56  CPU and platform profiles

56.1 The motivation

The computer ecosystem, especially in embedded systems is vast and growing and is thus diverse and is full of trivial differences. CPUs and development boards differ by minor and sometimes large differences in their design, ports, MMIO registers and other peripherals. Rizin handles these differences by storing the data regarding each CPUs and platforms in SDB files in a standard format, instead of hardcoding them with each of the disassembler plugins. This information will be parsed and added as flags and comments during the analysis loop and will show up in the disassembly and other places, making reverse engineering on those particular chips ets is much easier. This also helps in easy addition of a new port, in maintenance and in user-friendliness.

56.2 How it works

56.2.1 CPU profiles

All the specifics pertaining to a CPU is written down in a CPU profile. It is designed in a way that allows you to enter CPU specefic values like: size of the RAM (RAM_SIZE), size of the ROM (ROM_SIZE) and many more. CPU profiles can be selected using the configuration variable asm.cpu. Firstly, Rizin checks whether the a CPU profile exists for the selected CPU and architecture. If it exists, Rizin generates the filepath of the profile and gets to a stage where it’s ready to be loaded up. During analysis (aa), it’s loaded up and the values are parsed and handled. CPU profiles also allow you to add information regarding the IO and extended IO registers of a CPU. The information pertaining to the IO and extended IO registers are added as flags at their corresponding offsets.

The CPU profile of AVR’s ATTiny88 CPU looks like this:

RAM_SIZE=512
ROM_SIZE=8192
INTERRUPT_VECTOR_SIZE=2

PINB=io
PINB.address=0x03
DDRB=io
DDRB.address=0x04
PORTB=io

Here, PINB is the name and io is the type of the port and this will be added as a flag at the offset 0x03. The type can be ext_io if it’s an extended IO register, as well. Both will be added as flags and the only difference between them is that they will be added in different flagspaces.

CPU profiles also support mapping the ROM. According the ROM_ADDRESS and ROM_SIZE, a section named .rom will be added during analysis.

56.2.1.1 Adding CPU profiles

CPU profiles are stored in SDB files under the directory lirbz/asm/cpus and that’s where you will have to put your new profile. The files follow a naming convention like arch-cpu. You can see the complete list of things that are parsed at librz/analysis/arch_profile.c and if necessary, a new key can be easily added.

To add a new CPU profile, firstly make sure that you have the name of the CPU defined in the list of CPUs defined by the variable cpus in the corresponding architecture’s disassembler plugin (RzAsmPlugin). Then, simply add the SDB file in the directory, add the entry in the meson.build of the same directory and build again. Choose the right CPU and architecture and analyze again (aa) to load up the CPU profile.

For reference, you can see the previously added CPU profile of ATmega16 here: librz/asm/cpus/avr-ATmega16.sdb.txt.

56.2.2 Platform profiles

Platform profiles are designed with an idea to support adding information that is pertaining to a specific board or a micro controller. For example, most of the Raspberry Pi-s use a specific Broadcom chip and its peripherals like registers and interrupts will be the same for all Raspberry Pi-s. These profiles can be selected using the configuration variable asm.platforms and is loaded during analysis (aa). If you run e asm.platform=?, you can see the supported platforms by the selected architecture and CPU.

Let’s have a look at Broadcom 2835’s platform profile:

AUX_IRQ=name
AUX_IRQ.address=0x7e215000
AUX_IRQ.comment=Auxiliary Interrupt status

AUX_MU_IO_REG=name
AUX_MU_IO_REG.address=0x7e215040
AUX_MU_IO_REG.comment=Mini UART I/O Data

Just like in CPU profiles, the name will be added as a flag and the comment as a comment (CCu).

56.2.2.1 Adding platform profiles

Platform profiles are stored in librz/asm/platforms/ and that’s where you will have to put your profile. They follow a naming convention like arch-cpu-platform.

To add a new platform profile, we will have to define the name of the platform in the variable platforms in the RzAsmPlugin definition corresponding architecture’s disassembler plugin. You will also need to add the CPU is it’s not already added. Then, add the entry in the meson.build of the same directory and build again. Choose the right CPU, architecture and platform and analyze again (aa).

You can also check out the platform profiles that were previously added at librz/asm/platforms/arm-arm1176-bcm2835.sdb.txt.