The Android community is quite excited that Google has released its new system Android 9 Pie which brings a couple of new features and improved operations. However, recent reports from XDA developers suggests that Android 9 Pie, "rollback protection" changed from recommended features (on Oreo) to mandatory features (on Pie), including OEMs. This practically means that once you port to the Android 9 Pie system, there is no going back. Starting with Android 8.0 Oreo, Google introduced a "rollback protection" mechanism that prohibits rolling back from the new version to the old version. The purpose of this is to promote better security and to improve the fragmentation and speed of the system iteration in order to cooperate with Project Treble.
As of now, the four Pixels and Essential PH-1 are already getting Android 9 Pie update and the source code of has been submitted to AOSP. The implementation of "anti-rollback" relies on the new Android Verified Boot 2.0 (Verified Boot 2.0). It is possible for a user to successfully downgrade to a lower version, but the startup mechanism of the system will not boot after detecting that the current version is lower than the previous version. In the compatibility guide document of the Android 9 Pie system, the boot/system/vendor partitions have added "rollback protection", but the baseband and camera are not included. It may be because most security deployments are for the first three partitions.
Bron: xda
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