-01 00:00   AndroidManifest.xml
       40  2009-01-01 00:00   resources.arsc
     1032  2009-01-01 00:00   apex_pubkey
      779  2009-01-01 00:00   META-INF/CERT.SF
     1722  2009-01-01 00:00   META-INF/CERT.RSA
   274432  2009-01-01 00:00   apex_payload.img
    57875  2009-01-01 00:00   assets/NOTICE.html.gz
       29  2009-01-01 00:00   apex_manifest.pb
      884  2009-01-01 00:00   apex_build_info.pb
---------                     -------
   338561                     10 files

I can understand that the apex_payload.img file is an ext4 file image. But the loop20 backing store is the whole zip file ?

I guess I was expecting that the apex_payload.img file would be extracted from the apex zip file somewhere and then that image file would be the backing_file store for the loop20 device. But that doesn’t seem to be the case.

Any insight on how the loop device can mount a zip file and be able to access the apex_payload.img file ?

Thanks
Chris