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

From: nerdopolis
Date: Thu Sep 12 2024 - 09:10:09 EST

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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@xxxxxxxxxxx> 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 guess that it would register both consoles in this case.
>
> > 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();
>
> 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 ?
> Another idea:
>
> 1. We could use the same trick as netconsole. I mean to use:
>
> ttynull_console.flags |= CON_ENABLED;
>
> to force register_console() to register the console even
> when it is not defined in the list of preferred consoles.
>
> It is a kind of hack. But it looks cleaner that adding
> ttynull console into the list of preferred consoles.
>
>
> 2. We need to decide whether the fallback to ttynull console
> is needed as late as possible. It should be done after
> all other drivers call register_console().
>
> I would do it in late_initcall_sync().
>
>
> 3. We need to detect when the fallback is needed. The check
> of /dev/console does not work, see
> the commit a91bd6223ecd46addc71ee6f ("Revert "init/console: Use
> ttynull as a fallback when there is no console").
>
> A solution might be to check if @console_list is empty.
>
>
> Something like (not even compile tested):
>
> diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
> index 6b2f7208b564..7cd7ba2ec33c 100644
> --- a/drivers/tty/ttynull.c
> +++ b/drivers/tty/ttynull.c
> @@ -59,6 +59,16 @@ static struct console ttynull_console = {
> .device = ttynull_device,
> };
>
> +void __init register_ttynull_console_force(void)
> +{
> + if (!ttynull_driver)
> + return;
> +
> + /* Force registration by setting the CON_ENABLED flag. */
> + ttynull_console.flags |= CON_ENABLED;
> + register_console(&ttynull_console);
> +}
> +
> static int __init ttynull_init(void)
> {
> struct tty_driver *driver;
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 054c0e7784fd..004612e6fc7f 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3857,6 +3857,25 @@ static int __init printk_late_init(void)
> }
> late_initcall(printk_late_init);
>
> +static int __init console_fallback(void)
> +{
> + bool need_fallback = false;
> +
> + console_list_lock();
> + /*
> + * Make sure that there is a console which can be associated
> + * with /dev/console
> + */
> + if (hlist_empty(&console_list))
> + need_fallback = true;
> +
> + console_list_unlock();
> +
> + if (need_fallback)
> + register_ttynull_console_force();
> +}
> +late_initcall_sync(console_fallback);
> +
> #if defined CONFIG_PRINTK
> /* If @con is specified, only wait for that console. Otherwise wait for all. */
> static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progress)
>
>
>
> The above code would need some love to compile without
> CONFIG_NULL_TTY.
>
> Also it still might break some system/configuration where the default
> console driver calls register_console() later.
>
> We might need to update the check in register_console() and
> call try_enable_default_console() even when ttynull console
> is already registered as a fallback. We might even want
> to unregister the ttynull console in this case.
>
> Best Regards,
> Petr
>

Next message: Mario Limonciello: "Re: [PATCH] PCI/PM: Put devices to low power state on shutdown"
Previous message: kernel test robot: "drivers/pinctrl/pinctrl-at91-pio4.c:136: warning: Function parameter or member 'pm_suspend_backup.imr' not described in 'atmel_pioctrl'"
Next in thread: Petr Mladek: "Re: VT-less kernels, and /dev/console on x86"
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