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<div id="content" class="content">
<h1 class="title">Mirai's Watchdog in rust.</h1>
<div id="table-of-contents" role="doc-toc">
<h2>Table of Contents</h2>
<div id="text-table-of-contents" role="doc-toc">
<ul>
<li><a href="#orgd3e9468">Disabling the watchdog.</a></li>
<li><a href="#org1392892">The magic close feature.</a></li>
<li><a href="#orgb70f00f">Additional resources</a></li>
</ul>
</div>
</div>


<div id="outline-container-orgd3e9468" class="outline-2">
<h2 id="orgd3e9468">Disabling the watchdog.</h2>
<div class="outline-text-2" id="text-orgd3e9468">
<p>
One of the first things that mirai does is disable the watchdog of the
host to ensure that it didnt reboot if the system begins to take too
long.
</p>

<pre class="example" id="org468b52a">

// Prevent watchdog from rebooting device
if ((wfd = open("/dev/watchdog", 2)) != -1 ||
    (wfd = open("/dev/misc/watchdog", 2)) != -1)
{
    int one = 1;

    ioctl(wfd, 0x80045704, &amp;one);
    close(wfd);
    wfd = 0;
}
</pre>

<p>
Lets break that down,
</p>

<pre class="example" id="org78095eb">
if ((wfd = open("/dev/watchdog", 2)) != -1 ||
</pre>

<p>
It opens the watchdog device read/write (#define O_RDWR 00000002).
</p>

<p>
It issues an 'ioctl' with a magic number.
</p>
<pre class="code"><code>ioctl(wfd, 0x80045704, &amp;one);
</code></pre>

<p>
Then closes the opened file descriptor.
</p>

<pre class="code"><code>close(wfd);
</code></pre>

<p>
While this does adhere to the spec, its also not the most supported
method of disabling the watchdog. there is another&#x2026;
</p>
</div>
</div>

<div id="outline-container-org1392892" class="outline-2">
<h2 id="org1392892">The magic close feature.</h2>
<div class="outline-text-2" id="text-org1392892">
<p>
The aptly named "magic close feature" (<a href="https://www.kernel.org/doc/Documentation/watchdog/watchdog-api.txt">https://www.kernel.org/doc/Documentation/watchdog/watchdog-api.txt</a>))
</p>


<div class="epigraph"><blockquote>
Magic Close feature:<br />
<br />
If a driver supports "Magic Close", the driver will not disable the<br />
watchdog unless a specific magic character 'V' has been sent to<br />
/dev/watchdog just before closing the file.  If the userspace daemon<br />
closes the file without sending this special character, the driver<br />
will assume that the daemon (and userspace in general) died, and will<br />
stop pinging the watchdog without disabling it first.  This will then<br />
cause a reboot if the watchdog is not re-opened in sufficient time.<br />
<br />

</blockquote></div>

<p>
Seems to be more widely supported than the ioctl method. There are
watchdog drivers in the tree that do not support ioctl manipulation,
but instead support the magic close feature.
</p>

<p>
Rust doesn't have a close() function, From the  std::fs::File docs:
</p>

<div class="epigraph"><blockquote>
Files are automatically closed when they go out of scope. Errors detected on<br />
closing are ignored by the implementation of Drop. Use the method sync_all if<br />
these errors must be manually handled.<br />

</blockquote></div>

<p>
This would leave us with a simple function : 
</p>

<pre class="code"><code>use std::fs;

fn disable_watchodg() -&gt; std::io::Result&lt;()&gt; {
    fs::write("/dev/watchdog", "V")?
}

fn main() {
 disable_watchdog();
}
</code></pre>

<p>
Wild right ?
</p>
</div>
</div>


<div id="outline-container-orgb70f00f" class="outline-2">
<h2 id="orgb70f00f">Additional resources</h2>
<div class="outline-text-2" id="text-orgb70f00f">
<p>
<a href="https://github.com/jgamblin/Mirai-Source-Code">Mirai Source code</a>
</p>
</div>
</div>
</div>
<div id="postamble" class="status">
<p class="author">Author: Wade Mealing</p>
<p class="date">Created: 2022-07-11 Mon 12:40</p>
</div>
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