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

[nerdopolis]

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 

Thanks