Re: ARC no console output (was Re: [PATCH 1/2] init/console: Use ttynull as a fallback when there is no console)

[Richard Weinberger]

[CC'in linux-um since there is a similar issue]




On Thu, Jan 7, 2021 at 12:38 AM Vineet Gupta <vgupta@xxxxxxxxxxxx> wrote:
>
> +CC Buildroot folks
>
> Hi Petr,
>
> On 11/11/20 5:54 AM, Petr Mladek wrote:
> > stdin, stdout, and stderr standard I/O stream are created for the init
> > process. They are not available when there is no console registered
> > for /dev/console. It might lead to a crash when the init process
> > tries to use them, see the commit 48021f98130880dd742 ("printk: handle
> > blank console arguments passed in.").
> >
> > Normally, ttySX and ttyX consoles are used as a fallback when no consoles
> > are defined via the command line, device tree, or SPCR. But there
> > will be no console registered when an invalid console name is configured
> > or when the configured consoles do not exist on the system.
> >
> > Users even try to avoid the console intentionally, for example,
> > by using console="" or console=null. It is used on production
> > systems where the serial port or terminal are not visible to
> > users. Pushing messages to these consoles would just unnecessary
> > slowdown the system.
> >
> > Make sure that stdin, stdout, stderr, and /dev/console are always
> > available by a fallback to the existing ttynull driver. It has
> > been implemented for exactly this purpose but it was used only
> > when explicitly configured.
> >
> > Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
>
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -1470,8 +1470,14 @@ void __init console_on_rootfs(void)
> >       struct file *file = filp_open("/dev/console", O_RDWR, 0);
> >
> >       if (IS_ERR(file)) {
> > -             pr_err("Warning: unable to open an initial console.\n");
> > -             return;
> > +             pr_err("Warning: unable to open an initial console. Fallback to ttynull.\n");
> > +             register_ttynull_console();
> > +
> > +             file = filp_open("/dev/console", O_RDWR, 0);
> > +             if (IS_ERR(file)) {
> > +                     pr_err("Warning: Failed to add ttynull console. No stdin, stdout, and stderr for the init process!\n");
> > +                     return;
> > +             }
>
>
> This breaks ARC booting (no output on console).
>
> Our Buildroot based setup has dynamic /dev where /dev/console doesn't
> exist statically and there's a primoridla /init shell script which does
> following
>
> /bin/mount -t devtmpfs devtmpfs /dev
> exec 0</dev/console
> exec 1>/dev/console
> exec 2>/dev/console
> exec /sbin/init "$@"
>
> Buildroot has had this way of handling missing /dev/console since 2011
> [1] and [2].
>
> Please advise what needs to be done to unbork boot. Otherwise this seems
> like a kernel change which breaks user-space and needs to be backed-out
> (or perhaps conditionalize on CONFIG_NULL_TTY. I'm surprised it hasn't
> been reported by any other  embedded folks
>
> Thx,
> -Vineet
>
> [1] http://lists.busybox.net/pipermail/buildroot/2011-July/044505.html
> [2] http://lists.busybox.net/pipermail/buildroot/2011-August/044832.html



--
Thanks,
//richard