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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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588 lines
35 KiB
HTML
588 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>>. 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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padding-right:60px;
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max-width:none;
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}
|
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</style>
|
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<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;}
|
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.nav-footer > p > a {white-space: nowrap;}</style>
|
||
</head>
|
||
<body id="poll" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect3">
|
||
<h4 id="poll"><a class="anchor" href="#poll"></a><a class="link" href="#poll">17.8.3. poll</a></h4>
|
||
<div class="paragraph">
|
||
<p>The poll system call allows an user process to do a non-busy wait on a kernel event.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Sources:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/kernel_modules/poll.c">kernel_modules/poll.c</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/rootfs_overlay/lkmc/poll.sh">rootfs_overlay/lkmc/poll.sh</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Example:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./poll.sh</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Outcome: <code>jiffies</code> gets printed to stdout every second from userland, e.g.:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>poll
|
||
<6>[ 4.275305] poll
|
||
<6>[ 4.275580] return POLLIN
|
||
revents = 1
|
||
POLLIN n=10 buf=4294893337
|
||
poll
|
||
<6>[ 4.276627] poll
|
||
<6>[ 4.276911] return 0
|
||
<6>[ 5.271193] wake_up
|
||
<6>[ 5.272326] poll
|
||
<6>[ 5.273207] return POLLIN
|
||
revents = 1
|
||
POLLIN n=10 buf=4294893588
|
||
poll
|
||
<6>[ 5.276367] poll
|
||
<6>[ 5.276618] return 0
|
||
<6>[ 6.275178] wake_up
|
||
<6>[ 6.276370] poll
|
||
<6>[ 6.277269] return POLLIN
|
||
revents = 1
|
||
POLLIN n=10 buf=4294893839</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Force the poll <a href="file-operations.html"><code>file_operation</code></a> to return 0 to see what happens more clearly:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>./poll.sh pol0=1</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Sample output:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>poll
|
||
<6>[ 85.674801] poll
|
||
<6>[ 85.675788] return 0
|
||
<6>[ 86.675182] wake_up
|
||
<6>[ 86.676431] poll
|
||
<6>[ 86.677373] return 0
|
||
<6>[ 87.679198] wake_up
|
||
<6>[ 87.680515] poll
|
||
<6>[ 87.681564] return 0
|
||
<6>[ 88.683198] wake_up</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>From this we see that control is not returned to userland: the kernel just keeps calling the poll <code>file_operation</code> again and again.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Typically, we are waiting for some hardware to make some piece of data available available to the kernel.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The hardware notifies the kernel that the data is ready with an interrupt.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>To simplify this example, we just fake the hardware interrupts with a <a href="kthread.html">kthread</a> that sleeps for a second in an infinite loop.</p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Bibliography:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><a href="https://stackoverflow.com/questions/30035776/how-to-add-poll-function-to-the-kernel-module-code/44645336#44645336" class="bare">https://stackoverflow.com/questions/30035776/how-to-add-poll-function-to-the-kernel-module-code/44645336#44645336</a></p>
|
||
</li>
|
||
<li>
|
||
<p><a href="https://stackoverflow.com/questions/30234496/why-do-we-need-to-call-poll-wait-in-poll/44645480#44645480" class="bare">https://stackoverflow.com/questions/30234496/why-do-we-need-to-call-poll-wait-in-poll/44645480#44645480</a></p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>← Previous: <a href="seq-file-single-open.html">seq_file single_open</a> | ↑ Up: <a href="pseudo-files.html">Pseudo files</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="ioctl.html">ioctl</a> →</p>
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