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

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Feb 22 2016 at  1:55pm -0500,
Kees Cook <keescook@chromium.org> wrote:

> On Sun, Feb 21, 2016 at 2:08 PM, Alasdair G Kergon <agk@redhat.com> 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:
> >> https://patchwork.kernel.org/patch/104860/
> >> https://patchwork.kernel.org/patch/104861/
> >
> > 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?

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)

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.

Mike