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

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Message-ID: <ZuLsN7yviWQPFUvf@pathway.suse.cz>
Date: Thu, 12 Sep 2024 15:27:19 +0200
From: Petr Mladek <pmladek@...e.com>
To: nerdopolis <bluescreen_avenger@...izon.net>
Cc: Steven Rostedt <rostedt@...dmis.org>,
	Greg KH <gregkh@...uxfoundation.org>, john.ogness@...utronix.de,
	senozhatsky@...omium.org, tglx@...utronix.de, tony@...mide.com,
	linux-kernel@...r.kernel.org
Subject: Re: VT-less kernels, and /dev/console on x86

On Thu 2024-09-12 08:29:26, nerdopolis wrote:
> On Tuesday, August 20, 2024 9:29:36 AM EDT Petr Mladek wrote:
> > On Mon 2024-08-19 11:50:39, nerdopolis wrote:
> > > 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@...izon.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:  
> > > > > > > > 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.
> > > > > > > > 
> > > 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();
> > 
> > This is better. But it does not handle some situations. For example,
> > default console might also by defined by:
> > 
> >    + scpr, see acpi_parse_spcr()
> >    + device tree, see of_console_check()
> >    + netconsole=, it is hidden in init_netconsole()
> > 
> > I tried to handle this another way. The "ttynull" console was
> > added when /dev/console could not be opened in console_on_rootfs(),
> > see the commit 757055ae8dedf5333af17b ("init/console: Use ttynull
> > as a fallback when there is no console").
> > 
> > But it did not work well and we had to revert the change, see
> > the commit a91bd6223ecd46addc71ee6f ("Revert "init/console: Use
> > ttynull as a fallback when there is no console").
> > 
> Out of curiosity, would the upcoming nbcon work fix the race conditions or
> problems that were seen in 757055ae8dedf5333af17b ?

Not really. The nbcon work affects the context and locking used for
flushing of the consoles. It does _not_ affect the registration. And
it does _not_ affect how the consoles are associated with /dev/console.

Best Regards,
Petr