linux-kernel - [Resend/Correction] 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: <3245549.5fSG56mABF@nerdopolis2>
Date: Fri, 12 Jul 2024 13:15:48 -0400
From: nerdopolis <bluescreen_avenger@...izon.net>
To: gregkh@...uxfoundation.org, jirislaby@...nel.org,
 linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org
Subject: [Resend/Correction] When /dev/console is a disconnected /dev/ttyS0,
 TCGETS on /dev/console results in EIO (Even if TIOCCONS is called on a PTY)

Hi

I am resending as I realized I sent in Rich Text instead of Plain Text.
I am sorry if any of you got this duplicate email

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
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)

I have not been able to come up with a proposed fix

Thanks