Commit 757055ae authored by Petr Mladek's avatar Petr Mladek
Browse files

init/console: Use ttynull as a fallback when there is no console 



stdin, stdout, and stderr standard I/O stream are created for the init
process. They are not available when there is no console registered
for /dev/console. It might lead to a crash when the init process
tries to use them, see the commit 48021f98 ("printk: handle
blank console arguments passed in.").

Normally, ttySX and ttyX consoles are used as a fallback when no consoles
are defined via the command line, device tree, or SPCR. But there
will be no console registered when an invalid console name is configured
or when the configured consoles do not exist on the system.

Users even try to avoid the console intentionally, for example,
by using console="" or console=null. It is used on production
systems where the serial port or terminal are not visible to
users. Pushing messages to these consoles would just unnecessary
slowdown the system.

Make sure that stdin, stdout, stderr, and /dev/console are always
available by a fallback to the existing ttynull driver. It has
been implemented for exactly this purpose but it was used only
when explicitly configured.

Reviewed-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: default avatarGuenter Roeck <linux@roeck-us.net>
Tested-by: default avatarGuenter Roeck <linux@roeck-us.net>
Acked-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20201111135450.11214-2-pmladek@suse.com
parent 8119c433

drivers/tty/Kconfig

+0 −14
Original line number Diff line number Diff line
@@ -428,20 +428,6 @@ config MIPS_EJTAG_FDC_KGDB_CHAN 
	help

	  FDC channel number to use for KGDB.



config NULL_TTY

	tristate "NULL TTY driver"

	help

	  Say Y here if you want a NULL TTY which simply discards messages.



	  This is useful to allow userspace applications which expect a console

	  device to work without modifications even when no console is

	  available or desired.



	  In order to use this driver, you should redirect the console to this

	  TTY, or boot the kernel with console=ttynull.



	  If unsure, say N.



config TRACE_ROUTER

	tristate "Trace data router for MIPI P1149.7 cJTAG standard"

	depends on TRACE_SINK

drivers/tty/Makefile

+1 −2
Original line number Diff line number Diff line
@@ -2,7 +2,7 @@ 
obj-$(CONFIG_TTY)		+= tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \

				   tty_buffer.o tty_port.o tty_mutex.o \

				   tty_ldsem.o tty_baudrate.o tty_jobctrl.o \

				   n_null.o

				   n_null.o ttynull.o

obj-$(CONFIG_LEGACY_PTYS)	+= pty.o

obj-$(CONFIG_UNIX98_PTYS)	+= pty.o

obj-$(CONFIG_AUDIT)		+= tty_audit.o
@@ -25,7 +25,6 @@ obj-$(CONFIG_ISI) += isicom.o 
obj-$(CONFIG_MOXA_INTELLIO)	+= moxa.o

obj-$(CONFIG_MOXA_SMARTIO)	+= mxser.o

obj-$(CONFIG_NOZOMI)		+= nozomi.o

obj-$(CONFIG_NULL_TTY)	        += ttynull.o

obj-$(CONFIG_ROCKETPORT)	+= rocket.o

obj-$(CONFIG_SYNCLINK_GT)	+= synclink_gt.o

obj-$(CONFIG_SYNCLINKMP)	+= synclinkmp.o

drivers/tty/ttynull.c

+18 −0
Original line number Diff line number Diff line
@@ -2,6 +2,13 @@ 
/*

 * Copyright (C) 2019 Axis Communications AB

 *

 * The console is useful for userspace applications which expect a console

 * device to work without modifications even when no console is available

 * or desired.

 *

 * In order to use this driver, you should redirect the console to this

 * TTY, or boot the kernel with console=ttynull.

 *

 * Based on ttyprintk.c:

 *  Copyright (C) 2010 Samo Pogacnik

 */
@@ -59,6 +66,17 @@ static struct console ttynull_console = { 
	.device = ttynull_device,

};



void __init register_ttynull_console(void)

{

	if (!ttynull_driver)

		return;



	if (add_preferred_console(ttynull_console.name, 0, NULL))

		return;



	register_console(&ttynull_console);

}



static int __init ttynull_init(void)

{

	struct tty_driver *driver;

include/linux/console.h

+3 −0
Original line number Diff line number Diff line
@@ -187,9 +187,12 @@ extern int braille_register_console(struct console *, int index, 
extern int braille_unregister_console(struct console *);

#ifdef CONFIG_TTY

extern void console_sysfs_notify(void);

extern void register_ttynull_console(void);

#else

static inline void console_sysfs_notify(void)

{ }

static inline void register_ttynull_console(void)

{ }

#endif

extern bool console_suspend_enabled;

init/main.c

+8 −2
Original line number Diff line number Diff line
@@ -1468,9 +1468,15 @@ void __init console_on_rootfs(void) 
	struct file *file = filp_open("/dev/console", O_RDWR, 0);



	if (IS_ERR(file)) {

		pr_err("Warning: unable to open an initial console.\n");

		pr_err("Warning: unable to open an initial console. Fallback to ttynull.\n");

		register_ttynull_console();



		file = filp_open("/dev/console", O_RDWR, 0);

		if (IS_ERR(file)) {

			pr_err("Warning: Failed to add ttynull console. No stdin, stdout, and stderr for the init process!\n");

			return;

		}

	}

	init_dup(file);

	init_dup(file);

	init_dup(file);

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