Re: [PATCH v5 0/3] init: add support to directly boot to a mapped device

From: Mike Snitzer
Date: Fri Feb 26 2016 - 14:21:54 EST

On Fri, Feb 26 2016 at 1:52pm -0500,
Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> On Fri, Feb 26, 2016 at 8:53 AM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> > On Mon, Feb 22 2016 at 1:55pm -0500,
> > Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> >> On Sun, Feb 21, 2016 at 2:08 PM, Alasdair G Kergon <agk@xxxxxxxxxx> wrote:
> >> > On Sat, Feb 20, 2016 at 10:13:49AM -0800, Kees Cook wrote:
> >> >> This is a resurrection of a patch series from a few years back, first
> >> >> brought to the dm maintainers in 2010. It creates a way to define dm
> >> >> devices on the kernel command line for systems that do not use an
> >> >> initramfs, or otherwise need a dm running before init starts.
> >> >>
> >> >> This has been used by Chrome OS for several years, and now by Brillo
> >> >> (and likely Android soon).
> >> >>
> >> >> The last version was v4:
> >> >>
> >> >>
> >> >
> >> > Inconsistencies in the terminology here can be sorted out during review,
> >> > and I see that you've taken on board some of my review comments from
> >> > 2010, but what are your responses to the rest of them?
> >>
> >> Ah, sorry, the threads I could find were incomplete, so I wasn't able
> >> to find those comments that were made to Will's 2010 submission. In
> >> some of the cleanups I did I was very confused about "target" vs
> >> "table", and tried to fix that. Regardless, I'm open to fixing
> >> whatever is needed. :)
> >>
> >> Thanks for looking at this again!
> >
> > This work isn't going to fly as is. I appreciate the effort and the
> > goal (without understanding _why_) but: you're open-coding, duplicating
> > and/or reinventing way too much in do_mounts_dm.c
> >
> > 1) You first need to answer: _why_ is using a proper initramfs not
> > viable? A very simple initramfs that issues dmsetup commands, etc,
> > isn't so daunting is it? Why is it so important for the kernel to
> > natively provide a dmsetup interface? Chrome, Android, etc cannot use
> > initramfs?
> That is correct: Chrome OS does not (and won't) use an initramfs. This
> is mainly for reasons of boot speed, verified boot block size, and
> maybe some other things I don't remember.

Not sure what "verified boot block size" means but...

Sorry I really don't buy that using a custom initramfs would be the
source of slow boot. initramfs is _not_ this hugely inefficient
mechanism you'd have us believe.

And if that is the justification for this early boot dm= support then
the Chrome OS project/team will have to continue to carry the hack
locally. It has no place upstream. But I'm open to revisiting this if
it can be implemented in a very cheap way.

> > 2) If you are able to adequately justify the need for dm=:
> > I'd much rather the dm= kernel commandline be a simple series of
> > comma-delimited dmsetup-like commands.
> >
> > You'd handle each command with extremely basic parsing:
> > <dm_ioctl_cmd> <args> [, <dm_ioctl_cmd> <args>]
> > (inventing a special token to denote <newline>, to support tables with
> > multiple entries, rather than relying on commas and counts, etc)
> Sure, changing the syntax is fine by me. We'd need to plumb access to
> the ioctl interface, though.

I was hoping to avoid any extra hacks but yes... seems you'd need a new
API to issue the equivalent of a DM ioctl programatically. Hopefully
it'd be quite a small wrapper.

> > and you'd then have do_mounts_dm.c open /dev/mapper/control directly and
> > issue proper DM ioctls rather than adding all your shim code. This last
> > bit of opening /dev/mapper/control from init needs more research -- not
> > sure if doing such a thing from kernel is viable/safe/acceptable.
> Well, there's no /dev and no init since our dm is the root device
> (dm-verity). We need everything up and running before we mount the
> root filesystem, very similar to do_mount_md.c's purpose.

Ah yes, microoptimization associated with no udev or normal Linux boot
comes full circle and limits the use of existing standard interfaces.