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Date:   Wed, 26 May 2021 09:03:10 +0000
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     Menglong Dong <menglong8.dong@gmail.com>
Cc:     Josh Triplett <josh@joshtriplett.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        Sami Tolvanen <samitolvanen@google.com>, ojeda@kernel.org,
        johan@kernel.org, Bjorn Helgaas <bhelgaas@google.com>,
        masahiroy@kernel.org, Menglong Dong <dong.menglong@zte.com.cn>,
        joe@perches.com, Jens Axboe <axboe@kernel.dk>, hare@suse.de,
        Jan Kara <jack@suse.cz>, tj@kernel.org,
        gregkh@linuxfoundation.org, song@kernel.org,
        NeilBrown <neilb@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        f.fainelli@gmail.com, arnd@arndb.de,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        wangkefeng.wang@huawei.com, Barret Rhoden <brho@google.com>,
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        mingo@kernel.org, linux-fsdevel@vger.kernel.org,
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Subject: Re: [PATCH v2 2/3] init/do_cmounts.c: introduce 'user_root' for
 initramfs
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On Wed, May 26, 2021 at 04:33:00PM +0800, Menglong Dong wrote:
> On Wed, May 26, 2021 at 12:33 PM Josh Triplett <josh@joshtriplett.org> wrote:
> >
> > On Tue, May 25, 2021 at 10:23:09PM -0500, Eric W. Biederman wrote:
> > > If we are going to do this something that is so small and clean it can
> > > be done unconditionally always.
> > [...]
> > > The net request as I understand it: Make the filesystem the initramfs
> > > lives in be an ordinary filesystem so it can just be used as the systems
> > > primary filesystem.
> >
> > Including the ability to pivot_root it away, which seems like the main
> > sticking point.
> >
> > If this can be done without any overhead, that seems fine, but if this
> > involves mounting an extra filesystem, that may add an appreciable
> > amount of boot time for systems trying to boot in milliseconds. (Such
> > systems would not use an initramfs if they're going to go on and boot a
> > separate root filesystem, but they can use an initramfs as their *only*
> > filesystem.)
> 
> Compared to the time the unpacking spent, a mounting seems nothing. In the
> scene above, this change can be disabled by kconfig, if pivot_root
> is not needed in initramfs.

I asked for the kconfig entry. And it would be good to document then
also the worst case expected on boot for what this could do to you. I
mean, we are opening a different evil universe. So that's why the
kconfig exists.  How bad and evil can this be?

I don't think anyone has clarified that yet.

  Luis