On Sun, Feb 23, 2025 at 03:44:56PM -0500, adamsimonelli@xxxxxxxxx wrote: > From: Adam Simonelli <adamsimonelli@xxxxxxxxx> > > 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@xxxxxxxxx> > --- > 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. thanks, greg k-h
