advpi – Week 6 – Code Execution

This week was hyper-focused on getting normal, proper code execution.

Code layout

The first thing to get right was the code execution.

I found this wonderful project kvm-arm32-on-arm64 that provided a style for doing this.

So, it turns out that ARM converter prints the code in the format its in memory. So, to write it correct, I used the big endian format and then stored it in code in the uint32_t format. This way, the compiler will set it to little endian format as required, and I don’t need to think about endianness for a while.

So if I take this bit of ARM code, that loops,

nop
nop
nop
mov r0,0x0
mov r1,0x1000
mov r2,0x1
add r3,r1,r2
str r3,[r1]
mov r15,0x2000000

We convert it to the following array.

uint32_t CODE[] = {
    0xE320F000,
    0xE320F000,
    0xE320F000,
    0xE3A00000,
    0xE3A01A01,
    0xE3A02001,
    0xE0813002,
    0xE5813000,
    0xE3A0F402,
};

I would expect it to exit from MMIO (as it attempts to write to a read only page) at 0x1000, and exit per the program. If it loops or does anything else, something is wrong.

Thankfully, we get this wonderful(ly confusing) output:

Hello, from advpi!
Opened bios
Attempted mmio
Attempted write=yes of value=4097 and at address=4096
Register(0)=0
Register(1)=0
Register(2)=0
Register(3)=0
Register(4)=0
Register(5)=0
Register(6)=0
Register(7)=0
Register(8)=0
Register(9)=0
Register(10)=0
Register(11)=0
Register(12)=0
Register(13)=0
Register(14)=0
Register(15)=0
Closing the Virtual Machine

So, the MMIO output was correct, but we’re not able to see the registers. I’m thinking it is more related to the exception levels and register banks that are available, but I shall check more on it as required.

So although technically we can’t see the registers, we can get some rudimentary output through MMIO, or shared memory.

If I remove the MMIO causing statement, we get an endless loop – showing that we’re stopping as required.

What’s next

This week was short as well. Following this, I’ll be setting up a GBA cart, and the video device. I’m thinking of using SDL2 to show a video device and handle device I/O.