Hey there, let's dig into this loop device issue you're facing—since you've already covered the basic permission checks, let's dive into some kernel and platform-specific angles that are easy to miss on MTK devices running older Android kernels.

1. Verify Kernel Loop Module Support #

First, confirm if your kernel actually supports loop devices—older MTK kernels sometimes omit this feature or lock it down:

• Run this command to check kernel config:

  zcat /proc/config.gz | grep CONFIG_BLK_DEV_LOOP
  

  • If you see CONFIG_BLK_DEV_LOOP=y, loop support is built-in.
  • If it shows =m, you'll need to load the module manually (path may vary):

    insmod /system/lib/modules/loop.ko
    

  • If there's no output at all, your kernel doesn't support loop devices—you'll need a custom kernel with this flag enabled.

2. SELinux Contexts (Critical for Android 7) #

Even with root, SELinux can silently block loop operations on Android 7. Let's rule this out:

• Check current SELinux mode:

  getenforce
  

  • If it returns Enforcing, switch to Permissive temporarily to test:

    setenforce 0
    

    If this fixes the issue, you'll need to add a custom SELinux policy rule to make the change permanent.
• Verify the security context of your image file and mount point:

  ls -Z /data/adb/your-image.img
  ls -Z /path/to/your/mount-point
  

  You may need to align the image's context with the mount point, e.g.:

  chcon u:object_r:system_file:s0 /data/adb/your-image.img
  

3. Explicit Loop Device Configuration #

You mentioned using losetup, but let's try more explicit commands to avoid auto-detection quirks:

• First, detach any existing loop devices:

  losetup -D
  

• Manually create the loop device node if it doesn't exist:

  mknod /dev/block/loop0 b 7 0
  

• Assign the image to the loop device:

  losetup /dev/block/loop0 /data/adb/your-image.img
  

• Try mounting with specific flags:

  mount -t ext4 -o rw,noatime /dev/block/loop0 /mnt/test-mount
  

• If this fails, check kernel logs for detailed errors:

  dmesg | grep -i loop
  dmesg | grep -i ext4
  

  This will reveal issues like incompatible EXT4 features or block device access denials.

4. Check EXT4 Feature Compatibility #

Your 4.4 kernel might not support newer EXT4 features used to create the image. Let's inspect:

• List the image's EXT4 features:

  dumpe2fs /data/adb/your-image.img | grep -i features
  

• Compare with features your kernel supports (check dmesg | grep ext4). If you see features like metadata_csum or 64bit, recreate the image with compatible options:

  dd if=/dev/zero of=/data/adb/new-image.img bs=1M count=64
  mkfs.ext4 -O ^metadata_csum,^64bit /data/adb/new-image.img
  

5. MTK-Specific Kernel Restrictions #

MTK often adds custom patches that restrict loop device usage. Here's what to check:

• Look for loop device entries in sysfs:

  ls /sys/block/loop*
  

  If no entries exist, your kernel likely blocks loop devices entirely.
• Test direct write access to the loop device (if you created it):

  echo "test" > /dev/block/loop0
  

  If this fails, check dmesg for MTK-specific error messages about block device access.

6. Test a Different Image Location #

/data/adb might have special security handling on Android 7. Try moving the image to a more permissive directory:

mv /data/adb/your-image.img /data/local/tmp/
chmod 777 /data/local/tmp/your-image.img

Repeat your losetup and mount tests from this location.

内容的提问来源于stack exchange，提问作者Sebastl