Default [Q] [REF] [Android 4.4] sdcard symlinks

I'm looking for a smooth way to use symlinks from/to both the emulated sdcard and an ext4 formatted external sdcard in order to easily redirect data between the two.
However the way those file systems are both mounted and linked on various locations in the directory tree isn't very straight forward:


Mount Points

The external sdcard gets mounted natively on /mnt/media_rw/sdcard1 ... and once again virtually with a fuse layer on top on /storage/sdcard1
Code:
root@cm-11:/ # mount | grep sdcard1
/dev/block/vold/179:33 on /mnt/media_rw/sdcard1 type ext4 (rw,dirsync,context=u:object_r:sdcard_external:s0,nosuid,nodev,noexec,noatime,data=ordered)
/dev/fuse on /storage/sdcard1 type fuse (rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other)

root@cm-11:/ # df -m | grep sdcard1
/dev/block/vold/179:33   14953       174     14627   1% /mnt/media_rw/sdcard1
/dev/fuse                14953       174     14627   1% /storage/sdcard1
The emulated sdcard (origin: /data/media/*) is virtually fuse mounted on /mnt/shell/emulated (and /storage/emulated/**)
Code:
root@cm-11:/ # mount | grep emulated | grep fuse                           
/dev/fuse on /mnt/shell/emulated type fuse (rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other)

/dev/fuse on /storage/emulated/0 type fuse (rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /storage/emulated/0/Android/obb type fuse (rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /storage/emulated/legacy type fuse (rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other)
/dev/fuse on /storage/emulated/legacy/Android/obb type fuse (rw,nosuid,nodev,relatime,user_id=1023,group_id=1023,default_permissions,allow_other)

root@cm-11:/ # df -m | grep emulated | grep fuse                                       
/dev/fuse                 5465       164      5301   3% /mnt/shell/emulated

/dev/fuse                 5465       164      5301   3% /storage/emulated/0
/dev/fuse                 5465       164      5301   3% /storage/emulated/0/Android/obb
/dev/fuse                 5465       164      5301   3% /storage/emulated/legacy
/dev/fuse                 5465       164      5301   3% /storage/emulated/legacy/Android/obb


Links

Existing symlinks pointing to sdcards:
Code:
/external_sd -> /storage/sdcard1
/mnt/external_sd -> /storage/sdcard1

/sdcard -> /storage/emulated/legacy
/mnt/sdcard -> /storage/emulated/legacy
/storage/sdcard0 -> /storage/emulated/legacy
/storage/emulated/legacy -> /mnt/shell/emulated/0


Symlink Creation

Fuse mount points don't allow for creation of symlinks:
Code:
root@cm-11:/ # ln -s /storage/sdcard0 /storage/sdcard1/link_to_sdcard0
ln: /storage/sdcard1/link_to_sdcard0: Function not implemented
But native mount points do:
Code:
root@cm-11:/ # ln -sv /storage/sdcard0 /mnt/media_rw/sdcard1/link_to_sdcard0
'/mnt/media_rw/sdcard1/link_to_sdcard0' -> '/storage/sdcard0'


Permissions

Code:
drwx------		4 1023		1023		/mnt/media_rw
drwxr-xr-x		4 root		root		/mnt/media_rw/sdcard1**		# when mounted
drwx------		2 1023		1023		/mnt/media_rw/usbdisk		# when unmounted
drwxr-x--x		5 root		1028		/storage
drwxrwx--x		4 root		1028		/storage/sdcard1**		# when mounted
drwx------		2 root		root		/storage/usbdisk		# when unmounted


drwx------		3 shell		shell		/mnt/shell
drwxrwx--x		5 root		1028		/mnt/shell/emulated**		# only 0 populated (legacy, obb empty)
drwx------		2 shell   	shell		/mnt/shell/emulated		# when unmounted
drwxr-x--x		5 root		1028		/storage
dr-xr-xr-x 		2 root		root		/storage/emulated**

1015=sdcard_rw
1023=media_rw
1028=sdcard_r

After applying these permission fixes to overcome external storage restrictions introduced in 4.4 the link becomes accessible in File Manager, but again only via the native mount point.



Storage Variables

Am I wrong assuming that apps rather use environment variables to determine storage paths than looking in the directory tree!? Well, obviously they point to the unwanted fuse mount points:
Code:
root@cm-11:/ # env | grep sdcard1
EXTERNAL_SD=/storage/sdcard1
SECONDARY_STORAGE=/storage/sdcard1:/storage/usbdisk

root@cm-11:/ # env | grep emulated
EMULATED_STORAGE_SOURCE=/mnt/shell/emulated
EMULATED_STORAGE_TARGET=/storage/emulated
EXTERNAL_STORAGE=/storage/emulated/legacy


Solution?

Here is some official information on how storage is used: http://source.android.com/devices/te...g-example.html


I've started messing around building a new ramdisk in order to gain control over the cards. Ideas so far are:
- remove fuse stuff from init.rc --> use bind mounts for the emulated sdcard?
- remove vold stuff from init.rc?
- setting up mount points manually in fstab
- changing variables to the native mount point in case of the external sdcard

Ideas for solutions causing the least headache?



---
Notes:
/etc/init* script
/system/build.prop: persist.fuse_sdcard=false # emulated sdcard does not get mounted at all