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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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674 lines
40 KiB
HTML
674 lines
40 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>>. EVERYTHING is built from source. <<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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|
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|
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|
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|
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|
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|
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</style>
|
||
<style>.toc-current{font-weight: bold;} .toc-root{font-family: "Open Sans","DejaVu Sans",sans-serif;
|
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font-size: 0.9em;} #content{display: flex; flex-direction: column; flex: 1 1 auto;}
|
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.nav-footer{text-align: center; margin-top: auto;}
|
||
.nav-footer > p > a {white-space: nowrap;}</style>
|
||
</head>
|
||
<body id="count-boot-instructions" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect3">
|
||
<h4 id="count-boot-instructions"><a class="anchor" href="#count-boot-instructions"></a><a class="link" href="#count-boot-instructions">17.13.4. Count boot instructions</a></h4>
|
||
<div class="paragraph">
|
||
<p>TODO: didn’t port during refactor after 3b0a343647bed577586989fb702b760bd280844a. Reimplementing should not be hard.</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://www.quora.com/How-many-instructions-does-a-typical-Linux-kernel-boot-take" class="bare">https://www.quora.com/How-many-instructions-does-a-typical-Linux-kernel-boot-take</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/chat/issues/31" class="bare">https://github.com/cirosantilli/chat/issues/31</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://rwmj.wordpress.com/2016/03/17/tracing-qemu-guest-execution/" class="bare">https://rwmj.wordpress.com/2016/03/17/tracing-qemu-guest-execution/</a></p>
|
||
</li>
|
||
<li>
|
||
<p><code>qemu/docs/tracing.txt</code> and <code>qemu/docs/replay.txt</code></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://stackoverflow.com/questions/39149446/how-to-use-qemus-simple-trace-backend/46497873#46497873" class="bare">https://stackoverflow.com/questions/39149446/how-to-use-qemus-simple-trace-backend/46497873#46497873</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Results (boot not excluded) are shown at: <a href="count-boot-instructions.html#table-boot-instruction-counts">Table 1, “Boot instruction counts for various setups”</a></p>
|
||
</div>
|
||
<table id="table-boot-instruction-counts" class="tableblock frame-all grid-all stretch">
|
||
<caption class="title">Table 1. Boot instruction counts for various setups</caption>
|
||
<colgroup>
|
||
<col style="width: 25%;">
|
||
<col style="width: 25%;">
|
||
<col style="width: 25%;">
|
||
<col style="width: 25%;">
|
||
</colgroup>
|
||
<thead>
|
||
<tr>
|
||
<th class="tableblock halign-left valign-top">Commit</th>
|
||
<th class="tableblock halign-left valign-top">Arch</th>
|
||
<th class="tableblock halign-left valign-top">Simulator</th>
|
||
<th class="tableblock halign-left valign-top">Instruction count</th>
|
||
</tr>
|
||
</thead>
|
||
<tbody>
|
||
<tr>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">7228f75ac74c896417fb8c5ba3d375a14ed4d36b</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">arm</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">QEMU</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">680k</p></td>
|
||
</tr>
|
||
<tr>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">7228f75ac74c896417fb8c5ba3d375a14ed4d36b</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">arm</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">gem5 AtomicSimpleCPU</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">160M</p></td>
|
||
</tr>
|
||
<tr>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">7228f75ac74c896417fb8c5ba3d375a14ed4d36b</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">arm</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">gem5 HPI</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">155M</p></td>
|
||
</tr>
|
||
<tr>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">7228f75ac74c896417fb8c5ba3d375a14ed4d36b</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">x86_64</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">QEMU</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">3M</p></td>
|
||
</tr>
|
||
<tr>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">7228f75ac74c896417fb8c5ba3d375a14ed4d36b</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">x86_64</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">gem5 AtomicSimpleCPU</p></td>
|
||
<td class="tableblock halign-left valign-top"><p class="tableblock">528M</p></td>
|
||
</tr>
|
||
</tbody>
|
||
</table>
|
||
<div class="paragraph">
|
||
<p>QEMU:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./trace-boot --arch x86_64</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>sample output:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>instructions 1833863
|
||
entry_address 0x1000000
|
||
instructions_firmware 20708</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>gem5:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --arch aarch64 --emulator gem5 --eval 'm5 exit'
|
||
# Or:
|
||
# ./run --arch aarch64 --emulator gem5 --eval 'm5 exit' -- --cpu-type=HPI --caches
|
||
./gem5-stat --arch aarch64 sim_insts</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Notes:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><code>0x1000000</code> is the address where QEMU puts the Linux kernel at with <code>-kernel</code> in x86.</p>
|
||
<div class="paragraph">
|
||
<p>It can be found from:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run-toolchain readelf -- -e "$(./getvar vmlinux)" | grep Entry</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO confirm further. If I try to break there with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run-gdb *0x1000000</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>but I have no corresponding source line. Also note that this line is not actually the first line, since the kernel messages such as <code>early console in extract_kernel</code> have already shown on screen at that point. This does not break at all:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run-gdb extract_kernel</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>It only appears once on every log I’ve seen so far, checked with <code>grep 0x1000000 trace.txt</code></p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Then when we count the instructions that run before the kernel entry point, there is only about 100k instructions, which is insignificant compared to the kernel boot itself.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO <code>--arch arm</code> and <code>--arch aarch64</code> does not count firmware instructions properly because the entry point address of the ELF file (<code>ffffff8008080000</code> for <code>aarch64</code>) does not show up on the trace at all. Tested on <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/commit/f8c0502bb2680f2dbe7c1f3d7958f60265347005">f8c0502bb2680f2dbe7c1f3d7958f60265347005</a>.</p>
|
||
</div>
|
||
</li>
|
||
<li>
|
||
<p>We can also discount the instructions after <code>init</code> runs by using <code>readelf</code> to get the initial address of <code>init</code>. One easy way to do that now is to just run:</p>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run-gdb --userland "$(./getvar userland_build_dir)/linux/poweroff.out" main</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>And get that from the traces, e.g. if the address is <code>4003a0</code>, then we search:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>grep -n 4003a0 trace.txt</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>I have observed a single match for that instruction, so it must be the init, and there were only 20k instructions after it, so the impact is negligible.</p>
|
||
</div>
|
||
</li>
|
||
<li>
|
||
<p>to disable networking. Is replacing <code>init</code> enough?</p>
|
||
<div class="openblock">
|
||
<div class="content">
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://superuser.com/questions/181254/how-do-you-boot-linux-with-networking-disabled" class="bare">https://superuser.com/questions/181254/how-do-you-boot-linux-with-networking-disabled</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://superuser.com/questions/684005/how-does-one-permanently-disable-gnu-linux-networking/1255015#1255015" class="bare">https://superuser.com/questions/684005/how-does-one-permanently-disable-gnu-linux-networking/1255015#1255015</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p><code>CONFIG_NET=n</code> did not significantly reduce instruction counts, so maybe replacing <code>init</code> is enough.</p>
|
||
</div>
|
||
</li>
|
||
<li>
|
||
<p>gem5 simulates memory latencies. So I think that the CPU loops idle while waiting for memory, and counts will be higher.</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="kprobes.html">Kprobes</a> | ↑ Up: <a href="linux-kernel-tracing.html">Linux kernel tracing</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="linux-kernel-hardening.html">Linux kernel hardening</a> →</p>
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