Call To ARMs

My 4th semester involves ARM programming. And proprietary tooling (Keil C). But we don’t do that here.

Building

Assembling and linking ARM binaries on non-ARM architecture devices is fairly trivial. I went along with the GNU cross bare metal toolchain binutils, which provides arm-as and arm-ld (among a bunch of other utils that I don’t care about for now).

Assemble .s files with:

arm-none-eabi-as main.s -g -march=armv8.1-a -o main.out

The -g flag generates extra debugging information that gdb picks up. The -march option establishes target architecture.

Link .o files with:

arm-none-eabi-ld main.out -o main

Running (and Debugging)

Things get interesting here. gdb on your x86 machine cannot read nor execute binaries compiled for ARM. So, we simulate an ARM processor using qemu. Now qemu allows you to run gdbserver on startup. Connecting our local gdb instance to gdbserver gives us a view into the program’s execution. Easy!

Run qemu, with gdbserver on port 1234, with our ARM binary, main:

qemu-arm -singlestep -g 1234 main

Start up gdb on your machine, and connect to qemu’s gdbserver:

(gdb) set architecture armv8-a
(gdb) target remote localhost:1234
(gdb) file main
Reading symbols from main...  # yay!

GDB Enhanced

gdb is cool, but it’s not nearly as comfortable as well fleshed out emulators/IDEs like Keil. Watching registers, CPSR and memory chunks update is pretty fun.

I came across gdb’s TUI mode (hit C-x C-a or type tui enable at the prompt). TUI mode is a godsend. It highlights the current line of execution, shows you disassembly outputs, updated registers, active breakpoints and more.

But, it is an absolute eyesore.

Say hello to GEF! “GDB Enhanced Features” teaches our old dog some cool new tricks. Here are some additions that made my ARM debugging experience loads better:

• Memory watches
• Register watches, with up to 7 levels of deref (overkill, I agree)
• Stack tracing

And it’s pretty! See for yourself:

Editing

Vim, with syntax off because it dosen’t handle GNU ARM syntax too well.