[RFC PATCH v2 0/4] init: boot image-based systems without an initramfs (rootimage=)

Changes since v1 (https://lore.kernel.org/all/[email protected]/),
following Christian Brauner's review
(https://lore.kernel.org/all/20260727-gepaukt-eislauf-waran-7c03f0e47609@brauner/):

The goal is unchanged: A/B image updates from a single partition,
without paying for a second userspace spin-up at boot. What changed is
how rootimage=/rootimageverity= get from "here is a path" to "here is
the verified, mounted root", which is what most of the v1 review was
about.

 - Patches 1-2 are new: they add generic, reusable VFS infrastructure
   (struct fs_context::source_file, path_mount_file()/
   init_mount_file(), vfs_parse_fs_param_file()) that lets an in-kernel
   caller hand a filesystem an already-open struct file as its mount
   source instead of a path, and convert erofs's existing file-backed
   mount support to use it when given. This is the "new infrastructure
   available to any fs" asked for, so that init/do_mounts.c never has
   to independently re-resolve a path that something else already
   opened and validated.

 - Patch 3 (rootimage=) now opens the image file exactly once and
   mounts that same struct file via the above, instead of opening it,
   doing nothing with the open, and separately handing erofs a path
   that it re-resolved on its own with no guaranteed relationship to
   what was checked - which is what v1 did, and which is where most of
   the "second lookup" / "weak assumptions" findings came from. There
   is no fd-vs-path race left because there is no second lookup left.

   rootimagesrcdir= is now mandatory (rootimagesrcdir=none opts out
   explicitly) instead of silently detaching the carrier by default,
   so there's no more zombie superblock/dangling mountpoint by
   default; you have to ask for that outcome by name.

 - Patch 4 (rootimageverity=) is mostly unchanged in what it checks,
   but now runs on the exact file patch 3 mounts, so the digest check
   and the mount are guaranteed to agree on what "the image" is. It
   also now refuses rootimageverity= together with rootimageflags=
   containing device= (multi-device erofs images), since the digest
   only ever covers the primary image file and silently ignoring the
   rest would be a false sense of integrity.

Not changed / not attempting to fix in this version, per the v1
discussion:

 - The carrier filesystem itself is still fully parsed (superblock,
   directory entries, extents) before rootimageverity= gets to check
   anything, since reaching the image file at all requires that. This
   is structural to a file-backed image sitting on a general-purpose
   writable filesystem, the same way a dm-verity root still needs a
   trusted block layer under it; rootimage=/rootimageverity= are meant
   to sit on top of an already-appropriately-trusted carrier (e.g.
   itself dm-verity/LUKS-backed, or a well-audited always-read-only
   fs), not conjure trust in an arbitrary writable one out of thin air.

 - A/B slot selection and fallback/rollback bookkeeping are still the
   bootloader's job, exactly as with dm-mod.create= today; this series
   only does the "resolve the one path the bootloader already chose,
   verify it, make it root" mechanical step for the file-backed case,
   not image-based deployment policy in general.

 - There is still no sysfs equivalent of
   /sys/block/loopX/loop/backing_file to discover which file backs the
   root after boot (the mandatory rootimagesrcdir= at least means the
   carrier itself stays reachable). This would need its own generic
   piece of infrastructure and is left for a follow-up.

Eric Curtin (4):
  fs: allow in-kernel mounters to hand filesystems an already-open
    source file
  erofs: use fs_context source_file for file-backed mounts when given
  init: support mounting the root filesystem from an image file
  init: support pinning the root image's fsverity digest

 .../admin-guide/kernel-parameters.txt         |  56 ++++
 Documentation/filesystems/mount_api.rst       |  18 ++
 fs/erofs/super.c                              |  19 ++
 fs/fs_context.c                               |  39 ++-
 fs/init.c                                     |  18 ++
 fs/internal.h                                 |   2 +
 fs/namespace.c                                |  96 ++++++-
 include/linux/fs_context.h                    |   4 +
 include/linux/init_syscalls.h                 |   2 +
 init/do_mounts.c                              | 269 +++++++++++++++++-
 10 files changed, 514 insertions(+), 9 deletions(-)

-- 
2.43.0