Android OverlayFS Integration with adb Remount

Introduction

Users working with userdebug or eng builds expect to be able to remount the system partition as read-write and then add or modify any number of files without reflashing the system image, which is efficient for a development cycle.

Limited memory systems use read-only types of file systems or dynamic Android partitions (DAPs). These file systems land system partition images right-sized, and have been deduped at the block level to compress the content. This means that a remount either isn’t possible, or isn't useful because of space limitations or support logistics.

OverlayFS resolves these debug scenarios with the adb disable-verity and adb remount commands, which set up backing storage for a writable file system as an upper reference, and mount the lower reference on top.

Performing a remount

Use the following sequence to perform the remount.

$ adb root
$ adb disable-verity
$ adb reboot
$ adb wait-for-device
$ adb root
$ adb remount

Then enter one of the following sequences:

$ adb shell stop
$ adb sync
$ adb shell start
$ adb reboot

or

$ adb push <source> <destination>
$ adb reboot

Note that you can replace these two lines in the above sequence:

$ adb disable-verity
$ adb reboot

with this line:

$ adb remount -R

Note: adb remount -R won’t reboot if the device is already in the adb remount state.

None of this changes if OverlayFS needs to be engaged. The decisions whether to use traditional direct file-system remount, or one wrapped by OverlayFS is automatically determined based on a probe of the file-system types and space remaining.

Backing Storage

When OverlayFS logic is feasible, it uses either the /cache/overlay/ directory for non-A/B devices, or the /mnt/scratch/overlay directory for A/B devices that have access to LRAP. It is also possible for an A/B device to use the system_ partition for backing storage. eg: if booting off system_a+vendor_a, use system_b. The backing store is used as soon as possible in the boot process and can occur at first stage init, or when the mount_all commands are run in init RC scripts.

By attaching OverlayFS early, SEpolicy or init can be pushed and used after the exec phases of each stage.

Caveats

• Backing storage requires more space than immutable storage, as backing is done file by file. Be mindful of wasted space. For example, defining BOARD_IMAGE_PARTITION_RESERVED_SIZE has a negative impact on the right-sizing of images and requires more free dynamic partition space.
• The kernel requires CONFIG_OVERLAY_FS=y. If the kernel version is higher than 4.4, it requires source to be in line with android-common kernels.  The patch series is available on the upstream mailing list and the latest as of Sep 5 2019 is https://www.spinics.net/lists/linux-mtd/msg08331.html This patch adds an override_creds mount option to OverlayFS that permits legacy behavior for systems that do not have overlapping sepolicy rules, principals of least privilege, which is how Android behaves. For 4.19 and higher a rework of the xattr handling to deal with recursion is required. https://patchwork.kernel.org/patch/11117145/ is a start of that adjustment.
• adb enable-verity frees up OverlayFS and reverts the device to the state prior to content updates. The update engine performs a full OTA.
• adb remount overrides are incompatible with OTA resources, so the update engine may not run if fs_mgr_overlayfs_is_setup() returns true.
• If a dynamic partition runs out of space, making a logical partition larger may fail because of the scratch partition. If this happens, clear the scratch storage by running either either fastboot flashall or adb enable-verity. Then reinstate the overrides and continue.
• For implementation simplicity on retrofit dynamic partition devices, take the whole alternate super (eg: if "a" slot, then the whole of "system_b"). Since landing a filesystem on the alternate super physical device without differentiating if it is setup to support logical or physical, the alternate slot metadata and previous content will be lost.
• There are other subtle caveats requiring complex logic to solve. Have evaluated them as too complex or not worth the trouble, please File a bug if a use case needs to be covered.
  • The backing storage is treated fragile, if anything else has issue with the space taken, the backing storage will be cleared out and we reserve the right to not inform, if the layering does not prevent any messaging.
  • Space remaining threshold is hard coded. If 1% or more space still remains, OverlayFS will not be used, yet that amount of space remaining is problematic.
  • Flashing a partition via bootloader fastboot, as opposed to user space fastbootd, is not detected, thus a partition may have override content remaining. adb enable-verity to wipe.
  • Space is limited, there is near unlimited space on userdata, we have made an architectural decision to not utilize /data/overlay/ at this time. Acquiring space to use for backing remains an ongoing battle.
  • First stage init, or ramdisk, can not be overriden.
  • Backing storage will be discarded or ignored on errors, leading to confusion. When debugging using adb remount it is currently advised to confirm update is present after a reboot to develop confidence.
• File bugs or submit fixes for review.