linux-kernel - [PATCH v8 3/4] tty: Change order of ttynull to be linked sooner if enabled as a console.

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Message-ID: <20250311033133.1859963-4-adamsimonelli@gmail.com>
Date: Mon, 10 Mar 2025 23:31:32 -0400
From: adamsimonelli@...il.com
To: linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jiri Slaby <jirislaby@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Petr Mladek <pmladek@...e.com>
Cc: Adam Simonelli <adamsimonelli@...il.com>
Subject: [PATCH v8 3/4] tty: Change order of ttynull to be linked sooner if enabled as a console.

From: Adam Simonelli <adamsimonelli@...il.com>

If CONFIG_NULL_TTY_DEFAULT_CONSOLE is enabled, and CONFIG_VT is disabled,
ttynull will become the default primary console device, based on the link
order.

ttynull will be the only console device usually with this option enabled.
Some architectures do call add_preferred_console() which may add another
console though.

Many distributions ship with CONFIG_VT enabled. On tested desktop hardware
if CONFIG_VT is disabled, the default console device falls back to
/dev/ttyS0 instead of /dev/tty.

This could cause issues in user space, and hardware problems:

1. The user space issues include the case where  /dev/ttyS0 is
disconnected, and the TCGETS ioctl, which some user space libraries use
as a probe to determine if a file is a tty, is called on /dev/console and
fails. Programs that call isatty() on /dev/console and get an incorrect
false value may skip expected logging to /dev/console.

2. The hardware issues include the case if a user has a science instrument
or other device connected to the /dev/ttyS0 port, and they were to upgrade
to a kernel that is disabling the CONFIG_VT option, kernel logs will then be
sent to the device connected to /dev/ttyS0 unless they edit their kernel
command line manually.

The new CONFIG_NULL_TTY_CONSOLE option will give users and distribution
maintainers an option to avoid this. Disabling CONFIG_VT and enabling
CONFIG_NULL_TTY_CONSOLE will ensure the default kernel console behavior
is not dependant on hardware configuration by default, and avoid
unexpected new behavior on devices connected to the /dev/ttyS0 serial
port.

Signed-off-by: Adam Simonelli <adamsimonelli@...il.com>
---
 drivers/tty/Makefile | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
index 07aca5184a55..7d0414dc31ed 100644
--- a/drivers/tty/Makefile
+++ b/drivers/tty/Makefile
@@ -11,6 +11,12 @@ obj-$(CONFIG_N_HDLC)		+= n_hdlc.o
 obj-$(CONFIG_N_GSM)		+= n_gsm.o

 obj-y				+= vt/
+
+# If ttynull is configured to be a console by default, ensure that it is linked
+# earlier before a real one is selected.
+obj-$(CONFIG_NULL_TTY_DEFAULT_CONSOLE) \
+				+= ttynull.o
+
 obj-$(CONFIG_HVC_DRIVER)	+= hvc/
 obj-y				+= serial/
 obj-$(CONFIG_SERIAL_DEV_BUS)	+= serdev/
@@ -20,7 +26,13 @@ obj-$(CONFIG_AMIGA_BUILTIN_SERIAL) += amiserial.o
 obj-$(CONFIG_MOXA_INTELLIO)	+= moxa.o
 obj-$(CONFIG_MOXA_SMARTIO)	+= mxser.o
 obj-$(CONFIG_NOZOMI)		+= nozomi.o
+
+# If ttynull is enabled, but not as a boot console, it is linked and used later
+# after the real ones.
+ifneq ($(CONFIG_NULL_TTY_DEFAULT_CONSOLE),y)
 obj-$(CONFIG_NULL_TTY)	        += ttynull.o
+endif
+
 obj-$(CONFIG_SYNCLINK_GT)	+= synclink_gt.o
 obj-$(CONFIG_PPC_EPAPR_HV_BYTECHAN) += ehv_bytechan.o
 obj-$(CONFIG_GOLDFISH_TTY)	+= goldfish.o
-- 
2.45.2