linux-kernel - Re: [RFC PATCH] rootfs: force mounting rootfs as tmpfs

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Date:   Thu, 01 Feb 2018 14:41:26 -0800
From:   Taras Kondratiuk <takondra@...co.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Rob Landley <rob@...dley.net>
Cc:     initramfs <initramfs@...r.kernel.org>,
        Victor Kamensky <kamensky@...co.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] rootfs: force mounting rootfs as tmpfs

Quoting Mimi Zohar (2018-02-01 13:51:52)
> On Thu, 2018-02-01 at 11:09 -0600, Rob Landley wrote:
> > On 02/01/2018 09:55 AM, Mimi Zohar wrote:
> > > On Thu, 2018-02-01 at 09:20 -0600, Rob Landley wrote:
> > > 
> > >>> With your patch and specifying "root=tmpfs", dracut is complaining:
> > >>>
> > >>> dracut: FATAL: Don't know how to handle 'root=tmpfs'
> > >>> dracut: refusing to continue
> > >>
> > >> [googles]... I do not understand why this package exists.
> > >>
> > >> If you're switching to another root filesystem, using a tool that
> > >> wikipedia[citation needed] says has no purpose but to switch to another
> > >> root filesystem, (so let's reproduce the kernel infrastructure in
> > >> userspace while leaving it the kernel too)... why do you need initramfs
> > >> to be tmpfs? You're using it for half a second, then discarding it,
> > >> what's the point of it being tmpfs?
> > > 
> > > Unlike the kernel image which is signed by the distros, the initramfs
> > > doesn't come signed, because it is built on the target system.  Even
> > > if the initramfs did come signed, it is beneficial to measure and
> > > appraise the individual files in the initramfs.
> > 
> > You can still shoot yourself in the foot with tmpfs. People mount a /run
> > and a /tmp and then as a normal user you can go
> > https://twitter.com/landley/status/959103235305951233 and maybe the
> > default should be a little more clever there...
> > 
> > I'll throw it on the todo heap. :)
> > 
> > >> Sigh. If people are ok with having rootfs just be tmpfs whenever tmpfs
> > >> is configured in, even when you're then going to overmount it with
> > >> something else like you're doing, let's just _remove_ the test. If it
> > >> can be tmpfs, have it be tmpfs.
> > > 
> > > Very much appreciated!
> > 
> > Not yet tested, but something like the attached? (Sorry for the
> > half-finished doc changes in there, I'm at work and have a 5 minute
> > break. I can test properly this evening if you don't get to it...)
> 
> Yes, rootfs is being mounted as tmpfs.

I don't think you can unconditionally replace ramfs with initramfs by
default. Their behavior is different in some cases (e.g. pivot_root vs
switch_root) and it can break many systems that expect ramfs by default.