Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console

Guenter Roeck
2020-11-12T01:33:46Z

On Wed, Nov 11, 2020 at 02:54:49PM +0100, Petr Mladek wrote:
> 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 48021f98130880dd742 ("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.
>
> Signed-off-by: Petr Mladek <pmladek@suse.com>

Reviewed-and-tested-by: Guenter Roeck <linux@roeck-us.net>

> ---
> drivers/tty/Kconfig | 14 --------------
> drivers/tty/Makefile | 3 +--
> drivers/tty/ttynull.c | 18 ++++++++++++++++++
> include/linux/console.h | 3 +++
> init/main.c | 10 ++++++++--
> 5 files changed, 30 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
> index 93fd984eb2f5..ca359bbd62f5 100644
> --- a/drivers/tty/Kconfig
> +++ b/drivers/tty/Kconfig
> @@ -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
> diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
> index 020b1cd9294f..f6b6bee0422d 100644
> --- a/drivers/tty/Makefile
> +++ b/drivers/tty/Makefile
> @@ -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
> diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
> index 17f05b7eb6d3..eced70ec54e1 100644
> --- a/drivers/tty/ttynull.c
> +++ b/drivers/tty/ttynull.c
> @@ -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;
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 4b1e26c4cb42..9c662e41cde5 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -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;
>
> diff --git a/init/main.c b/init/main.c
> index 130376ec10ba..24413c055a85 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1470,8 +1470,14 @@ 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");
> - return;
> + 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);
> --
> 2.26.2
>