linux-kernel - Re: [PATCH v6 2/2] init/do_mounts.c: create second mount for initramfs

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Date:   Wed, 28 Jul 2021 10:07:40 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Menglong Dong <menglong8.dong@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>, johan@...nel.org,
        ojeda@...nel.org, jeyu@...nel.org, masahiroy@...nel.org,
        joe@...ches.com, Jan Kara <jack@...e.cz>, hare@...e.de,
        Jens Axboe <axboe@...nel.dk>, tj@...nel.org,
        gregkh@...uxfoundation.org, song@...nel.org,
        NeilBrown <neilb@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Barret Rhoden <brho@...gle.com>, f.fainelli@...il.com,
        palmerdabbelt@...gle.com, wangkefeng.wang@...wei.com,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, vbabka@...e.cz,
        Alexander Potapenko <glider@...gle.com>,
        johannes.berg@...el.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>, jojing64@...il.com,
        terrelln@...com, geert@...ux-m68k.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>, arnd@...db.de,
        Chris Down <chris@...isdown.name>, mingo@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH v6 2/2] init/do_mounts.c: create second mount for
 initramfs

On Tue 2021-07-27 14:37:01, Christian Brauner wrote:
> On Tue, Jul 27, 2021 at 08:24:03PM +0800, Menglong Dong wrote:
> > Hello Christian,
> > 
> > On Thu, Jun 17, 2021 at 10:38 PM Christian Brauner
> > <christian.brauner@...ntu.com> wrote:
> > >
> > [...]
> > >
> > > Hey Menglong,
> > >
> > > Since we're very close to the next kernel release it's unlikely that
> > > anything will happen before the merge window has closed.
> > > Otherwise I think we're close. I haven't had the time to test yet but if
> > > nothing major comes up I'll pick it up and route it through my tree.
> > > We need to be sure there's no regressions for anyone using this.
> > >
> > 
> > Seems that it has been a month, and is it ok to move a little
> > further? (knock-knock :/)
> 
> Yep, sorry.
> When I tested this early during the merge window it regressed booting a
> regular system for me meaning if I compiled a kernel with this feature
> enabled it complained about not being being able to open an initial
> console and it dropped me right into initramfs instead of successfully
> booting. I haven't looked into what this is caused or how to fix it for
> lack of time.

I guess that you have seen the following message printed by
console_on_rootfs():

      "Warning: unable to open an initial console."

This function is responsible for opening stdin, stdout, stderr
file to be used by the init process.

I am not sure how this is supposed to work with the pivot_root
and initramfs.


Some more details:

console_on_rootfs() tries to open /dev/console. It is created
by tty_init(). The open() callback calls:

  + tty_kopen()
    + tty_lookup_driver()
      + console_device()

, where console_device() iterates over all registered consoles
  and returns the first with tty binding.

There is ttynull_console that might be used as a fallback. But I
am not sure if this is what you want.

Best Regards,
Petr