linux-kernel - Re: [PATCH v12 1/1] printk: Add an option to allow ttynull to be a default console device

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Message-ID: <5112655.tIAgqjz4sF@nerdopolis2>
Date: Mon, 17 Mar 2025 19:26:08 -0400
From: Adam Simonelli <adamsimonelli@...il.com>
To: Petr Mladek <pmladek@...e.com>
Cc: 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>
Subject:
 Re: [PATCH v12 1/1] printk: Add an option to allow ttynull to be a default
 console device

On Monday, March 17, 2025 11:42:25 AM EDT Petr Mladek wrote:
> On Fri 2025-03-14 12:07:49, adamsimonelli@...il.com wrote:
> > From: Adam Simonelli <adamsimonelli@...il.com>
> > 
> > The new option is CONFIG_NULL_TTY_DEFAULT_CONSOLE.
> > 
> > if enabled, and CONFIG_VT is disabled, ttynull will become the default
> > primary console device.
> > 
> > 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.
> > 
> > Motivation:
> > 
> > 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_DEFAULT_CONSOLE option will give users and
> > distribution maintainers an option to avoid this. Disabling CONFIG_VT and
> > enabling CONFIG_NULL_TTY_DEFAULT_CONSOLE will ensure the default kernel
> > console behavior is not dependent on hardware configuration by default, and
> > avoid unexpected new behavior on devices connected to the /dev/ttyS0 serial
> > port.
> > 
> > Reviewed-by: Petr Mladek <pmladek@...e.com>
> > Tested-by: Petr Mladek <pmladek@...e.com>
> > Signed-off-by: Adam Simonelli <adamsimonelli@...il.com>
> 
> JFYI, the patch have been comitted into printk/linux.git,
> branch for-6.15.
> 
> Best Regards,
> Petr
> 
Oh wow! Thank you so much!