linux-kernel - Re: When /dev/console is a disconnected /dev/ttyS0, TCGETS on /dev/console results in EIO (Even if TIOCCONS is called on a PTY)

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Message-ID: <2024071238-underarm-impulsive-dc1b@gregkh>
Date: Fri, 12 Jul 2024 14:59:58 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: nerdopolis <bluescreen_avenger@...izon.net>
Cc: jirislaby@...nel.org, linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: When /dev/console is a disconnected /dev/ttyS0, TCGETS on
 /dev/console results in EIO (Even if TIOCCONS is called on a PTY)

On Fri, Jul 12, 2024 at 08:52:15AM -0400, nerdopolis wrote:
> Hi
> 
> Background:--------------------------------------------------------------------
> This issue becomes evident on VT-less kernels. As when there is no /dev/tty0
> device, the console defaults to being /dev/ttyS0 instead. Although this can
> also be replicated if booting a standard kernel with 'console=ttyS0' and ensure
> nothing is plugged into /dev/ttyS0.
> 
> This issue prevents systemd from logging to the console.
> systemd when logging to /dev/console, long story short it calls isatty() on
> /dev/console, and when /dev/console is actually /dev/ttyS0, and nothing is
> connected to /dev/ttyS0, isatty() fails on /dev/console due to an input/output
> error, causing systemd to not log the console output, because it rejects
> /dev/console as not being a terminal.
> 
> 
> This is noticed on a VT-less system with Plymouth. Plymouth calls the TIOCCONS
> ioctl on a pty device it requests, to redirect console output, and in newer
> versions, it displays the console logs on its own without the assistance of a
> VT.
> 
> This part of it works, Plymouth is able to 'see' what gets written to
> /dev/console, log output from processes that write to /dev/console directly
> (for example 'echo hi > /dev/console") do appear in plymouth's
> /var/log/boot.log, it is just that systemd is not writing to /dev/console
> because isatty() fails to report /dev/console as a tty device.
> 
> The alternate fix in for systemds https://github.com/systemd/systemd/pull/33690[1]
> is believed to be that when TIOCCONS is called on a PTY, or another terminal
> device, that trying to call TCGETS on /dev/console should no longer result
> in an error.
> 
> 
> Replicating the issue:---------------------------------------------------------
> 
> This program replicates it:
> -------------------------------------------------------------------------------
> #include <stdio.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <errno.h>
> #include <string.h>
> 
> int main(void)
> {
>         int fd;
> 
>         if (getuid() != 0) {
>                 printf("Must be root\n");
>                 return 1;
>         }
> 
>         fd = open ("/dev/console", O_RDONLY);
>         if (!isatty(fd)) {
>                 printf("err on /dev/console: %s\n", strerror(errno));
>         }
>         return 0;
> }
> -------------------------------------------------------------------------------
> 
> When the kernel console is /dev/ttyS0 and /dev/ttySO has no device connected,
> it prints "err on /dev/console: Input/output error"
> 
> When I strace it, the relevant line is:
> ioctl(3</dev/console<char 5:1>>, TCGETS, 0x7f...) = -1 EIO (Input/output error)

Do you have a proposed kernel change for this that solves this for your
tests here?

thanks,

greg k-h