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Storage

Android external storage HAL icon

Android has evolved over time to support a wide variety of storage device types and features. All versions of Android support devices with traditional storage, which includes portable and emulated storage. Portable storage can be provided by physical media, like an SD card or USB, that is for temporary data transfer/ file storage. The physical media may remain with the device for an extended period of time, but is not tied to the device and may be removed. SD cards have been available as portable storage since Android 1.0; Android 6.0 added USB support. Emulated storage is provided by exposing a portion of internal storage through an emulation layer and has been available since Android 3.0.

Starting in Android 6.0, Android supports adoptable storage, which is provided by physical media, like an SD card or USB, that is encrypted and formatted to behave like internal storage. Adoptable storage can store all types of application data.

Permissions

Access to external storage is protected by various Android permissions. Starting in Android 1.0, write access is protected with the WRITE_EXTERNAL_STORAGE permission. Starting in Android 4.1, read access is protected with the READ_EXTERNAL_STORAGE permission.

Starting in Android 4.4, the owner, group and modes of files on external storage devices are now synthesized based on directory structure. This enables apps to manage their package-specific directories on external storage without requiring they hold the broad WRITE_EXTERNAL_STORAGE permission. For example, the app with package name com.example.foo can now freely access Android/data/com.example.foo/ on external storage devices with no permissions. These synthesized permissions are accomplished by wrapping raw storage devices in a FUSE daemon.

Starting in Android 10, apps that target Android 9 and lower default to legacy storage, and can opt in to isolated storage. Apps that target Android 10 and default to isolated storage can temporarily opt out of it. Use the manifest attribute requestLegacyExternalStorage, which controls the storage model, to change the default state.

Since both READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE permissions are soft-restricted, if the installer didn’t whitelist the app, the permission controls access to the aural and visual collections only, with no access to the SD card. This applies even if the app requests legacy storage. For more information about both hard restrictions and soft restrictions, see Hard and soft restrictions in Android 10.

If the installer whitelisted the permission, an app running in legacy mode gets the nonisolated permission behavior. The permission controls SD card access, and the aural and visual collections. This happens when either the app targets Android 9 or lower and doesn’t opt in to isolated storage, or it targets Android 10 and opts out.

The whitelist state can be specified only at install time, and can't be changed until the app has been installed.

For more information on setting the READ_EXTERNAL_STORAGE permission, see setWhitelistedRestrictedPermissions() in the PackageInstaller.SessionParams class.

Runtime permissions

Android 6.0 introduces a new runtime permissions model where apps request capabilities when needed at runtime. Because the new model includes the READ/WRITE_EXTERNAL_STORAGE permissions, the platform needs to dynamically grant storage access without killing or restarting already-running apps. It does this by maintaining three distinct views of all mounted storage devices:

  • /mnt/runtime/default is shown to apps with no special storage permissions, and to the root namespace where adbd and other system components live.
  • /mnt/runtime/read is shown to apps with READ_EXTERNAL_STORAGE (Set LEGACY_STORAGE for Android 10)
  • /mnt/runtime/write is shown to apps with WRITE_EXTERNAL_STORAGE

At Zygote fork time, we create a mount namespace for each running app and bind mount the appropriate initial view into place. Later, when runtime permissions are granted, vold jumps into the mount namespace of already-running apps and bind mounts the upgraded view into place. Note that permission downgrades always result in the app being killed.

The setns() functionality used to implement this feature requires at least Linux 3.8, but patches have been backported successfully to Linux 3.4. The PermissionsHostTest CTS test can be used to verify correct kernel behavior.