In preparation for reusing the image dissector in the GPT auto-discovery
logic, only optionally fail the dissection when we can't identify a root
partition.
In the GPT auto-discovery we are completely fine with any kind of root,
given that we run when it is already mounted and all we do is find some
additional auxiliary partitions on the same disk.
This is useful for reusing the dissector logic in the gpt-auto-discovery logic:
there we really don't want to use MBR or naked file systems as root device.
This adds support for discovering and making use of properly tagged dm-verity
data integrity partitions. This extends both systemd-nspawn and systemd-dissect
with a new --root-hash= switch that takes the root hash to use for the root
partition, and is otherwise fully automatic.
Verity partitions are discovered automatically by GPT table type UUIDs, as
listed in
https://www.freedesktop.org/wiki/Specifications/DiscoverablePartitionsSpec/
(which I updated prior to this change, to include new UUIDs for this purpose.
mkosi with https://github.com/systemd/mkosi/pull/39 applied may generate images
that carry the necessary integrity data. With that PR and this commit, the
following simply lines suffice to boot up an integrity-protected container image:
```
# mkdir test
# cd test
# mkosi --verity
# systemd-nspawn -i ./image.raw -bn
```
Note that mkosi writes the image file to "image.raw" next to a a file
"image.roothash" that contains the root hash. systemd-nspawn will look for that
file and use it if it exists, in case --root-hash= is not specified explicitly.
This adds two new APIs to systemd:
- loop-util.h is a simple internal API for allocating, setting up and releasing
loopback block devices.
- dissect-image.h is an internal API for taking apart disk images and figuring
out what the purpose of each partition is.
Both APIs are basically refactored versions of similar code in nspawn. This
rework should permit us to reuse this in other places than just nspawn in the
future. Specifically: to implement RootImage= in the service image, similar to
RootDirectory=, but operating on a disk image; to unify the gpt-auto-discovery
generator code with the discovery logic in nspawn; to add new API to machined
for determining the OS version of a disk image (i.e. not just running
containers). This PR does not make any such changes however, it just provides
the new reworked API.
The reworked code is also slightly more powerful than the nspawn original one.
When pointing it to an image or block device with a naked file system (i.e. no
partition table) it will simply make it the root device.
