Android mount external sdcard obb to internal SD card succeed

Question

(I posted this question on Stack Overflow before(https://stackoverflow.com/questions/42738392/android-mount-obb-to-external-sd-card-failed), then I found here may be a better place to post this question.)

I want to move a game's obb(1GB) to SD card.
Copied the obb file to SD card, and symbolic link by:

su

ln -sf /storage/sdcard1/Android/obb/com.game /sdcard/Android/obb/com.game

but failed with "function not implemented".

I googled it, and got "some filesystem doesn't support symbolic link".

So I tried to use mount by command:

su

mount -o bind /storage/sdcard1/Android/obb/com.game /sdcard/Android/obb/com.game

but I found only root can see the files in the folder.
Then I do:

cd /system/bin/

chmod 4777 ./toolbox #make anyone can use mount(in toolbox)

su u0_aXXX #the game's uid

mount -o bind /storage/sdcard1/Android/obb/com.game /sdcard/Android/obb/com.game

ls -l /sdcard/Android/obb/com.game

$ -rwxrwx--- u0_aXXX sdcard_r com.game.obb

But when I

cat /sdcard/Android/obb/com.game/com.game.obb

$ cat: /sdcard/â€¦ame.obb permission denied

I'm using android5, any solution?
Thanks.

— KirkSu | Sources

Answer

(If you're finding how to move obb to external sdcard and mount it back to internal, here's the right place!)
Finally I found the solution of this question by myself!
The app can't access the obb by itself, it requires media_rw to access it.
Found by lsof:

sdcard 340 media_rw 15 ??? ??? ??? ???

/data/media/obb/com.game/com.game.obb

sdcard 340 media_rw 17 ??? ??? ??? ???

/data/media/obb/com.game/com.game.obb

com.game 26335 u0_a266 36 ??? ??? ??? ???

/storage/emulated/0/Android/obb/com.game/com.game.obb

com.game 26335 u0_a266 37 ??? ??? ??? ???

/storage/emulated/0/Android/obb/com.game/com.game.obb

So I have to mount the external sdcard folder to two places and each has to have its right owner:

owner
 folder

u0_a266  /storage/emulated/legacy/Android/obb/com.game

media_rw /data/media/obb/com.game

But if I mount /storage/sdcard1/Android/obb/com.game to those two places, it will become:

owner
 folder

u0_a266  /storage/emulated/legacy/Android/obb/com.game

u0_a266  /data/media/obb/com.game

Then I found if I do:

mount -o bind /storage/sdcard1/Android/obb/com.game /storage/emulated/legacy/Android/obb/com.game

mount -o bind /mnt/media_rw/sdcard1/Android/obb/com.game /data/media/obb/com.game

I found this works fine, because the mount command's source and destination is their original owner.
The app can run very well!

— KirkSu | Sources

Android mount external sdcard obb to internal SD card succeed

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