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https://github.com/cirosantilli/linux-kernel-module-cheat.git
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670 lines
38 KiB
HTML
670 lines
38 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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</head>
|
||
<body id="procsyskernelprintk" class="article">
|
||
<div id="header">
|
||
<h1>Linux Kernel Module Cheat</h1>
|
||
</div>
|
||
<div id="content">
|
||
<div class="sect3">
|
||
<h4 id="procsyskernelprintk"><a class="anchor" href="#procsyskernelprintk"></a><a class="link" href="#procsyskernelprintk">17.4.1. /proc/sys/kernel/printk</a></h4>
|
||
<div class="paragraph">
|
||
<p>The current printk level can be obtained with:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>cat /proc/sys/kernel/printk</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>As of <code>87e846fc1f9c57840e143513ebd69c638bd37aa8</code> this prints:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>7 4 1 7</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which contains:</p>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
<li>
|
||
<p><code>7</code>: current log level, modifiable by previously mentioned methods</p>
|
||
</li>
|
||
<li>
|
||
<p><code>4</code>: documented as: "printk’s without a loglevel use this": TODO what does that mean, how to call <code>printk</code> without a log level?</p>
|
||
</li>
|
||
<li>
|
||
<p><code>1</code>: minimum log level that still prints something (<code>0</code> prints nothing)</p>
|
||
</li>
|
||
<li>
|
||
<p><code>7</code>: default log level</p>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>We start at the boot time default after boot by default, as can be seen from:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>insmod myprintk.ko</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which outputs something like:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre><1>[ 12.494429] pr_alert
|
||
<2>[ 12.494666] pr_crit
|
||
<3>[ 12.494823] pr_err
|
||
<4>[ 12.494911] pr_warning
|
||
<5>[ 12.495170] pr_notice
|
||
<6>[ 12.495327] pr_info</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>Source: <a href="https://github.com/cirosantilli/linux-kernel-module-cheat/blob/master/kernel_modules/myprintk.c">kernel_modules/myprintk.c</a></p>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>This proc entry is defined at: <a href="https://github.com/torvalds/linux/blob/v5.1/kernel/sysctl.c#L839" class="bare">https://github.com/torvalds/linux/blob/v5.1/kernel/sysctl.c#L839</a></p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>#if defined CONFIG_PRINTK
|
||
{
|
||
.procname = "printk",
|
||
.data = &console_loglevel,
|
||
.maxlen = 4*sizeof(int),
|
||
.mode = 0644,
|
||
.proc_handler = proc_dointvec,
|
||
},</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which teaches us that printk can be completely disabled at compile time:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>config PRINTK
|
||
default y
|
||
bool "Enable support for printk" if EXPERT
|
||
select IRQ_WORK
|
||
help
|
||
This option enables normal printk support. Removing it
|
||
eliminates most of the message strings from the kernel image
|
||
and makes the kernel more or less silent. As this makes it
|
||
very difficult to diagnose system problems, saying N here is
|
||
strongly discouraged.</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p><code>console_loglevel</code> is defined at:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>#define console_loglevel (console_printk[0])</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and <code>console_printk</code> is an array with 4 ints:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>int console_printk[4] = {
|
||
CONSOLE_LOGLEVEL_DEFAULT, /* console_loglevel */
|
||
MESSAGE_LOGLEVEL_DEFAULT, /* default_message_loglevel */
|
||
CONSOLE_LOGLEVEL_MIN, /* minimum_console_loglevel */
|
||
CONSOLE_LOGLEVEL_DEFAULT, /* default_console_loglevel */
|
||
};</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>and then we see that the default is configurable with <code>CONFIG_CONSOLE_LOGLEVEL_DEFAULT</code>:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>/*
|
||
* Default used to be hard-coded at 7, quiet used to be hardcoded at 4,
|
||
* we're now allowing both to be set from kernel config.
|
||
*/
|
||
#define CONSOLE_LOGLEVEL_DEFAULT CONFIG_CONSOLE_LOGLEVEL_DEFAULT
|
||
#define CONSOLE_LOGLEVEL_QUIET CONFIG_CONSOLE_LOGLEVEL_QUIET</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The message loglevel default is explained at:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>/* printk's without a loglevel use this.. */
|
||
#define MESSAGE_LOGLEVEL_DEFAULT CONFIG_MESSAGE_LOGLEVEL_DEFAULT</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>The min is just hardcoded to one as you would expect, with some amazing kernel comedy around it:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>/* We show everything that is MORE important than this.. */
|
||
#define CONSOLE_LOGLEVEL_SILENT 0 /* Mum's the word */
|
||
#define CONSOLE_LOGLEVEL_MIN 1 /* Minimum loglevel we let people use */
|
||
#define CONSOLE_LOGLEVEL_DEBUG 10 /* issue debug messages */
|
||
#define CONSOLE_LOGLEVEL_MOTORMOUTH 15 /* You can't shut this one up */</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>We then also learn about the useless <code>quiet</code> and <code>debug</code> kernel parameters at:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>config CONSOLE_LOGLEVEL_QUIET
|
||
int "quiet console loglevel (1-15)"
|
||
range 1 15
|
||
default "4"
|
||
help
|
||
loglevel to use when "quiet" is passed on the kernel commandline.
|
||
|
||
When "quiet" is passed on the kernel commandline this loglevel
|
||
will be used as the loglevel. IOW passing "quiet" will be the
|
||
equivalent of passing "loglevel=<CONSOLE_LOGLEVEL_QUIET>"</pre>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph">
|
||
<p>which explains the useless reason why that number is special. This is implemented at:</p>
|
||
</div>
|
||
<div class="literalblock">
|
||
<div class="content">
|
||
<pre>static int __init debug_kernel(char *str)
|
||
{
|
||
console_loglevel = CONSOLE_LOGLEVEL_DEBUG;
|
||
return 0;
|
||
}
|
||
|
||
static int __init quiet_kernel(char *str)
|
||
{
|
||
console_loglevel = CONSOLE_LOGLEVEL_QUIET;
|
||
return 0;
|
||
}
|
||
|
||
early_param("debug", debug_kernel);
|
||
early_param("quiet", quiet_kernel);</pre>
|
||
</div>
|
||
</div>
|
||
<div class="ulist">
|
||
<ul>
|
||
</ul>
|
||
</div>
|
||
</div>
|
||
<div class="paragraph nav-footer">
|
||
<p>↑ Up: <a href="printk.html">printk</a> | ⌂ Home: <a href="README.html">Linux Kernel Module Cheat</a> | Next: <a href="ignore-loglevel.html">ignore_loglevel</a> →</p>
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