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 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.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20201111135450.11214-2-pmladek@suse.com

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 93fd984eb2f5a43b4972b3a13bff726d215fd029..ca359bbd62f5f393cc118004933e5035d05ae1a3 100644 (file)
--- 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 020b1cd9294fdc3342cb145dda766cf4e00127a7..f6b6bee0422d753d84cd8826c4aa2b9c56bc169c 100644 (file)
--- 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 17f05b7eb6d3ede3790d98d9527c7221eb907405..eced70ec54e174e0ae6d48ba90777ebcf08f5e4e 100644 (file)
--- 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 4b1e26c4cb4286603b0a381022fef0468bb8728c..9c662e41cde519ece7b9c382411a9b3fd94ae272 100644 (file)
--- 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 1af84337cb18d5dacd6907ff57b579dafbbf8045..b3fcf446f99e6a55f3c5f1560f1f5692c9c1fb24 100644 (file)
--- a/init/main.c
+++ b/init/main.c
@@ -1468,8 +1468,14 @@ 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);