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Subject: Re: [RFC PATCH] rootfs: force mounting rootfs as tmpfs
From:   Mimi Zohar <zohar@linux.vnet.ibm.com>
To:     Rob Landley <rob@landley.net>,
        Taras Kondratiuk <takondra@cisco.com>,
        Arvind Sankar <nivedita@alum.mit.edu>
Cc:     initramfs <initramfs@vger.kernel.org>,
        Victor Kamensky <kamensky@cisco.com>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Al Viro <viro@ZenIV.linux.org.uk>,
        linux-kernel <linux-kernel@vger.kernel.org>
Date:   Mon, 16 Apr 2018 11:46:05 -0400
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Hi Rob,

On Thu, 2018-02-01 at 17:34 -0600, Rob Landley wrote:
> 
> On 02/01/2018 04:41 PM, Taras Kondratiuk wrote:
> > 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)
> 
> Both are switch_root, you can't pivot_root off of either one. (Yes, I
> hit that bug and reported it, and they fixed it, back in the day...
> http://lists.busybox.net/pipermail/busybox/2006-March/053529.html )
> 
> > and it can break many systems that expect ramfs by default.
> 
> The use case I told Mimi about off-list (since they stopped cc:ing the
> list in one of their replies but the conversation continued) was the guy
> who was extracting an initramfs bigger than 50% of system memory, which
> worked with initramfs but failed with initmpfs. A quick google didn't
> find the original message but it resulted in this blog entry from the
> affected party:
> 
> http://www.lightofdawn.org/blog/?viewDetailed=00128
> 
> I.E. yeah, I know, I need to redo these patches tonight.

I'd really like to be able to have rootfs be a tmpfs filesystem.  Any
time estimate on this patch?

thanks!

Mimi