linux-kernel - Re: [RFC][Patch] Allow not mounting a root fs

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Message-ID: <Pine.LNX.4.64.0707072335160.1536@fbirervta.pbzchgretzou.qr>
Date:	Sat, 7 Jul 2007 23:39:17 +0200 (CEST)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Bodo Eggert <7eggert@....de>
cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][Patch] Allow not mounting a root fs

On Jul 7 2007 23:18, Bodo Eggert wrote:
>> >> BTW: Is it possible to mount a tmpfs on / before extracting the cpio?
>> >
>> >Not in the stock kernel.  There have been some patches floating around
>> >for that, I think.
>> 
>> What would it buy? rootfs is a tmpfs, is not it?
>
>As far as I understand, it is a ramfs aka. non-swappable tmpfs without any 
>limit and including a chance of DoSing the system.
>
>Since I'm toying with root-on-initcpio, I'm looking for things that might 
>make the task some easier.
>
>Currently I'm thinking about changing the root=initramfs to root=tmpfs and
>additionally mounting a tmpfs on / before unpacking the cpios. OTOH, this 
>seems to be a little bit messy. OTOH again, in _that_ boot code, nobody
>should complain :)

Come to speak of it, I think you can have it much easier by having the kernel
exporting the cpio image as a virtual file inside rootfs, so that you could
re-extract it inside a tmpfs. In other words:

mount -t tmpfs tmpfs /mnt;
cd /mnt;
cpio -diuv </initramfs.cpio;
#
# switch_root nukes rootfs, pivot_roots and execs some init
klibc_switch_root .; # or something like that

Of course this needs double the memory than directly mounting tmpfs as 
rootfs, but it's an idea too.

	Jan
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