Linux Kernel Module Cheat

17.4.1. /proc/sys/kernel/printk

The current printk level can be obtained with:

cat /proc/sys/kernel/printk

As of 87e846fc1f9c57840e143513ebd69c638bd37aa8 this prints:

7       4       1       7

which contains:

7: current log level, modifiable by previously mentioned methods
4: documented as: "printk’s without a loglevel use this": TODO what does that mean, how to call printk without a log level?
1: minimum log level that still prints something (0 prints nothing)
7: default log level

We start at the boot time default after boot by default, as can be seen from:

insmod myprintk.ko

which outputs something like:

<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

Source: kernel_modules/myprintk.c

This proc entry is defined at: https://github.com/torvalds/linux/blob/v5.1/kernel/sysctl.c#L839

#if defined CONFIG_PRINTK
	{
		.procname	= "printk",
		.data		= &console_loglevel,
		.maxlen		= 4*sizeof(int),
		.mode		= 0644,
		.proc_handler	= proc_dointvec,
	},

which teaches us that printk can be completely disabled at compile time:

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.

console_loglevel is defined at:

#define console_loglevel (console_printk[0])

and console_printk is an array with 4 ints:

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 */
};

and then we see that the default is configurable with CONFIG_CONSOLE_LOGLEVEL_DEFAULT:

/*
 * 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

The message loglevel default is explained at:

/* printk's without a loglevel use this.. */
#define MESSAGE_LOGLEVEL_DEFAULT CONFIG_MESSAGE_LOGLEVEL_DEFAULT

The min is just hardcoded to one as you would expect, with some amazing kernel comedy around it:

/* 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 */

We then also learn about the useless quiet and debug kernel parameters at:

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>"

which explains the useless reason why that number is special. This is implemented at:

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);