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: <2169369.OBFZWjSADL@nerdopolis2>
Date: Wed, 24 Jul 2024 23:17:54 -0400
From: nerdopolis <bluescreen_avenger@...izon.net>
To: Greg KH <gregkh@...uxfoundation.org>
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 Thursday, July 18, 2024 7:52:15 AM EDT nerdopolis wrote:
> On Friday, July 12, 2024 8:59:58 AM EDT Greg KH wrote:
> > 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
> > 
> Hi
> 
> Sorry if this is a duplicate, I accidentally had rich text sending turned on,
> and did not realize until too late. Anyway, to answer the question, unfortunately
> I have been unable to come up with a fix on my own 
> 
Would the fix be to make /sys/class/tty/console/active report the terminal
that TIOCCONS was called against? I am probably wrong there.
> Thanks
>