Mount external sdcard obb to internal SD card

Question

I want to move a game's obb (1GB) to an 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/…ame.obb permission denied

I'm using Android 5, any solution?

---

(I posted this question on Stack Overflow before, then I found here may be a better place to post this question.)

Answer 1

Finally, I found the solution to 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!