Lennart Poettering
d83d804863
So far we already had the CopyFiles= option in systemd-repart drop-in
files, as a mechanism for populating freshly formatted file systems with
files and directories. This adds MakeDirectories= in similar style, and
creates simple directories as listed. The option is of course entirely
redundant, since the same can be done with CopyFiles= simply by copying
in a directory. It's kinda nice to encode the dirs to create directly in
the drop-in files however, instead of providing a directory subtree to
copy in somehere, to make the files more self-contained — since often
just creating dirs is entirely sufficient.
The main usecase for this are GPT OS images that carry only a /usr/
tree, and for which a root file system is only formatted on first boot
via repart. Without any additional CopyFiles=/MakeDirectories=
configuration these root file systems are entirely empty of course
initially. To mount in the /usr/ tree, a directory inode for /usr/ to
mount over needs to be created. systemd-nspawn will do so automatically
when booting up the image, as will the initrd during boot. However, this
requires the image to be writable – which is OK for npawn and
initrd-based boots, but there are plenty tools where read-only operation
is desirable after repart ran, before the image was booted for the first
time. Specifically, "systemd-dissect" opens the image in read-only to
inspect its contents, and this will only work of /usr/ can be properly
mounted. Moreover systemd-dissect --mount --read-only won't succeed
either if the fs is read-only.
Via MakeDirectories= we now provide a way that ensures that the image
can be mounted/inspected in a fully read-only way immediately after
systemd-repart completed. Specifically, let's consider a GPT disk image
shipping with a file usr/lib/repart.d/50-root.conf:
[Partition]
Type=root
Format=btrfs
MakeDirectories=/usr
MakeDirectories=/efi
With this in place systemd-repart will create a root partition when run,
and add /usr and /efi into it as directory inods. This ensures that the
whole image can then be mounted truly read-only anf /usr and /efi can be
overmounted by the /usr partition and the ESP.
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