Can someone help me mount my Android/data and Android/obb in my internal storage from my portable storage? I'm on a Pie ROM. I found this: How to bind mount a folder inside /sdcard with correct permissions? but I don't know what should I do.

When I type:

ls -dZ /sdcard

I get

u:object_r:rootfs:s0 /sdcard

Sub-directories in /sdcard/Android/data or /sdcard/Android/obb (called apps' private directories on primary external storage) have synthesized permissions based on directory structure. While public directories on the rest of the /sdcard have fixed permissions. See details in Android's Storage Journey and What is /storage/emulated/0/?.

My answer to your linked question: How to bind mount a folder inside /sdcard with correct permissions? addresses the latter case i.e. bind mounting public directories which are shared with all apps. But private directories cannot be bind-mounted from external storage with fixed permissions. However using the same approach you can bind-mount every app's private directory separately.

Let's take example of Termux app. Its private directories (Android/data/com.termux and Android/obb/com.termux) are owned by the UID of Termux app assigned at the time of installation. Common ways to get UID value is to read from /data/system/packages.list or do ls -ld /sdcard/Android/data/com.termux or using stat command. A simple approach is to bind-mount the directory as world-writable so that every app can write to it. But it's not a good idea to set such open permissions. So we go the Android's way; bind-mount a directory from external SD card using FUSE:

~# bindfs -u $(stat -c %u /sdcard/Android/data/com.termux) -g 9997 -p a-rwx,ug+rw,ug+X /mnt/media_rw/[UUID]/Android/data/com.termux /mnt/runtime/write/emulated/0/Android/data/com.termux
  • For further mount options and details see the above linked answer.
  • Replace /mnt/media_rw/[UUID] with /mnt/expand/[UUID]/media/0 if SD card is formatted as Adoptable Storage but data is not migrated.
  • In the same way obb directories can be bind-mounted.
  • If your device supports sdcardfs, the same mount should propagate to default and read emulated VIEWS as well. Otherwise if your device is older you can bind-mount the same way to other VIEWS.
  • Some apps by default prefer writing private data to secondary external storage (i.e. external SD card), so you don't need to bind-mount their data or obb directories.
  • when i use the code i get "Not a valid user ID: 10187" , i found the same user id in "ls -n /sdcard/Android/obb"
    – mahanr
    Dec 19 '19 at 12:47
  • @mahanr please go through the linked answer once, I don't want to copy paste everything. You need to touch files /etc/passwd and /etc/group. Dec 19 '19 at 12:56
  • @mahanr as per the new change to bindfs, /etc/passwd and /etc/group is no more necessary. I'm adding the link to new binary in linked answer. Dec 21 '19 at 17:23
  • Can I mount whole obb folder?
    – Jtnqr
    Dec 24 '20 at 14:01
  • @Jtnqr I can't think of a reason for it to not work. Just take care of the ownership, mode and SELinux context properly. Dec 24 '20 at 14:10

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