Messages in this thread | | | Date | Wed, 11 Nov 2020 14:52:15 -0800 | From | Guenter Roeck <> | Subject | Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console |
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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 >
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