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08/02 — 2020
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Call To ARMs
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<p>My 4th semester involves ARM programming. And proprietary
tooling (Keil C). But we don't do that here.</p>
<h3 id="Building">Building</h3>
<p>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 <code>arm-as</code> and
<code>arm-ld</code> (among a bunch of other utils that I don't care
about for now). </p>
<p>Assemble <code>.s</code> files with:</p>
<pre><code class="language-shell">arm-none-eabi-as main.s -g -march=armv8.1-a -o main.out
</code></pre>
<p>The <code>-g</code> flag generates extra debugging information that
<code>gdb</code> picks up. The <code>-march</code> option establishes target
architecture.</p>
<p>Link <code>.o</code> files with:</p>
<pre><code class="language-shell">arm-none-eabi-ld main.out -o main
</code></pre>
<h3 id="Running%20(and%20Debugging)">Running (and Debugging)</h3>
<p>Things get interesting here. <code>gdb</code> on your x86 machine
cannot read nor execute binaries compiled for ARM. So, we
simulate an ARM processor using <code>qemu</code>. Now qemu allows you
to run <code>gdbserver</code> on startup. Connecting our local <code>gdb</code>
instance to <code>gdbserver</code> gives us a view into the program’s
execution. Easy!</p>
<p>Run <code>qemu</code>, with <code>gdbserver</code> on port <code>1234</code>, with our ARM
binary, <code>main</code>:</p>
<pre><code class="language-shell">qemu-arm -singlestep -g 1234 main
</code></pre>
<p>Start up <code>gdb</code> on your machine, and connect to <code>qemu</code>’s
<code>gdbserver</code>:</p>
<pre><code>(gdb) set architecture armv8-a
(gdb) target remote localhost:1234
(gdb) file main
Reading symbols from main... # yay!
</code></pre>
<h3 id="GDB%20Enhanced">GDB Enhanced</h3>
<p><code>gdb</code> is cool, but it's not nearly as comfortable as well
fleshed out emulators/IDEs like Keil. Watching registers,
CPSR and memory chunks update <em>is</em> pretty fun. </p>
<p>I came across <code>gdb</code>'s TUI mode (hit <code>C-x C-a</code> or type <code>tui
enable</code> 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.</p>
<p><em>But</em>, it is an absolute eyesore.</p>
<p>Say hello to <a href="https://github.com/hugsy/gef">GEF</a>! “GDB
Enhanced Features” teaches our old dog some cool new tricks.
Here are some additions that made my ARM debugging
experience loads better:</p>
<ul>
<li>Memory watches</li>
<li>Register watches, with up to 7 levels of deref (overkill,
I agree)</li>
<li>Stack tracing</li>
</ul>
<p>And it's pretty! See for yourself:</p>
<p><a href="https://u.peppe.rs/wq.png"><img src="https://u.peppe.rs/wq.png" alt="gef.png" /></a></p>
<h3 id="Editing">Editing</h3>
<p>Vim, with <code>syntax off</code> because it
dosen't handle GNU ARM syntax too well.</p>
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