Fernly - Fernvale Reversing OS

Fernly is a simple operating system designed for use in the reverse engineering of the Fernvale CPU. It will likely be disposed of when the system has been understood well enough to implement a full operating system.

Setting up cross compilation

Linux

git clone https://github.com/robertfoss/setup_codesourcery.git
sudo setup_codesourcery/setup.sh
/usr/local/bin/codesourcery-arm-2014.05.sh

Building Fernly

To compile, simply run "make". If you're cross-compiling, set CROSS_COMPILE to the prefix of your cross compiler. This is very similar to how to compile for Linux.

For example:

make CROSS_COMPILE=arm-none-linux-gnueabi-

Running Fernly

To run, connect the target device and run the following command:

./build/fernly-usb-loader -s /dev/fernvale ./build/usb-loader.bin ./build/firmware.bin

This will open up /dev/fernvale, load usb-loader.bin as a stage 1 bootloader, and then load (and jump to) firmware.bin as stage 2. Optionally, you can add a stage 3 file by specifying it as an additional argument.

Many 3rd-party devices enter bootloader mode only for a short window (~1s) after being connected to USB. A device almost certainly should be "off". Some devices require that battery is removed, while some - don't. To accommodate such cases, there's -w (wait) option. Run fernly-usb-loader, and only then connect a device to USB. This will allow to try various combinations mentioned above with greater comfort (you need to disconnect and poweroff device after each try, and restart fernly-usb-loader).

./build/fernly-usb-loader -w -s /dev/ttyUSB0 ./build/usb-loader.bin ./build/firmware.bin

Linux Notes

Since Fernvale is based on a Mediatek chip, ModemManager will, by default, try to treat it as a modem and make it available for network connections. This is undesirable.

To work around this problem, create a udev rule under /etc/udev/rules.d/ called 98-fernvale.rules with the following contents:

SUBSYSTEM=="tty", ATTRS{idVendor}=="0e8d",\
    ATTRS{idProduct}=="0003",\
    MODE="0660", SYMLINK+="fernvale"

ACTION=="add|change", SUBSYSTEM=="usb",\
    ENV{DEVTYPE}=="usb_device", ATTRS{idVendor}=="0e8d",\
    ATTRS{idProduct}=="0003",\
    ENV{ID_MM_DEVICE_IGNORE}="1"

OSX Notes

The default OSX CDC matching seems to miss the Fernvale board. Use fernvale-osx-codeless to get a com port.

SPI and Flashrom

Fernly includes a special 'flashrom' mode that allows for direct communication with the flashrom program to manipulate the onboard SPI. The protocol is binary, and can be entered by issuing the following command:

spi flashrom

Fernly will respond with a binary 0x05, indicating it is ready.

The format of the protocol is very simple. The host writes the number of bytes to write, then the number of bytes to read, and then writes the data to send to the flash chip. It then reads the requested number of bytes. For example, to send a 2-byte command '0xfe 0xfa' followed by a 3-byte response, write the following data to the serial port:

| 02 03 fe fa |

Then read three bytes of data from the serial port.

A maximum of 255 bytes may be transmitted and received at one time, though in practice these numbers may be smaller.

To exit 'spi flashrom' mode and return to fernly, read/write zero bytes. That is, send the following packet:

| 00 00 |

See ROM-BACKUP.txt for user-level instructions how to backup/restore FlashROM of your device.

Licensing

Fernly is licensed under the BSD 2-clause license (see LICENSE).

Previous versions of fernly linked against division libraries taken from U-Boot, which were licensed under GPL-2. These files have been removed.

Instead, we supply a version of libgcc.a. This file was extracted from a standard gcc toolchain, specifically:

https://code.google.com/p/yus-repo/downloads/detail?name=arm-none-eabi-4.6-armv5.tar.gz

It has not been modified, and its distribution here should be covered under the "runtime exception".

Memory Map