Re: revocation of /dev/console at the end of rc sequence

On Wed, Mar 26, 2025 at 10:23 AM Andriy Gapon <avg@freebsd.org> wrote:

>
> Not sure what would be the best forum to talk about this issue, so posting
> here.
>
> I observe a kind of unexpected problem related to /dev/console.
> Here is my understanding of what is happening.
>
> Thanks to DTrace I was able to capture /dev/console getting "revoked", its
> vnode
> getting recycled (at least, destroyed):
>
>    1  15949                     vgonel:entry
> 2025 Mar 25 21:45:33 vgone vnode fffff8000607b540, console rdev by sh (21)
>
>                kernel`vgone+0x2f
>                kernel`devfs_revoke+0x41
>                kernel`VOP_REVOKE_APV+0x71
>                kernel`killjobc+0x15a
>                kernel`exit1+0x707
>                kernel`0xffffffff808bd00d
>                kernel`amd64_syscall+0x189
>                kernel`0xffffffff80c4fbfb
>
> My understanding is that this is the rc shell process exiting.
>
> init forks a process that exec-s bin/sh (or alternative init shell) to run
> /etc/rc (or alternative init script).
> The process sets up itself as a session leader and /dev/console as its
> controlling terminal (as well as stdin, stdout and stderr).
> It does that by calling login_tty() which internally uses setsid() and
> tcsetsid() to do the job.
> See runcom -> run_script -> execute_script -> open_console in
> sbin/init/init.c.
>
> I guess that this makes total sense given the nature of /etc/rc.
>
> When a session leader process exits, it revokes its controlling terminal.
> That's the standard behavior.
> I guess that the idea is that any descendant processes in the session get
> disassociated from the terminal.
>
> Both things make sense, but they lead to a problem as well.
> The problem is that by the time /etc/rc finishes there can be a daemon
> (so, not
> a member of the session) that opened /dev/console for its own needs.
> The daemon would then lose its /dev/console access which it may not expect.
>
> One such daemon is console-kit-daemon.
> Here is how /dev/console descriptor looks afterwards (as reported by
> procstat -f):
>    PID COMM                FD T V FLAGS    REF  OFFSET PRO NAME
>   3323 console-kit-daemon    10 v x r-------  10       0 -   -
>
> More details here:
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=285394
> (Note that some earlier comments have incorrect information and
> assumptions).
>
> Can we do something better here given the special nature of /dev/console?
> May something in FreeSBD?
>
> For console-kit-daemon I have an idea of using /dev/ttyv0 instead of
> /dev/console.  The daemon needs the device for things like observing VT
> switching and similar operations
>

So what is it using /dev/console for? I think the answer to this question
will help
understand the right solution. What does console-kit think it can do with
/dev/console?
/dev/console, even if we didn't do the revoke, seems like the wrong thing
for it
to be opening.

Seems like
     TIOCCONS int *on
                 If on points to a non-zero integer, redirect kernel console
                 output (kernel printf's) to this terminal.  If on points
to a
                 zero integer, redirect kernel console output back to the
                 normal console.  This is usually used on workstations to
                 redirect kernel messages to a particular window.
is what it should be doing.
Is the standard way to redirect / scrape the console.

Also, ttyv0 is always wrong to hard-code. It might have nothing at all
to do with the console.

There's a couple of different notions of "the console" today, and they
are a bit poorly defined. There's the "console" for the kernel. And it's
really
a collection of devices that get kernel output and are prompted for kernel
input.

There's the "console" used for /etc/rc. This is /dev/console. It's just an
alias for the first tty device in the kernel console list (sometimes called
the
primary console). there's little special beyond that, though Kyle and I
keep threatening to make it a mux of all the kernel consoles on that list.
It's functionality is somewhat limited (and has been since phk separated
it out from the tty that underlies it around FreeBSD 2 or 3).

So what is it that console-kit-daemon is wanting to do?

Warner