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<ol class="chapter"><li class="chapter-item expanded "><a href="../intro/index.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="../ebpf/index.html" class="active"><strong aria-hidden="true">2.</strong> eBPF Program Limitiations</a></li><li class="chapter-item expanded "><a href="../start/index.html"><strong aria-hidden="true">3.</strong> Getting Started</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../start/development.html"><strong aria-hidden="true">3.1.</strong> Development Environment</a></li><li class="chapter-item expanded "><a href="../start/hello-xdp.html"><strong aria-hidden="true">3.2.</strong> Hello XDP!</a></li><li class="chapter-item expanded "><a href="../start/logging-packets.html"><strong aria-hidden="true">3.3.</strong> Logging Packets</a></li></ol></li></ol>
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<h1 class="menu-title">Building eBPF Programs With Aya</h1>
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<main>
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<h1 id="ebpf-program-constraints"><a class="header" href="#ebpf-program-constraints">eBPF Program Constraints</a></h1>
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<p>The eBPF Virtual Machine, where our eBPF programs will be run, is a constrained runtime environment:</p>
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<ul>
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<li>There is only 512 bytes of stack (or 256 bytes if we are using tail calls).</li>
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<li>There is no access to heap space and data must instead be written to maps.</li>
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</ul>
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<p>Even applications written in C are restricted to a subset of language features:</p>
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<ul>
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<li>no loops</li>
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<li>no global variables</li>
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<li>no variadic functions</li>
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<li>no floating-point numbers</li>
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<li>no passing structures as function arguments</li>
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</ul>
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<p>While these limitations do not map 1:1 with Rust, we are still constrained:</p>
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<ul>
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<li>We may not use the standard library. We use <code>core</code> instead.</li>
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<li><code>core::fmt</code> may not be used and neither can traits that rely on it, for example <code>Display</code> and <code>Debug</code></li>
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<li>As there is no heap, we cannot use <code>alloc</code> or <code>collections</code>.</li>
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<li>We must not <code>panic</code> as the eBPF VM does not support stack unwinding, or the <code>abort</code> instruction.</li>
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<li>There is no <code>main</code> function</li>
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</ul>
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<p>Alongside this, a lot of the code that we write is <code>unsafe</code>, as we are reading directly from kernel memory.</p>
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