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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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590 lines
36 KiB
HTML
590 lines
36 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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|
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</head>
|
||
<body id="initrd" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect1">
|
||
<h2 id="initrd"><a class="anchor" href="#initrd"></a><a class="link" href="#initrd">8. initrd</a></h2>
|
||
<div class="sectionbody">
|
||
<div class="paragraph">
|
||
<p>The kernel can boot from an CPIO file, which is a directory serialization format much like tar: <a href="https://superuser.com/questions/343915/tar-vs-cpio-what-is-the-difference" class="bare">https://superuser.com/questions/343915/tar-vs-cpio-what-is-the-difference</a></p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The bootloader, which for us is provided by QEMU itself, is then configured to put that CPIO into memory, and tell the kernel that it is there.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This is very similar to the kernel image itself, which already gets put into memory by the QEMU <code>-kernel</code> option.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>With this setup, you don’t even need to give a root filesystem to the kernel: it just does everything in memory in a ramfs.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>To enable initrd instead of the default ext2 disk image, do:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./build-buildroot --initrd
|
||
./run --initrd</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>By looking at the QEMU run command generated, you can see that we didn’t give the <code>-drive</code> option at all:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>cat "$(./getvar run_dir)/run.sh"</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Instead, we used the QEMU <code>-initrd</code> option to point to the <code>.cpio</code> filesystem that Buildroot generated for us.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Try removing that <code>-initrd</code> option to watch the kernel panic without rootfs at the end of boot.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>When using <code>.cpio</code>, there can be no <a href="disk-persistency.html">filesystem persistency</a> across boots, since all file operations happen in memory in a tmpfs:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>date >f
|
||
poweroff
|
||
cat f
|
||
# can't open 'f': No such file or directory</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which can be good for automated tests, as it ensures that you are using a pristine unmodified system image every time.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Not however that we already disable disk persistency by default on ext2 filesystems even without <code>--initrd</code>: <a href="disk-persistency.html">Section 23.3, “Disk persistency”</a>.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>One downside of this method is that it has to put the entire filesystem into memory, and could lead to a panic:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>end Kernel panic - not syncing: Out of memory and no killable processes...</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This can be solved by increasing the memory as explained at <a href="memory-size.html">Memory size</a>:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./run --initrd --memory 256M</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The main ingredients to get initrd working are:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><code>BR2_TARGET_ROOTFS_CPIO=y</code>: make Buildroot generate <code>images/rootfs.cpio</code> in addition to the other images.</p>
|
||
<div class="paragraph">
|
||
<p>It is also possible to compress that image with other options.</p>
|
||
</div>
|
||
</li>
|
||
<li>
|
||
<p><code>qemu -initrd</code>: make QEMU put the image into memory and tell the kernel about it.</p>
|
||
</li>
|
||
<li>
|
||
<p><code>CONFIG_BLK_DEV_INITRD=y</code>: Compile the kernel with initrd support, see also: <a href="https://unix.stackexchange.com/questions/67462/linux-kernel-is-not-finding-the-initrd-correctly/424496#424496" class="bare">https://unix.stackexchange.com/questions/67462/linux-kernel-is-not-finding-the-initrd-correctly/424496#424496</a></p>
|
||
<div class="paragraph">
|
||
<p>Buildroot forces that option when <code>BR2_TARGET_ROOTFS_CPIO=y</code> is given</p>
|
||
</div>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>TODO: how does the bootloader inform the kernel where to find initrd? <a href="https://unix.stackexchange.com/questions/89923/how-does-linux-load-the-initrd-image" class="bare">https://unix.stackexchange.com/questions/89923/how-does-linux-load-the-initrd-image</a></p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="initrd-in-desktop-distros.html">initrd in desktop distros</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="initramfs.html">initramfs</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="rootfs.html">rootfs</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="gem5-initrd.html">gem5 initrd</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="gem5-initramfs.html">gem5 initramfs</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="busybox-shell-initrc-files.html">BusyBox shell initrc files</a> | ↑ Up: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="initrd-in-desktop-distros.html">initrd in desktop distros</a> →</p>
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