I am trying to create some volatile (tmpfs) storage for my Android, to avoid clutter.
I am trying to follow the previous questions:
making manually mounted partition folders write accessible to all apps
and
How to bind mount a folder inside /sdcard with correct permissions?
The way I am doing is by adding an "init script" into a magisk "service.d" directory:
# cat /sbin/.magisk/img/.core/service.d/001-tmpfs.sh
#!/system/bin/sh
while [ "$(getprop sys.boot_completed)" != 1 ];
do
sleep 1
done
su -mm -c mount -v none -t tmpfs -o size=4g,nosuid,nodev,noexec,noatime,context=u:object_r:sdcardfs:s0,uid=0,gid=9997,mode=0777 /mnt/runtime/write/emulated/0/Download/tmpfs-cleared-on-reboot
Seems plain and simple, only doesn't work in the sense that the folder is mounted successfully, and files disappear after a reboot, if copied there by root. But an ordinary app cannot copy anything there. FX Explorer, for example, says "Access was denied."
It does work though if I set setenforce 0, which seems to indicate that there is a SELinux problem.
The only relevant error in dmesg looks the following:
[ 3181.763111] [20200731_13:36:57.002188]@1 type=1400 audit(1596173815.897:12245): avc: denied { create } for pid=5870 comm="Thread-139" name=233339202D5370656E63657220477265656E62657267206F6E20696E7472696E7369632076732E20696E737472756D656E74616C2076616C7565732C206F766572636F6E666964656E63652C20616E6420686F7720746F2061637475616C6C79207570646174652062656C6965667320696D70726F76656420617564696F2E6D7033 scontext=u:r:untrusted_app:s0:c104,c256,c512,c768 tcontext=u:object_r:sdcardfs:s0 tclass=file permissive=0
It appears if I try to copy some file into this directory.
ls -lahZ /mnt/runtime/write/emulated/0/Download/ gives
drwxrwxrwx 2 root everybody u:object_r:sdcardfs:s0 40 2020-07-31 13:25 tmpfs-cleared-on-reboot
ls -larhtZ /storage/emulated/0/Download/
drwxrwxrwx 2 root everybody u:object_r:sdcardfs:s0 40 2020-07-31 13:25 tmpfs-cleared-on-reboot
Which is strange, because the other files in the directory have permissions like this:
drwxrwx--x 2 root sdcard_rw u:object_r:sdcardfs:s0 4.0K 2020-07-30 20:04 pdfs-scientific_papers-toread
the group is different. I am not sure whether this is related or not.
But anyway, I cannot copy anything into this directory as a normal app.
I can copy stuff there as root, and it disappears on reboot, as expected. But where is the permissions problem?
This is Android 9 on OnePlus 5t.
uid=0,gid=1015,mode=0771instead? – alecxs Aug 01 '20 at 10:20cdinto it. – Vladimir Nikishkin Aug 03 '20 at 04:42