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Reverse-engineering tools for Allwinner firmware
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README.md

README.md

Overview

The goal of this project is to understand the various blobs provided by Allwinner for their sunxi series of ARM SoCs. This includes understanding the interfaces the blobs provide and the bits of undocumented hardware they use. Currently, the tools in this repository are set up to process two types of blobs: the boot ROMs and the ARISC firmware.

From the original, untouched blobs, the tools here can generate ELF objects with symbols, annotated assembly listings, and SVG graphs documenting the control flow of code within the blob.

Boot ROMs (BROM)

The boot ROM, is flashed into the SoC hardware, and is generally the same for every chip of the same model. It comes in two flavors: normal (NBROM) and secure (SBROM). The secure boot ROM is much larger and more complicated because it must verify the next stage of firmware before running it. Unfortunately, only one of the ROMs is visible at a time, depending on the state of the device's "secure boot" eFuse.

The BROM blobs in this repository were dumped from running devices using md from within u-boot.

ARISC firmware

These files are closed-source firmware provided by Allwinner with their board support package (BSP), usually with the name scp.bin. Slightly different versions of the firmware are provided in the BSPs from different board manufacturers. By looking at the changes between versions (and knowing the order from the embedded version number), we can determine the history of changes made to the firmware.

While the firmware is proprietary, source code is provided in an AES-encrypted tarball in various released Linux kernel trees.

Prerequisites

These tools require a cross binutils (or full toolchain) for each architecture of blobs you want to process. Note that even on SoCs that support AArch64, the BROM is still AArch32, so you will need ARM cross tools for them. Cross toolchains can be downloaded from Linaro for ARM, GitHub for OpenRISC, or they can be built manually with musl-cross-make. If your cross toolchain is named differently, edit the top of the Makefile to provide your path or prefix.

Beyond that, the scripts only require bash and standard POSIX utilities.

Using the tools

make BLOB=<soc>/<blob_name> will build a single blob all the way from the hexdump to the call graph. You can simply run make to build all files for all blobs. You can run make <soc>/<blob_name>/<file> for any file (annotated.s blob.elf, blob.s, callgraph.dot, and callgraph.svg) to only make that file and its dependencies.

There are some additional scripts that are useful for processing newly-found blobs or forr other specific use cases. They are not hooked up to the Makefile, so they must be run manually. Documentation is at the top of each script.

Contributing

After modifying the annotations in annotated.s, run make save to copy them back to the comments file. You can then commit your changes to comments. If you find a new, previously-unmarked function, create a symbol for it in the symbols file. Add two blank lines before the first instruction in annotated.s and run make save before running make again. This is important to keep the comments aligned, as objdump will place a blank line and a line with the symbol name before the function.

Please run make check to check the consistency of your symbols and sections files.

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