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linux-kernel-module-cheat/ctrl-alt-del.html
Ciro Santilli 0a2a05fe28 8e6cdcb14b
2025-05-10 07:50:53 +01:00

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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;. EVERYTHING is built from source. &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="ctrl-alt-del" class="article">
<div id="header">
<h1>Linux Kernel Module Cheat</h1>
</div>
<div id="content">
<div class="sect4">
<h5 id="ctrl-alt-del"><a class="anchor" href="#ctrl-alt-del"></a><a class="link" href="#ctrl-alt-del">17.17.2.1. Ctrl Alt Del</a></h5>
<div class="paragraph">
<p>If you run in <a href="qemu-graphic-mode.html">QEMU graphic mode</a>:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>./run --graphic</pre>
</div>
</div>
<div class="paragraph">
<p>and then from the graphic window you enter the keys:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>Ctrl-Alt-Del</pre>
</div>
</div>
<div class="paragraph">
<p>then this runs the following command on the guest:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>/sbin/reboot</pre>
</div>
</div>
<div class="paragraph">
<p>This is enabled from our <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/rootfs_overlay/etc/inittab">rootfs_overlay/etc/inittab</a>:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>::ctrlaltdel:/sbin/reboot</pre>
</div>
</div>
<div class="paragraph">
<p>This leads Linux to try to reboot, and QEMU shutdowns due to the <code>-no-reboot</code> option which we set by default for, see: <a href="exit-emulator-on-panic.html">Section 17.6.1.3, &#8220;Exit emulator on panic&#8221;</a>.</p>
</div>
<div class="paragraph">
<p>Here is a minimal example of Ctrl Alt Del:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>./run --kernel-cli 'init=/lkmc/linux/ctrl_alt_del.out' --graphic</pre>
</div>
</div>
<div class="paragraph">
<p>Source: <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/userland/linux/ctrl_alt_del.c">userland/linux/ctrl_alt_del.c</a></p>
</div>
<div class="paragraph">
<p>When you hit <code>Ctrl-Alt-Del</code> in the guest, our tiny init handles a <code>SIGINT</code> sent by the kernel and outputs to stdout:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>cad</pre>
</div>
</div>
<div class="paragraph">
<p>To map between <code>man 2 reboot</code> and the uClibc <code>RB_*</code> magic constants see:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>less "$(./getvar buildroot_build_build_dir)"/uclibc-*/include/sys/reboot.h"</pre>
</div>
</div>
<div class="paragraph">
<p>The procfs mechanism is documented at:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>less linux/Documentation/sysctl/kernel.txt</pre>
</div>
</div>
<div class="paragraph">
<p>which says:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>When the value in this file is 0, ctrl-alt-del is trapped and
sent to the init(1) program to handle a graceful restart.
When, however, the value is &gt; 0, Linux's reaction to a Vulcan
Nerve Pinch (tm) will be an immediate reboot, without even
syncing its dirty buffers.
Note: when a program (like dosemu) has the keyboard in 'raw'
mode, the ctrl-alt-del is intercepted by the program before it
ever reaches the kernel tty layer, and it's up to the program
to decide what to do with it.</pre>
</div>
</div>
<div class="paragraph">
<p>Under the hood, behaviour is controlled by the <code>reboot</code> syscall:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>man 2 reboot</pre>
</div>
</div>
<div class="paragraph">
<p><code>reboot</code> system calls can set either of the these behaviours for <code>Ctrl-Alt-Del</code>:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>do a hard shutdown syscall. Set in uClibc C code with:</p>
<div class="literalblock">
<div class="content">
<pre>reboot(RB_ENABLE_CAD)</pre>
</div>
</div>
<div class="paragraph">
<p>or from procfs with:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>echo 1 &gt; /proc/sys/kernel/ctrl-alt-del</pre>
</div>
</div>
<div class="paragraph">
<p>Done by BusyBox' <code>reboot -f</code>.</p>
</div>
</li>
<li>
<p>send a SIGINT to the init process. This is what BusyBox' init does, and it then execs the string set in <code>inittab</code>.</p>
<div class="paragraph">
<p>Set in uclibc C code with:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>reboot(RB_DISABLE_CAD)</pre>
</div>
</div>
<div class="paragraph">
<p>or from procfs with:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>echo 0 &gt; /proc/sys/kernel/ctrl-alt-del</pre>
</div>
</div>
<div class="paragraph">
<p>Done by BusyBox' <code>reboot</code>.</p>
</div>
</li>
</ul>
</div>
<div class="paragraph">
<p>When a BusyBox init is with the signal, it prints the following lines:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>The system is going down NOW!
Sent SIGTERM to all processes
Sent SIGKILL to all processes
Requesting system reboot</pre>
</div>
</div>
<div class="paragraph">
<p>On busybox-1.29.2&#8217;s init at init/init.c we see how the kill signals are sent:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>static void run_shutdown_and_kill_processes(void)
{
/* Run everything to be run at "shutdown". This is done _prior_
* to killing everything, in case people wish to use scripts to
* shut things down gracefully... */
run_actions(SHUTDOWN);
message(L_CONSOLE | L_LOG, "The system is going down NOW!");
/* Send signals to every process _except_ pid 1 */
kill(-1, SIGTERM);
message(L_CONSOLE, "Sent SIG%s to all processes", "TERM");
sync();
sleep(1);
kill(-1, SIGKILL);
message(L_CONSOLE, "Sent SIG%s to all processes", "KILL");
sync();
/*sleep(1); - callers take care about making a pause */
}</pre>
</div>
</div>
<div class="paragraph">
<p>and <code>run_shutdown_and_kill_processes</code> is called from:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>/* The SIGPWR/SIGUSR[12]/SIGTERM handler */
static void halt_reboot_pwoff(int sig) NORETURN;
static void halt_reboot_pwoff(int sig)</pre>
</div>
</div>
<div class="paragraph">
<p>which also prints the final line:</p>
</div>
<div class="literalblock">
<div class="content">
<pre> message(L_CONSOLE, "Requesting system %s", m);</pre>
</div>
</div>
<div class="paragraph">
<p>which is set as the signal handler via TODO.</p>
</div>
<div class="paragraph">
<p>Bibliography:</p>
</div>
<div class="ulist">
<ul>
<li>
<p><a href="https://superuser.com/questions/193652/does-linux-have-a-ctrlaltdel-equivalent/1324415#1324415" class="bare">https://superuser.com/questions/193652/does-linux-have-a-ctrlaltdel-equivalent/1324415#1324415</a></p>
</li>
<li>
<p><a href="https://unix.stackexchange.com/questions/42573/meaning-and-commands-for-ctrlaltdel/444969#444969" class="bare">https://unix.stackexchange.com/questions/42573/meaning-and-commands-for-ctrlaltdel/444969#444969</a></p>
</li>
</ul>
</div>
<div class="ulist">
<ul>
</ul>
</div>
</div>
<div class="paragraph nav-footer">
<p> Up: <a href="linux-kernel-magic-keys.html">Linux kernel magic keys</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="sysrq.html">SysRq</a> →</p>
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