linux-kernel - Re: [PATCH v7 3/3] tty: Change order of ttynull to be linked sooner if enabled as a console.

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Message-ID: <8533590.alqRGMn8q6@nerdopolis2>
Date: Wed, 05 Mar 2025 23:32:09 -0500
From: Adam Simonelli <adamsimonelli@...il.com>
To: Andy Shevchenko <andy.shevchenko@...il.com>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jiri Slaby <jirislaby@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Steven Rostedt <rostedt@...dmis.org>,
 John Ogness <john.ogness@...utronix.de>,
 Sergey Senozhatsky <senozhatsky@...omium.org>, Petr Mladek <pmladek@...e.com>
Subject:
 Re: [PATCH v7 3/3] tty: Change order of ttynull to be linked sooner if
 enabled as a console.

On Wednesday, March 5, 2025 2:22:06 PM EST Andy Shevchenko wrote:
> On Wed, Mar 5, 2025 at 6:30 AM <adamsimonelli@...il.com> wrote:
> >
> > 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.
> >
> > 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.
> 
> ...
> 
> Since it touches the link order only under drivers/tty the commit
> message should mention what the effect will be on the consoles drivers
> for which are located elsewhere.
> 
> 

OK, I will think of what to say. Based on what I tested in throwing a preferred
console in x86 though, it seems like ttynull was still at the top of the list
of /proc/consoles still, I just have to come up with a good way to mention that
in the commit message.