[PATCH 0/7] ZFS/other CoW FS save_env support

[Paul Dagnelie]

Hey all, I previously discussed my concept for this patch in my email
https://lists.gnu.org/archive/html/grub-devel/2020-01/msg00004.html .
I'm pleased to announce that I've gotten it into a working state and
it is ready to review.  There are a number of changes here, which I
will break down below.

First, the concept of "special files" is introduced into grub. These
are files that are manipulated using different functions from the
remainder of the filesystem. A filesystem advertises its support for
these files by filling in new entries in the grub_fs struct.

Second, the loadenv command was modified to use the special file
functions of the root filesystem if they are detected. If that
happens, these functions are used to open, read, and write to the
loadenv file instead of the normal grub file functions.  A variable
was also added that allows the user to force a file to be used instead
of the special files, or vice versa.

Third, the grub-editenv command was modified to teach it to probe the
root filesystem and then check in an array of structures that contain
functions that will read and modify the filesystem's special file in
userland. These functions are very similar to normal read and write
functions, and so don't result in a very different code flow.

Finally, the GRUB ZFS logic was modified to have new special_file
functions. These functions manipulate the pad2 area of the ZFS label,
which was previously unused. It now stores a version number, the raw
contents of the grubenv file, and an embedded checksum for integrity
purposes. GRUB was taught to read and verify these areas, and also to
modify them, computing the embeddded checksum appropriately.  In
addition, if GRUB is build with libzfs support, an entry is added to
the grub-editenv table that points GRUB to the appropriate libzfs
functions, and init and fini functions are built into grub-editenv
itself.

Future work:
* In the ZFS code could store a packed nvlist instead of a raw file,
but this would involve further changes to the grub environment file
code and was deferred for when it would be more useful and important.
* For the special files code, there is only one special file allowed
per filesystem, but a specification interface (using either an enum or
a set of names) could be added in the future.
* Other filesystem support was not built into this change, but
extensibility was a primary goal, so adding support for btrfs or other
filesystems should be relatively straightforward.
* I did not implement the proposed UEFI variable support, but I did
develop this with that future in mind, so adding it should be a
relatively simple extension of these changes.

This patchset has been tested on systems with and without ZFS as the boot
filesystem, and built with and without ZFS libraries installed. It
worked in each case I tried, including with manual corruption applied
to the ZFS labels to force GRUB to read from the other label.  This
was tested in conjunction with
https://github.com/zfsonlinux/zfs/pull/10009 , the ZoL patch that
enables ZFS to read from and write to the padding areas we reserved
for the data.

Paul Dagnelie (7):
  Factor out file filter function for reuse
  Add support for special envblk functions in GRUB
  Factor out envblk buffer creation for reuse
  Use envblk file and fs functions to implement reading the grubenv file
    from special FS regions
  Add ZFS envblock functions
  Refactor default envblk size out for reuse
  Update editenv to support editing zfs envblock, fix whitespace, and
    autodetect bootenv support in libzfs

 configure.ac                 |   8 +
 grub-core/commands/loadenv.c | 122 +++++++++++++--
 grub-core/fs/zfs/zfs.c       | 134 +++++++++++++++--
 grub-core/kern/file.c        |  76 +++++++---
 include/grub/file.h          |  10 ++
 include/grub/fs.h            |  12 ++
 include/grub/lib/envblk.h    |   3 +
 include/grub/util/install.h  |   3 +
 include/grub/util/libzfs.h   |   7 +
 include/grub/zfs/vdev_impl.h |  33 ++---
 util/editenv.c               |  31 ++--
 util/grub-editenv.c          | 278 +++++++++++++++++++++++++++++++++--
 12 files changed, 632 insertions(+), 85 deletions(-)

-- 
2.19.0