87 lines
3.0 KiB
ReStructuredText
87 lines
3.0 KiB
ReStructuredText
==============
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Raspberry Pi 4
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==============
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The Raspberry Pi 4 is a single-board computer developed by the ``Raspberry Pi Foundation``. It is freely available and requires no exploit to run code on it.
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On top of that it is well supported in ``Qemu``, making this the cheapest and easiest target for gupje.
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Code
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----
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The github code for this `is here <https://github.com/EljakimHerrewijnen/rpi4_gupje>`_.
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.. hint::
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The code that runs on the Raspberry Pi4 is based on `this awesome repository <https://github.com/ethanfaust/rpi4-baremetal-uart>`_.
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Clone this repository *somewhere* and link the cloned folder in the ``devices/`` folder of gupje.
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.. code-block:: bash
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git clone --recursive https://github.com/EljakimHerrewijnen/rpi4_gupje
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cd /path/to/gupje/devices
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ln -s /path/to/rpi4_gupje rpi4
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You can now build the target by running the following command from gupje's root directory directory:
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.. code-block:: bash
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cd /path/to/gupje
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make -f devices/rpi4/Makefile
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In ``bin/rpi4/`` you will find the ``debugger.bin`` file, which will be used by the ``qemu.py`` script.
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Running Qemu
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############
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From ``devices/rpi4`` navigate to ``rpi4-baremetal-uart/`` folder and run make with a cross compiler.
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.. code-block:: bash
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cd /path/to/gupje/devices/rpi4/rpi4-baremetal-uart
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ARCH=arm64 PREFIX=aarch64-linux-gnu- make
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Implementation
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==============
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Only send and receive need to be implemented for this target. For this target UART is used.
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The debugger expects send/recv to be handled by the user so we need to build some logic to know that the data has been send and that the amount of expected data has been received. The following C code implements the send/recv for UART.
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.. code-block:: c
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void recv_data(void *address, uint32_t size){
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for(int i=0; i < size; i++){
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*((char *)address + i) = uart_get();
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}
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}
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void send(void *address, uint64_t size, uint32_t *error){
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for(int i=0; i < size; i++){
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uart_send(*((char *)address + i));
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}
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}
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However, in this implementation the debugger will need to link the uart_get and uart_send functions. In order to do that we copy the functions from the elf file and add them to the symbols.txt file.
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.. code-block:: bash
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$ readelf -a kernel8.elf | grep uart_
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25: 00000000000803a8 36 FUNC GLOBAL DEFAULT 1 uart_get
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27: 0000000000080384 36 FUNC GLOBAL DEFAULT 1 uart_send
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34: 0000000000080400 88 FUNC GLOBAL DEFAULT 1 uart_puts
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35: 000000000008024c 312 FUNC GLOBAL DEFAULT 1 uart_init
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36: 0000000000080458 84 FUNC GLOBAL DEFAULT 1 uart_hex
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39: 00000000000803cc 52 FUNC GLOBAL DEFAULT 1 uart_getc
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Add the symbols uart_get and uart_send to the symbols.txt file, along with the debugger regions:
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.. code-block:: text
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debugger_storage = 0x85000;
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debugger_stack = 0x83000;
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debugger_entry = 0x81000;
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uart_get = 0x00000000000803a8;
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uart_send = 0x0000000000080384;
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See the makefile for details on how the linking is done.
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