linux-kernel - Re: [PATCH v4 2/2] tty: Change order of ttynull to be linked sooner if enabled as a console.

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Message-ID: <5764492.44csPzL39Z@nerdopolis2>
Date: Mon, 24 Feb 2025 07:38:46 -0500
From: Adam Simonelli <adamsimonelli@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jiri Slaby <jirislaby@...nel.org>
Subject:
 Re: [PATCH v4 2/2] tty: Change order of ttynull to be linked sooner if
 enabled as a console.

On Monday, February 24, 2025 2:23:09 AM EST Greg Kroah-Hartman wrote:
> On Sun, Feb 23, 2025 at 03:44:56PM -0500, adamsimonelli@...il.com wrote:
> > 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
> 
> Nit, a " " needs to be after the "#" character, right?
> 
> And ick, this is going to be tricky, changing the link order depending
> on the configuration option setting?  This feels wrong, and messy, and
> very fragile.
> 
Yeah, it did feel kind of odd I will admit, but I was worried if there are any
ramifications that I don't know about if I move it up early for all cases now
or if I should just put it back to how it was in v1...
> thanks,
> 
> greg k-h
>