f57fd97df3
This patch introduces the fundamentals needed to support booting off dm-user. First, a method has been added to start snapuserd in first-stage init. It simply forks and execs, creates a specially named first-stage socket, then waits for requests. Next, a new method has been added to SnapshotManager to perform a second-stage handoff. This works by first launching a second copy of snapuserd using init's normal service management functionality. The new snapuserd runs alongside the original, but has correct privileges and a correct selinux context. Next, we inspect each COW device, and if its table uses dm-user, we replace the table with a renamed control device. The new control device is bound to the new snapuserd. device-mapper guarantees that such a table swap is safe. It flushes I/O to the old table and then replaces it with the new table. Once the new table is in place, the old dm-user control devices are automatically destroyed. Thus, once all dm-user devices has been transitioned, the first-stage daemon is idle and can gracefully exit. This patch does not modify init. A few changes will be needed on top of this patch: (1) CreateLogicalAndSnapshotPartitions will need further changes to start the first-stage daemon and track its pid. Additionally, it will need to ensure the named socket file is deleted, so there is no further IPC allowed after partitions are completed. (2) init will need to propagate the pid to second-stage init so the process can be killed (or signalled). (3) first-stage snapuserd will need to gracefully exit once it has no active handler threads. (4) second-stage init will need to invoke the transition helper on SnapshotMaanager, ideally as soon as feasible. Bug: 168259959 Test: manual test Change-Id: I54dec2edf85ed95f11ab4518eb3d7dbaf0bdcbfd |
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|---|---|---|
| .. | ||
| include | ||
| include_fstab/fstab | ||
| libdm | ||
| libfiemap | ||
| libfs_avb | ||
| liblp | ||
| libsnapshot | ||
| libstorage_literals | ||
| libvbmeta | ||
| tests | ||
| tools | ||
| .clang-format | ||
| Android.bp | ||
| clean_scratch_files.cpp | ||
| clean_scratch_files.rc | ||
| file_wait.cpp | ||
| fs_mgr.cpp | ||
| fs_mgr_boot_config.cpp | ||
| fs_mgr_dm_linear.cpp | ||
| fs_mgr_format.cpp | ||
| fs_mgr_fstab.cpp | ||
| fs_mgr_overlayfs.cpp | ||
| fs_mgr_priv.h | ||
| fs_mgr_priv_boot_config.h | ||
| fs_mgr_remount.cpp | ||
| fs_mgr_roots.cpp | ||
| fs_mgr_slotselect.cpp | ||
| fs_mgr_vendor_overlay.cpp | ||
| fs_mgr_verity.cpp | ||
| OWNERS | ||
| README.overlayfs.md | ||
| TEST_MAPPING | ||
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 logical resizable Android partitions (LRAPs). 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.