[RFC 1/2] printk: Add kernel parameter: mute_console

[Petr Mladek]

Users use various undocumented ways how to completely disable
console output. It is usually done by passing an invalid console
name, for example, console="", console=null.

It mostly works but just by chance. The console name is added to the list
of preferred consoles and the variable "preferred_console" is set.
As a result, any register_console() fails because the driver name
does not match the invalid name. And the console is not used as
a fallback because "preferred_console" is set.

It stops working, when another console is defined on the command line
or another way, for example, by SPCR or a device tree.

More importantly, the above approach might break the system. /dev/console
is used as stdin, stdout, and stderr for the init process [0]

Why yet another command line option?

console_loglevel is not reliable. It is manipulated also by user space
when it configures log daemons.

People might also want to just disable the kernel messages but still
use the console for login.

[0] https://lore.kernel.org/r/20201006025935.GA597@jagdpanzerIV.localdomain

Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 .../admin-guide/kernel-parameters.txt         |  6 ++++++
 kernel/printk/printk.c                        | 21 ++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 02d4adbf98d2..52b9e7f5468d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2974,6 +2974,12 @@
 			Used for mtrr cleanup. It is spare mtrr entries number.
 			Set to 2 or more if your graphical card needs more.
 
+	mute_console	[KNL]
+			Completely disable printing of kernel messages to
+			the console. It can still be used as stdin, stdout,
+			and stderr for the init process. Also it can be used
+			for login.
+
 	n2=		[NET] SDL Inc. RISCom/N2 synchronous serial card
 
 	netdev=		[NET] Network devices parameters
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fe64a49344bf..63fb96630767 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1207,6 +1207,19 @@ void __init setup_log_buf(int early)
 	memblock_free(__pa(new_log_buf), new_log_buf_len);
 }
 
+static bool mute_console;
+
+static int __init mute_console_setup(char *str)
+{
+	mute_console = true;
+	pr_info("All consoles muted.\n");
+
+	return 0;
+}
+
+early_param("mute_console", mute_console_setup);
+module_param(mute_console, bool, 0644);
+
 static bool __read_mostly ignore_loglevel;
 
 static int __init ignore_loglevel_setup(char *str)
@@ -1224,7 +1237,13 @@ MODULE_PARM_DESC(ignore_loglevel,
 
 static bool suppress_message_printing(int level)
 {
-	return (level >= console_loglevel && !ignore_loglevel);
+	if (unlikely(mute_console))
+		return true;
+
+	if (unlikely(ignore_loglevel))
+		return false;
+
+	return (level >= console_loglevel);
 }
 
 #ifdef CONFIG_BOOT_PRINTK_DELAY
-- 
2.26.2