linux-kernel - [PATCH v3 2/2] tty: Change order of ttynull to be linked sooner if enabled as a console.

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Message-ID: <20250223051441.1861603-3-adamsimonelli@gmail.com>
Date: Sun, 23 Feb 2025 00:14:41 -0500
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>
Cc: Adam Simonelli <adamsimonelli@...il.com>
Subject: [PATCH v3 2/2] tty: Change order of ttynull to be linked sooner if enabled as a console.

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

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

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 | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/tty/Makefile b/drivers/tty/Makefile
index 07aca5184a55..1a1051ecb1af 100644
--- a/drivers/tty/Makefile
+++ b/drivers/tty/Makefile
@@ -11,6 +11,10 @@ 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-$(CONFIG_NULL_TTY_CONSOLE)	+= ttynull.o
+
 obj-$(CONFIG_HVC_DRIVER)	+= hvc/
 obj-y				+= serial/
 obj-$(CONFIG_SERIAL_DEV_BUS)	+= serdev/
@@ -20,7 +24,13 @@ obj-$(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_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