[PATCH 0/2] Optionally allow ttynull to be selected as a default console

[PATCH 0/2] Optionally allow ttynull to be selected as a default console

[adamsimonelli]

From: Adam Simonelli <adamsimonelli@gmail.com>

When switching to a CONFIG_VT=n world, at least on x86 systems,
/dev/console becomes /dev/ttyS0. This can cause some undesired effects.
/dev/console's behavior is now tied to the physical /dev/ttyS0, which when
disconnected can cause isatty() to fail when /dev/ttyS0 is disconnected,
and users who upgrade to a theoretical vt-less kernel from their
distribution who have a device such as a science instrument connected to
their /dev/ttyS0 port will suddenly see it receive kernel log messages.

When the new CONFIG_NULL_TTY_CONSOLE option is turned on, this will allow
the ttynull device to be leveraged as the default console. Distributions
that had CONFIG_VT turned on before will be able to leverage this option
to where /dev/console is still backed by a psuedo device, avoiding these
issues, without needing to enable the entire VT subsystem.

Adam Simonelli (2):
  ttynull: Add an option to allow ttynull to be used as a console device
  tty: Change order of ttynull to be loaded sooner.

 drivers/tty/Kconfig   | 18 +++++++++++++++++-
 drivers/tty/Makefile  |  3 ++-
 drivers/tty/ttynull.c | 16 +++++++++++++++-
 3 files changed, 34 insertions(+), 3 deletions(-)

-- 
2.45.2

[PATCH 1/2] ttynull: Add an option to allow ttynull to be used as a console device

[adamsimonelli]

From: Adam Simonelli <adamsimonelli@gmail.com>

Add a config option CONFIG_NULL_TTY_CONSOLE that will have ttynull be
initialized by console_initcall() and selected as a possible console
device.

Signed-off-by: Adam Simonelli <adamsimonelli@gmail.com>
---
 drivers/tty/Kconfig   | 18 +++++++++++++++++-
 drivers/tty/ttynull.c | 16 +++++++++++++++-
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 63a494d36a1f..afe4e7722d4c 100644
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -383,7 +383,23 @@ config NULL_TTY
 	  available or desired.
 
 	  In order to use this driver, you should redirect the console to this
-	  TTY, or boot the kernel with console=ttynull.
+	  TTY, boot the kernel with console=ttynull, or enable
+	  CONFIG_NULL_TTY_CONSOLE.
+
+	  If unsure, say N.
+
+config NULL_TTY_CONSOLE
+        bool "Supports /dev/ttynull as a console automatically"
+        depends on NULL_TTY=y && !VT_CONSOLE
+	help
+	  Say Y here if you want the NULL TTY to be used as a /dev/console
+	  device.
+
+	  This is useful for userspace applications that expect a working
+	  console device, without changing the kernel boot options, such as a
+	  distribuition or environment that historically had CONFIG_VT_CONSOLE
+	  enabled, and have now disabled it, but still need /dev/console to be
+	  working for userspace applications.
 
 	  If unsure, say N.
 
diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
index 6b2f7208b564..8ba629ae426b 100644
--- a/drivers/tty/ttynull.c
+++ b/drivers/tty/ttynull.c
@@ -57,6 +57,10 @@ static struct tty_driver *ttynull_device(struct console *c, int *index)
 static struct console ttynull_console = {
 	.name = "ttynull",
 	.device = ttynull_device,
+#ifdef CONFIG_NULL_TTY_CONSOLE
+	.index = -1,
+	.flags = CON_PRINTBUFFER,
+#endif
 };
 
 static int __init ttynull_init(void)
@@ -90,10 +94,20 @@ static int __init ttynull_init(void)
 	}
 
 	ttynull_driver = driver;
-	register_console(&ttynull_console);
+	if (!console_is_registered(&ttynull_console))
+		register_console(&ttynull_console);
+
+	return 0;
+}
 
+#ifdef CONFIG_NULL_TTY_CONSOLE
+static int __init ttynull_register(void)
+{
+	register_console(&ttynull_console);
 	return 0;
 }
+console_initcall(ttynull_register);
+#endif
 
 static void __exit ttynull_exit(void)
 {
-- 
2.45.2

[PATCH 2/2] tty: Change order of ttynull to be loaded sooner.

[adamsimonelli]

From: Adam Simonelli <adamsimonelli@gmail.com>

If CONFIG_NULL_TTY_CONSOLE is enabled, and CONFIG_VT is disabled, ttynull
will become the default primary console device, based on the load order.
Users and distributions that are migrating away from CONFIG_VT will
benefit from this as /dev/console would not suddenly become /dev/ttyS0
which could otherwise cause some user space behavior changes, namely the
TCGETS ioctl failing, which causes libc's isatty() to incorrectly return
false when /dev/ttyS0 is disabled, and will prevent a device that is
connected to a user's /dev/ttyS0 to suddenly start getting kernel log
messages.

Signed-off-by: Adam Simonelli <adamsimonelli@gmail.com>
---
 drivers/tty/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
index 07aca5184a55..03bb47e11e1c 100644
--- a/drivers/tty/Makefile
+++ b/drivers/tty/Makefile
@@ -11,6 +11,8 @@ obj-$(CONFIG_N_HDLC)		+= n_hdlc.o
 obj-$(CONFIG_N_GSM)		+= n_gsm.o
 
 obj-y				+= vt/
+obj-$(CONFIG_NULL_TTY)		+= ttynull.o
+
 obj-$(CONFIG_HVC_DRIVER)	+= hvc/
 obj-y				+= serial/
 obj-$(CONFIG_SERIAL_DEV_BUS)	+= serdev/
@@ -20,7 +22,6 @@ obj-$(CONFIG_AMIGA_BUILTIN_SERIAL) += amiserial.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-$(CONFIG_SYNCLINK_GT)	+= synclink_gt.o
 obj-$(CONFIG_PPC_EPAPR_HV_BYTECHAN) += ehv_bytechan.o
 obj-$(CONFIG_GOLDFISH_TTY)	+= goldfish.o
-- 
2.45.2

[PATCH 2/2] tty: Change order of ttynull to be loaded sooner.

[n3rdopolis]

If CONFIG_NULL_TTY_CONSOLE is enabled, and CONFIG_VT is disabled, ttynull
will become the default primary console device, based on the load order.
Users and distributions that are migrating away from CONFIG_VT will
benefit from this as /dev/console would not suddenly become /dev/ttyS0
which could otherwise cause some user space behavior changes, namely the
TCGETS ioctl failing, which causes libc's isatty() to incorrectly return
false when /dev/ttyS0 is disabled, and will prevent a device that is
connected to a user's /dev/ttyS0 to suddenly start getting kernel log
messages.
Signed-off-by: n3rdopolis <bluescreen_avenger@verizon.net>
---
 drivers/tty/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
index 07aca5184a55..03bb47e11e1c 100644
--- a/drivers/tty/Makefile
+++ b/drivers/tty/Makefile
@@ -11,6 +11,8 @@ obj-$(CONFIG_N_HDLC)		+= n_hdlc.o
 obj-$(CONFIG_N_GSM)		+= n_gsm.o
 
 obj-y				+= vt/
+obj-$(CONFIG_NULL_TTY)		+= ttynull.o
+
 obj-$(CONFIG_HVC_DRIVER)	+= hvc/
 obj-y				+= serial/
 obj-$(CONFIG_SERIAL_DEV_BUS)	+= serdev/
@@ -20,7 +22,6 @@ obj-$(CONFIG_AMIGA_BUILTIN_SERIAL) += amiserial.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-$(CONFIG_SYNCLINK_GT)	+= synclink_gt.o
 obj-$(CONFIG_PPC_EPAPR_HV_BYTECHAN) += ehv_bytechan.o
 obj-$(CONFIG_GOLDFISH_TTY)	+= goldfish.o
-- 
2.45.2