liberatehongkong
/
linux-kernel-module-cheat


			
				
					
						
						
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<meta name="description" content="The perfect emulation setup to study and develop the &lt;&lt;linux-kernel&gt;&gt; v5.9.2, kernel modules, &lt;&lt;qemu-buildroot-setup,QEMU&gt;&gt;, &lt;&lt;gem5-buildroot-setup,gem5&gt;&gt; and x86_64, ARMv7 and ARMv8 &lt;&lt;userland-assembly,userland&gt;&gt; and &lt;&lt;baremetal-setup,baremetal&gt;&gt; assembly, &lt;&lt;c,ANSI C&gt;&gt;, &lt;&lt;cpp,C++&gt;&gt; and &lt;&lt;posix,POSIX&gt;&gt;. &lt;&lt;gdb&gt;&gt; and &lt;&lt;kgdb&gt;&gt; just work. Powered by &lt;&lt;about-the-qemu-buildroot-setup,Buildroot&gt;&gt; and &lt;&lt;about-the-baremetal-setup,crosstool-NG&gt;&gt;. Highly automated. Thoroughly documented. Automated &lt;&lt;test-this-repo,tests&gt;&gt;. "Tested" in an Ubuntu 20.04 host.">
<title>Linux Kernel Module Cheat</title>
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</head>
<body id="reverse-debug-the-emulator" class="article">
<div id="header">
<h1>Linux Kernel Module Cheat</h1>
</div>
<div id="content">
<div class="sect3">
<h4 id="reverse-debug-the-emulator"><a class="anchor" href="#reverse-debug-the-emulator"></a><a class="link" href="#reverse-debug-the-emulator">23.8.1. Reverse debug the emulator</a></h4>
<div class="paragraph">
<p>While step debugging any complex program, you always end up feeling the need to step in reverse to reach the last call to some function that was called before the failure point, in order to trace back the problem to the actual bug source.</p>
</div>
<div class="paragraph">
<p>While GDB "has" this feature, it is just too broken to be usable, and so we expose the amazing Mozilla RR tool conveniently in this repo: <a href="https://stackoverflow.com/questions/1470434/how-does-reverse-debugging-work/53063242#53063242" class="bare">https://stackoverflow.com/questions/1470434/how-does-reverse-debugging-work/53063242#53063242</a></p>
</div>
<div class="paragraph">
<p>Before the first usage setup rr with:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>echo 'kernel.perf_event_paranoid=1' | sudo tee -a /etc/sysctl.conf
sudo sysctl -p</pre>
</div>
</div>
<div class="paragraph">
<p>Then use it with your content of interest, for example:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>./run --debug-vm-rr --userland userland/c/hello.c</pre>
</div>
</div>
<div class="paragraph">
<p>This will:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>first run the program once until completion or crash</p>
</li>
<li>
<p>then restart the program at the very first instruction at <code>_start</code> and leave you in a GDB shell</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>From there, run the program until your point of interest, e.g.:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>break qemu_add_opts
continue</pre>
</div>
</div>
<div class="paragraph">
<p>and you can now reliably use reverse debugging commands such as <code>reverse-continue</code>, <code>reverse-finish</code> and <code>reverse-next</code>!</p>
</div>
<div class="paragraph">
<p>To restart debugging again after quitting <code>rr</code>, simply run on your host terminal:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>rr replay</pre>
</div>
</div>
<div class="paragraph">
<p>The use case of <code>rr</code> is often to go to the final crash and then walk back from there, so you often want to automate running until the end after record with <code>--debug-vm-args</code> as in:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>./run --debug-vm-args='-ex continue' --debug-vm-rr --userland userland/c/hello.c</pre>
</div>
</div>
<div class="paragraph">
<p>Programs often tend to blow up in very low frames that use values passed in from higher frames. In those cases, remember that just like with forward debugging, you can&#8217;t just go:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>up
up
up
reverse-next</pre>
</div>
</div>
<div class="paragraph">
<p>but rather, you must:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>reverse-finish
reverse-finish
reverse-finish
reverse-next</pre>
</div>
</div>
<div class="ulist">
<ul>
</ul>
</div>
</div>
<div class="paragraph nav-footer">
<p>↑ Up: <a href="debug-the-emulator.html">Debug the emulator</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="debug-gem5-python-scripts.html">Debug gem5 Python scripts</a> →</p>
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