pixel backup gang

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 an SSD mounted into the internal storage on my Pixel XL. the data is readable & writable in the Google Photos app.

why?

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)

the good

• 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

the bad

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

the ugly

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

prerequisites

• 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

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. you also probably want to disable Google Play Protect scanning in the Play Store menu.

via pixel terminal

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)"

via linux pc using adb

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

usage

setup

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
   • the Magisk "force the global mount namespace" doesn't work - maybe it only works for magisk modules?

mounting

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

unmounting

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.

notes

• currently, the mounting script disables selinux security controls entirely, which is quite unsafe - please don't have any kind of untrusted apps installed on your device while using this. selinux remains disabled until the next boot, or you can run the command setenforce 1 to re-enable it earlier. don't forget that the software on the pixel is severely out of date and there are a lot of serious known vulnerabilities. try to keep device radios off (especially bluetooth and NFC) to reduce the attack surface.
• my recommendation for regular usage is to find your drive's filesystem UUID using ./show_devices.sh and store it. you can then use this UUID in a script to always re-mount that same drive without having to figure out what the block device path is at e.g. something like ./mount_ext4.sh $(./find_device.sh ./my_drive_id.txt)
• excellent reference: https://android.stackexchange.com/questions/214288/how-to-stop-apps-writing-to-android-folder-on-the-sd-card/257401