/efi does not automount on Fedora 40 - various systemd-gpt-auto-generator issues

[marc-hb]

Summarizing a (minor?) problem discussed first in #74.

In some conditions, /efi does not automount. It does mount manually. Also, booting works both from the ESP partition and not (run_qemu.sh --no-direct-kernel and not). So this is considered a minor issue and will be closed as wontfix. Please re-open if you disagree.

On Fedora, /efi automounting is performed by these:

[root@fedora ~]# systemctl list-units --all | grep efi
  efi.automount                                loaded    active   running   EFI System Partition Automount
  efi.mount                                    loaded    active   mounted   EFI System Partition Automount

These are automatically generated at boot time. However, the auto detection has a number of apparently fragile requirements that are not always fullfilled. The complete list can be found in man systemd-gpt-auto-generator. You can test systemd-gpt-auto-generator on a running system like this:

mkdir temp
/usr/lib/systemd/system-generators/systemd-gpt-auto-generator temp temp temp
tree temp

temp
|-- -.mount.wants
|   `-- systemd-growfs-root.service -> /usr/lib/systemd/system/systemd-growfs-root.service
|-- efi.automount
|-- efi.mount
|-- local-fs.target.d
|   `-- 50-order-systemd-growfs-root.service.conf
`-- local-fs.target.wants
    `-- efi.automount -> ../efi.automount

Repeating temp 3 times means you want all 3 "things" to be generated in the same place, see man systemd.generator for more.

---

Fedora uses btrfs by default. That default used to be overridden and configured by run_qemu.sh to ext4 when using mkosi v14 but not anymore when upgrading to mkosi v15, see #74 why.

btrfs is simply INCOMPATIBLE with systemd-gpt-auto-generator, see systemd/systemd#16953 why.

Dec 10 09:31:43 fedora systemd-gpt-auto-generator[142]: File system behind root file system is reported by btrfs
    to be backed by pseudo-device /dev/root, which is not a valid userspace accessible device node. Cannot determine correct backing block device.
Dec 10 09:31:43 fedora (sd-exec-[136]: /usr/lib/systemd/system-generators/systemd-gpt-auto-generator
    failed with exit status 1.

Interestingly, this /dev/root / btrfs obfuscation problem disappears after boot. In other words, systemd-gpt-auto-generator is able to generate efi.*mount configuration files after boot. But it's too late now. Longer explanation in systemd/systemd#16953

---

Another thing I noticed: for systemd-gpt-auto-generator to consider /efi on my system, I MUST use --no-direct-kernel. In other words, I must actually use the ESP partition in order for systemd-gpt-auto-generator to recognize it. I don't know why. This happens with both mkosi14 and mkosi v22. It could be down to the kernel command line parameters? Which systemd-gpt-auto-generator definitely looks at - among many other things.

[marc-hb]

Adding to the confusion, I just discovered that the automount can be sometimes delayed:

$ mount | grep efi
efivarfs on

$ ls /efi
$ mount  | grep efi
efivars on ...
/dev/sda1 on /efi type vfat

The above was with mkosi v14, Fedora 40 and ext4 and booting the ESP with --no-direct-kernel.