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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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565 lines
35 KiB
HTML
565 lines
35 KiB
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<meta name="description" content="The perfect emulation setup to study and develop the <<linux-kernel>> v5.9.2, kernel modules, <<qemu-buildroot-setup,QEMU>>, <<gem5-buildroot-setup,gem5>> and x86_64, ARMv7 and ARMv8 <<userland-assembly,userland>> and <<baremetal-setup,baremetal>> assembly, <<c,ANSI C>>, <<cpp,C++>> and <<posix,POSIX>>. <<gdb>> and <<kgdb>> just work. Powered by <<about-the-qemu-buildroot-setup,Buildroot>> and <<about-the-baremetal-setup,crosstool-NG>>. Highly automated. Thoroughly documented. Automated <<test-this-repo,tests>>. "Tested" in an Ubuntu 20.04 host.">
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<title>Linux Kernel Module Cheat</title>
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</head>
|
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<body id="your-first-kernel-module-hack" class="article">
|
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<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect4">
|
||
<h5 id="your-first-kernel-module-hack"><a class="anchor" href="#your-first-kernel-module-hack"></a><a class="link" href="#your-first-kernel-module-hack">2.2.2.2. Your first kernel module hack</a></h5>
|
||
<div class="paragraph">
|
||
<p>Edit <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/kernel_modules/hello.c">kernel_modules/hello.c</a> to contain:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>pr_info("hello init hacked\n");</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and rebuild with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./build-modules</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Now there are two ways to test it out: the fast way, and the safe way.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The fast way is, without quitting or rebooting QEMU, just directly re-insert the module with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>insmod /mnt/9p/out_rootfs_overlay/lkmc/hello.ko</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and the new <code>pr_info</code> message should now show on the terminal at the end of the boot.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This works because we have a <a href="9p.html">9P</a> mount there setup by default, which mounts the host directory that contains the build outputs on the guest:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>ls "$(./getvar out_rootfs_overlay_dir)"</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The fast method is slightly risky because your previously insmodded buggy kernel module attempt might have corrupted the kernel memory, which could affect future runs.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Such failures are however unlikely, and you should be fine if you don’t see anything weird happening.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The safe way, is to fist <a href="rebuild-buildroot-while-running.html">quit QEMU</a>, rebuild the modules, put them in the root filesystem, and then reboot:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./build-modules
|
||
./build-buildroot
|
||
./run --eval-after 'insmod hello.ko'</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p><code>./build-buildroot</code> is required after <code>./build-modules</code> because it re-generates the root filesystem with the modules that we compiled at <code>./build-modules</code>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>You can see that <code>./build</code> does that as well, by running:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./build --dry-run</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>See also: <a href="dry-run.html">Dry run to get commands for your project</a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p><code>--eval-after</code> is optional: you could just type <code>insmod hello.ko</code> in the terminal, but this makes it run automatically at the end of boot, and then drops you into a shell.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>If the guest and host are the same arch, typically x86_64, you can speed up boot further with <a href="kvm.html">KVM</a>:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --kvm</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>All of this put together makes the safe procedure acceptably fast for regular development as well.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>It is also easy to GDB step debug kernel modules with our setup, see: <a href="gdb-step-debug-kernel-module.html">Section 3.4, “GDB step debug kernel module”</a>.</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="your-first-linux-kernel-hack.html">Your first Linux kernel hack</a> | ↑ Up: <a href="how-to-hack-stuff.html">How to hack stuff</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="your-first-glibc-hack.html">Your first glibc hack</a> →</p>
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