On Friday 13 July 2007 2:56:00 pm Bodo Eggert wrote:
> I toyed with setting up a diskless system in initramfs. In the process, I
> came across some things:
>
> 1) There is no way to have the kernel not mount a filesystem,
> unless you use /init or rdinit=.
Er, yes. By design.
The kernel has to run an init program in order to hand off control to
userspace. In initramfs, this is defined as /init. It looks in exactly one
documented place.
The older root= mechanism fell back to a half-dozen places (eventually trying
things like /bin/sh if it couldn't find anything else). This is because
there wasn't a documented standard for what should be executed, like there is
with initramfs.
> 1a) In the process of writing these patches, I found prepare_namespace not
> to be called if /init is present. prepare_namespace will call
> security_sb_post_mountroot after mounting the root fs. I did not yet
> see a way to call this from /init, and grepping kinit for "security" did
> not help, too.
I don't use selinux. I'm neither a Fortune 500 company nor the NSA.
However, if you don't trust your own initramfs, where everything starts out
running as root, you have bigger problems.
> This is probably a bug, but using the features of this patchset, you'll
> avoid hitting it. Therefore this patchset does nothing about that.
>
> 2) If you want to use tmpfs, you need a script which essentially
> duplicates the work the kernel just did: Mount the root fs, unpack or move
> the files.
mkdir sub
mount -t tmpfs sub sub
cp -ax / sub
switch_root sub /init-stage-2
I haven't tried it but it really doesn't sound like brain surgery. If your
switch_root is statically linked, you can use mv instead of cp.
> Using tmpfs instead for the first root mount is as cheap as
> using ramfs, as long as tmpfs is used anyway (and most likely it is).
*shrug* I don't object to doing so, but I've heard nebulous technical
objections from people who know more about the implementation details fo
tmpfs than I do.
> 2a) I figured if you prepared the root fs to contain a running system, you
> woud probably also set up a runnable system on it.
*scratches head*
That's a tautology, right?
> Therefore I changed
> the default to boot from tmpfs if there was no /init nor a root=
> option. (If there is a /init, it will be executed as usural.)
I have no idea what you just said. If there's an /init, we boot from it. If
there's no /init, they just violated the spec and we don't know what to boot
from.
They made an initramfs. They constructed it, explicitly, for the Linux
kernel, and /init is what the kernel will try to boot. I documented this
years ago. If they chose not to put an /init, then they didn't want us to
boot from initramfs. (Maybe they're supplying firmware and an /sbin/hotplug
for a statically linked device. I dunno.)
> Unfortunately the way I do it, this will override the rdev setting, but
> that should be OK, since rdev is dead. Isn't it?
>
> 3) While I was at it, I figured I would not need most of the init/mount*
> code anymore. Therefore I made patch 3, which ifdefs it out as far as
> possible while still aiming for a small change.
>
> Patch 1 adds the capability to use root=rootfs.
I've been using initramfs since back around 2.6.12. It works fine. Why are
you patching the kernel for something that already works?
> Patch 2 adds the capability to use tmpfs for root, default root=rootfs.
> ramfs becomes optional if rootfs=tmpfs.
> Patch 3 allows to remove the capability of automounting filesystems.
>
> All patches appyl to 2.6.22.1
Rob
--
"One of my most productive days was throwing away 1000 lines of code."
- Ken Thompson.
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