Call To ARMs
My 4th semester involves ARM programming. And proprietary tooling (Keil C). But we don’t do that here.
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-ld (among a bunch of other utils that I don’t care about for now).
.s files with:
arm-none-eabi-as main.s -g -march=armv8.1-a -o main.out
-g flag generates extra debugging information that
gdb picks up. The
-march option establishes target architecture.
.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!
gdbserver on port
1234, with our ARM binary,
qemu-arm -singlestep -g 1234 main
gdb on your machine, and connect to
(gdb) set architecture armv8-a (gdb) target remote localhost:1234 (gdb) file main Reading symbols from main... # yay!
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:
syntax off because it dosen’t handle GNU ARM syntax too well.
I'm Akshay, I go by nerd or nerdypepper on the internet.