mount ext4 drives into the google pixel's internal storage

WARNING: this code is experimental and there is no guarantee that it works. rooting your phone or running any commands as root can be very dangerous. you have been warned.

anyway here is a demo image of a portable SSD enclosure mounted into the internal storage on my Pixel XL. the data is readable & writable in the Google Photos app.

the main goal is to allow for easy backup of external media via Google Photos on original Google Pixel phones, which which have an unlimited storage benefit. the "usual" method is to copy every file to the device's internal storage, which can incur terabytes of unnecessary writes to the limited lifetime of the internal flash memory.

i first tried using FUSE (filesystem in user space) based solutions like bindfs (via termux root-packages) and fuse-nfs (complicated to compile for android so i built my own minimal version in Rust). this works and is especially good at sidestepping android's selinux policies. however the performance was not acceptable. (note: i have not tried fbind but i don't think that works out of the box here without using FUSE)

this method is basically a hack around the selinux policies + app permissions using the plain old mount command.

(if you don't care about using these scripts and just want to see how it's done, take a look at mount_ext4.sh)

• works with the stock kernel
• backs up external files larger than 4gb (stock OS only supports FAT32 for external drives)
• reduces wear on internal flash storage
• prevents device from overheating - the external drive gets hot instead
• makes 32gb pixels viable for mass backup

• phone needs to be rooted
• there's currently no way to auto-mount when the disk is connected

• there's no GUI, you need to execute shell scripts

• a Google Pixel (sailfish) or Google Pixel XL (marlin) on Android 10, rooted with Magisk. may work on other phones.
• a USB storage drive formatted with an ext4 filesystem. other filesystems not currently supported.

installation is essentially just copying the scripts to the device & making them executable. you can do this manually, or use one of the automated steps below.

1. start a terminal application and navigate to the directory where you want to install the scripts
2. run the following command:

sh -c "$(curl -fSs https://raw.githubusercontent.com/master-hax/pixel-backup-gang/master/install.sh)"

1. install Android Debug Bridge (adb) & connect the pixel
2. clone this repository
3. run make mobile-install. this installs the scripts to /data/local/tmp by default.
   • if your pixel has Termux installed, you can install the scripts to the Termux home directory with make mobile-install DEVICE_INSTALL_DIRECTORY=/data/data/com.termux/files/home
   • if you are running these steps on WSL, you should use the adb executable from windows (which has USB support) with make mobile-install HOST_ADB_COMMAND=/mnt/c/Users/someone/AppData/Local/Android/Sdk/platform-tools/adb.exe

1. start a shell on the device & navigate to the installation directory
   • from the device
     • launch Terminal, Termux, JuiceSSH, or some other terminal app
     • run su then allow sudo access to your terminal app in Magisk
   • from a PC
     • run adb shell
     • run su then allow sudo access to the shell process in Magisk
2. run ./start_global_shell.sh to enter the global mount namespace
   • you may not need this step if you use Magisk to force the global mount namespace

1. connect the ext4 formatted external drive to the pixel. you should get an os notification that says the drive is not supported. clear or ignore this notification.
   • this notification directs you to format the drive in FAT32 - don't do that
2. find the block device that you want to mount. it is usually found at /dev/block/sdg1 but changes when devices are connected and disconnected.
   • if you don't know the filesystem UUID, use ./show_devices.sh
   • if you know the filesystem UUID, use ./find_device.sh
3. run ./mount_ext4.sh <BLOCK_DEVICE> e.g. ./mount_ext4.sh /dev/block/sdg1

everything located under /the_binding on the external drive should now be visible by apps at /the_binding in the internal storage

1. make sure nothing important is reading from or writing to the drive
2. run ./unmount.sh

everything located under /the_binding in the internal storage should now be gone. you can disconnect the drive if you're sure all pending writes have been flushed.

TODO