Don't use heuristics to pick volumes to overlay

  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.

- The kernel requires **CONFIG_OVERLAY_FS=y**. overlayfs is used 'as is' as of

  android 16, no modifications are required.

- In order for overlayfs to work, overlays are mounted in the overlay_remounter

  domain, defined here: system/sepolicy/private/overlay_remounter.te. This domain

  must have full access to the files on the underlying volumes, add any other file

  and directory types here

- For devices with dynamic partitions, we use a simpler logic to decide which

  partitions to remount, being all logical ones. In case this isn't correct,

  we added the overlay=on and overlay=off mount flags to allow detailed control.

- _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

#include "fs_mgr_overlayfs_mount.h"

#include "fs_mgr_priv.h"

// Flag to simplify algorithm for choosing which partitions to overlay to simply overlay

// all dynamic partitions

constexpr bool overlay_dynamic_partitions_only = false;

using namespace std::literals;

using namespace android::fs_mgr;

using namespace android::storage_literals;

    Fstab candidates;

    for (const auto& entry : fstab) {

        // fstab overlay flag overrides all other behavior

        if (entry.fs_mgr_flags.overlay_off) continue;

        if (entry.fs_mgr_flags.overlay_on) {

            candidates.push_back(entry);

            continue;

        }

        // overlay_dynamic_partitions_only simplifies logic to overlay exactly dynamic partitions

        if (overlay_dynamic_partitions_only) {

            if (entry.fs_mgr_flags.logical) candidates.push_back(entry);

            continue;

        }

        // Filter out partitions whose type doesn't match what's mounted.

        // This avoids spammy behavior on devices which can mount different

        // filesystems for each partition.

        CheckFlag("metadata_csum", ext_meta_csum);

        CheckFlag("fscompress", fs_compress);

        CheckFlag("overlayfs_remove_missing_lowerdir", overlayfs_remove_missing_lowerdir);

        CheckFlag("overlay=on", overlay_on);

        CheckFlag("overlay=off", overlay_off);

#undef CheckFlag

        bool fs_compress : 1;

        bool overlayfs_remove_missing_lowerdir : 1;

        bool is_zoned : 1;

        bool overlay_on : 1;

        bool overlay_off : 1;

    } fs_mgr_flags = {};

    bool is_encryptable() const { return fs_mgr_flags.crypt; }