Kernelflinger

Overview

Kernelflinger is the Intel UEFI bootloader for Android^TM/Brillo^TM. It is compatible with the UEFI 2.4 specification.

Kernelflinger implements the Google Bootloader requirements for Android^TM L and M desserts.

The key features are:

1. Google verified boot support.
2. Fastboot support over USB and TCP.
3. Installer: Standalone EFI application that can be used to flash a device from the EFI shell using an external storage.
4. Crashmode: provides a simple access using adb commmand to retrieve data from memory, partitions, EFI variables or ACPI tables in case of OS crash.

Basic architecture

• libkernelflinger: library that provides all the tools necessary to access ACPI and SMBIOS tables, run image verification, use storage (SATA, eMMC, SDCard and UFS) and draw graphic widgets.
• libfastboot: Fastboot protocol implementation. fastboot protocol
• libadb: used by Crashmode.
• libefiusb: based on the non-standard DeviceMode protocol it provides easy to use USB configuration, read and write functions and TX/RX events callbacks.
• libefitcp: based on the standard UEFI TCP protocol, it provides easy to use TCP configuration, read and write functions and TX/RX events callbacks.
• libtransport: is a framework to abstract the transport layer. Used by both libfastboot and libadb to support USB and TCP transport.
• kernelflinger.c: main program that implements the boot flow.
• installer.c: main program of the Installer

Dependencies

Kernelflinger depends on the following libraries:

• gnu-efi (TODO: github link)
• openssl (TODO: github link)

Kernelflinger's compilation requires the following tools:

• sbsigntool: EFI binary signer.
• vendor_intel_build: EFI compilation definitions for Android^TM.

Compilation

Kernelflinger's compilation relies on the Android^TM compilation system. In an Android^TM tree, with all the dependencies checked out, run the following command to build $OUT/efi/kernelflinger.efi.

$ make kernelflinger-$TARGET_BUILD_VARIANT

Run the following command to build $OUT/efi/installer.efi:

$ make installer-$TARGET_BUILD_VARIANT

Kerneflinger specific configuration flags:

• TARGET_NO_DEVICE_UNLOCK: if true, any attempt to unlock the device (fastboot flashing unlock) will systematically fail.
• HAL_AUTODETECT: Cf. Autodetect.
• TARGET_BOOTLOADER_POLICY: Cf. Bootloader Policy and Factory Reset Protection
• KERNELFLINGER_ALLOW_UNSUPPORTED_ACPI_TABLE: makes kernelflinger ignore ACPI table oem_id, oem_table_id and revision fields.
• KERNELFLINGER_USE_POWER_BUTTON: makes kernelflinger use the power key as an input source.
• KERNELFLINGER_USE_WATCHDOG: makes kernelflinger start the "kernel" watchdog prior booting the kernel.
• KERNELFLINGER_USE_CHARGING_APPLET: makes Kernelflinger use the non-standard ChargingApplet protocol to get the battery and charger status, and modify the boot flow in consequence.
• KERNELFLINGER_IGNORE_RSCI: makes Kernelflinger ignore the non-standard RSCI ACPI table. This APCI table provides the reset and wake source reasons.
• KERNELFLINGER_IGNORE_NOT_APPLICABLE_RESET: makes Kernelflinger ignore the ACPI table RSCI reset source "not_applicable" when setting the bootreason.
• KERNELFLINGER_SL_BIND_ROOT_TRUST: makes Kernelflinger passing the devices state along with the SHA256 sum of the certificate used to verify the bootimage to Silent Lake Sec VMM that will use it as a root of trust for the Android Keychain API.

Command line parameters

• -f: enforce kernelfliner to enter Fastboot mode
• -U [test-suite-name]": run unittest test (see unittest.c).