Re: [PATCH v6 3/3] tty: Change order of ttynull to be linked sooner if enabled as a console.

[Adam Simonelli]

On Tuesday, March 4, 2025 1:51:52 AM EST Andy Shevchenko wrote:
> On Tue, Mar 4, 2025 at 5:55 AM <adamsimonelli@xxxxxxxxx> wrote:
> >
> > From: Adam Simonelli <adamsimonelli@xxxxxxxxx>
> >
> > 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
> 
> Missing period at the end.
> 
> > 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.
> 
> ...
> 
> >  obj-y                          += vt/
> 
> + blank line.
> 
> > +# 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
> 
> Here is the question: are you sure that all console drivers that exist
> in the kernel happen to be here? Have you grepped the source tree for
> checking this?
> 
Grepping for console_initcall, the only other places I see outside of
drivers/tty/ is 

arch/mips/fw/arc/arc_con.c
arch/mips/sibyte/common/cfe_console.c
arch/powerpc/kernel/legacy_serial.c
arch/powerpc/kernel/udbg.c
arch/powerpc/platforms/powermac/setup.c
arch/um/drivers/stderr_console.c
arch/xtensa/platforms/iss/console.c
drivers/s390/char/con3215.c
drivers/s390/char/con3270.c
drivers/s390/char/sclp_con.c
drivers/s390/char/sclp_vt220.c

> ...
> 
> > +# If ttynull is enabled, but not as a boot console, it is linked and used later
> > +# after the real ones.
> > +ifneq ($(CONFIG_NULL_TTY_DEFAULT_CONSOLE),y)
> 
> Also can be written as
> ifeq ($(...),)
> but it might be less explicit. Up to you.
> 
> >  obj-$(CONFIG_NULL_TTY)         += ttynull.o
> > +endif
> 
>