Re: VT-less kernels, and /dev/console on x86

[nerdopolis <bluescreen_avenger@verizon.net>]

On Monday, August 19, 2024 11:09:35 AM EDT Steven Rostedt wrote:
> On Sun, 18 Aug 2024 10:30:22 -0400
> nerdopolis <bluescreen_avenger@verizon.net> wrote:
> 
> > On Sunday, August 18, 2024 8:33:25 AM EDT nerdopolis wrote:
> > > On Sunday, August 18, 2024 1:12:14 AM EDT Greg KH wrote:  
> > > > On Sat, Aug 17, 2024 at 08:09:20PM -0400, nerdopolis wrote:  
> > > > > Hi
> > > > > 
> > > > > I originally brought this up on linux-serial, but I think it makes more sense
> > > > > that it's part of how printk console device selection works. Without VTs, while
> > > > > most software is able to handle the situation, some userspace programs expect
> > > > > /dev/console to still be responsive. Namely systemd. It calls isatty() against
> > > > > /dev/console, and since /dev/console on VT-less systems currently defaults to
> > > > > /dev/ttyS0, and when /dev/ttyS0 is disconnected, the ioctl's fail, and it
> > > > > refuses to write log messages to it.
> > > > > 
> > > > > There doesn't seem to be a mailing list for printk, so I had to use
> > > > > get_maintainer.pl. Hopefully this is correct
> > > > > 
> > > > > 
> > > > > After some grepping and guessing and testing, and playing around Something like
> > > > > diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
> > > > > index a45d423ad10f..f94a4632aab0 100644
> > > > > --- a/drivers/tty/Kconfig
> > > > > +++ b/drivers/tty/Kconfig
> > > > > @@ -384,9 +384,12 @@ config NULL_TTY
> > > > >  
> > > > >           In order to use this driver, you should redirect the console to this
> > > > >           TTY, or boot the kernel with console=ttynull.
> > > > > -
> > > > >           If unsure, say N.
> > > > >  
> > > > > +config NULL_TTY_CONSOLE
> > > > > +        bool "Supports /dev/ttynull as a console automatically"
> > > > > +        depends on NULL_TTY && !VT_CONSOLE
> > > > > +
> > > > >  config VCC
> > > > >         tristate "Sun Virtual Console Concentrator"
> > > > >         depends on SUN_LDOMS
> > > > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > > > > index dddb15f48d59..c1554a789de8 100644
> > > > > --- a/kernel/printk/printk.c
> > > > > +++ b/kernel/printk/printk.c
> > > > > @@ -3712,6 +3712,11 @@ void __init console_init(void)
> > > > >         initcall_t call;
> > > > >         initcall_entry_t *ce;
> > > > >  
> > > > > +#ifdef CONFIG_NULL_TTY_CONSOLE
> > > > > +       if (!strstr(boot_command_line, "console="))
> > > > > +               add_preferred_console("ttynull", 0, NULL);
> > > > > +#endif
> > > > > +
> > > > >         /* Setup the default TTY line discipline. */
> > > > >         n_tty_init();
> > > > >  
> > > > > 
> > > > > 
> > > > > 
> > > > > seems to work, it conflicts with CONFIG_VT_CONSOLE since it is effectively
> > > > > redundant, it is optional, so that it doesn't cause any changes to
> > > > > configurations, that historically had CONFIG_VT_CONSOLE turned off in the past,
> > > > > and for bootloader configs, it won't change any behavior if the kernel command
> > > > > line has a console device specified  
> > > > 
> > > > What is wrong with just setting the kernel command line for this
> > > > instead?
> > > >   
> > > When they eventually start shipping kernels without VTs, they will then have to
> > > include a script in their upgrade process that runs
> > > 
> > > sed -i "s/GRUB_CMDLINE_LINUX_DEFAULT=\"/GRUB_CMDLINE_LINUX_DEFAULT=\"nomodeset /g" /etc/default/grub  
> > Ugh, I meant
> > sed -i "s/GRUB_CMDLINE_LINUX_DEFAULT=\"/GRUB_CMDLINE_LINUX_DEFAULT=\"console=ttynull /g" /etc/default/grub
> > sorry
> 
> If you can modify the kernel .config for this, can you just update:
> 
>   CONFIG_CMDLINE_BOOL=y
>   CONFIG_CMDLINE="console=ttynull"
> 
> ?
> 
That could work, I think. I'll have to see how that works when a different
console= is specified on the command line from the bootloader though, I am
thinking that if console=ttyS0 is then manually specified by a user, there will
be two devices in /proc/consoles (ttyS0 on top of ttynull), but I admit I don't
know if there are actual ramifications of that, or not...


I am not sure if real distributions would want this to be the answer I guess I
will have to see if any others are using CONFIG_CMDLINE_BOOL/CONFIG_CMDLINE,
although this gives me an idea..

Would something like this below be more acceptable? I didn't test it yet, but
just the theory. I am thinking that this could have more use to allow a
preferred to be set...

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index dddb15f48d59..c1554a789de8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3712,6 +3712,11 @@ void __init console_init(void)
 	initcall_t call;
 	initcall_entry_t *ce;
 
+#ifdef CONFIG_DEFAULT_CONSOLE_HINT_BOOL
+       if (!strstr(boot_command_line, "console="))
+               add_preferred_console(CONFIG_DEFAULT_CONSOLE_HINT, 0, NULL);
+#endif
+
 	/* Setup the default TTY line discipline. */
 	n_tty_init();
 

> There's also bootconfig, that allows you to append command lines to the
> kernel image. See CONFIG_BOOT_CONFIG and Documentation/admin-guide/bootconfig.rst
> 
> -- Steve
>