linux-kernel - Re: VT-less kernels, and /dev/console on x86

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Message-ID: <ZuMkAiJz1YgdGpmJ@1wt.eu>
Date: Thu, 12 Sep 2024 19:25:22 +0200
From: Willy Tarreau <w@....eu>
To: nerdopolis <bluescreen_avenger@...izon.net>
Cc: linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg KH <gregkh@...uxfoundation.org>, john.ogness@...utronix.de,
        senozhatsky@...omium.org, tglx@...utronix.de, tony@...mide.com
Subject: Re: VT-less kernels, and /dev/console on x86

On Thu, Sep 12, 2024 at 12:48:01PM -0400, nerdopolis wrote:
> > > I still kind of lean to CONFIG_NULL_TTY_CONSOLE, that way if enabled, and in
> > > theory, only distributions that had CONFIG_VT_CONSOLE turned on would turn on
> > > this option. That could allow /dev/console will still work the same way for
> > > user space logging, while disabling vgacon and fbcon.
> > > 
> > > And it could still be overridden by console=ttyS0, which I think is needed
> > > anyway if you have CONFIG_VT_CONSOLE enabled
> > 
> > That sounds safer. And even then, I still don't understand why the application
> > logging to /dev/console needs to block on it instead of just dropping whatever
> > doesn't fit there since that's the primary intent of an optional logging
> > console, i.e. emit events but without preventing regular operations. Maybe
> > *this* is the thing that would require a setting: wait or drop.
> > 
> Sorry about the late reply, the application that is logging and dropping is
> kind of unintentional I think. From how I understand it, systemd wants to
> verify that /dev/console is actually /dev/console, so it calls isatty() on it.

OK.

> isatty() in turn calls the TCGETS ioctl on /dev/console, which when the console
> device is actually /dev/ttyS0, and /dev/ttyS0 is unplugged, it the ioctl fails,
> and isatty() returns false, and systemd assumes that it is not a serial device,
> and in turn it doesn't log because of that.

I'm no tty expert at all, but on this machine I'm having this:

  # dmesg|grep console
  [    0.000000] Kernel command line: boot_image=l0 console=ttyS0,115200 root=/dev/mmcblk0p2 rw rootwait
  [    1.638242] printk: console [ttyS0] disabled
  [    2.848759] printk: console [ttyS0] enabled
  [   12.950334] systemd[1]: Starting Set the console keyboard layout...

  # cat /proc/consoles 
  ttyS0                -W- (EC p a)    4:64

  # strace -e trace=ioctl stty -a < /dev/console 
  ioctl(0, TCGETS, {B115200 opost -isig -icanon -echo ...}) = 0
  ioctl(1, TIOCGWINSZ, {ws_row=30, ws_col=152, ws_xpixel=1534, ws_ypixel=604}) = 0
  ioctl(0, TIOCGWINSZ, {ws_row=24, ws_col=80, ws_xpixel=0, ws_ypixel=0}) = 0
  speed 115200 baud; rows 24; columns 80; line = 0;
  intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R;
  werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
  -parenb -parodd -cmspar cs8 hupcl -cstopb cread clocal -crtscts
  -ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel iutf8
  opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
  -isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt -echoctl -echoke -flusho -extproc
  +++ exited with 0 +++

So TCGETS succeeded above, and the console is unplugged. Even if I enable
HW flow control (crtscts) the result is the same BTW (I preferred to check).
Same if I enable carrier detection (-clocal).

So I'm unsure what would cause TCGETS to fail in your case.

Willy