bootc install to-filesystem uses the wrong efi partition

[hecknt]

I'm running bootc install to-filesystem /mnt. I am doing this with the composefs backend, but the same issue occurs on both composefs and ostree. The mount structure I have is as follows:

• /dev/sda4 (btrfs): /mnt
• /dev/sda3 (vfat): /mnt/boot/efi

My partitions overall look like this:

• /dev/sda1: vfat, type EFI System
• /dev/sda2: btrfs, type Linux filesystem
• /dev/sda3: vfat, type EFI System
• /dev/sda4: btrfs, type Linux filesystem

When I run the bootc install to-filesystem command, it completes successfully, but it doesn't populate /mnt/boot/efi at all (/dev/sda3). That's because it's actually populating /dev/sda1, which is a completely different EFI partition. This means that it is creating boot entries and installing systemd-boot to a partition that is not mounted under the directory that i invoked bootc install to-filesystem on. This is very bad.

When I set the type of /dev/sda1 to anything that isn't EFI System and then run bootc install to-filesystem, it uses the correct filesystem. My assumption was that whatever EFI partition was mounted in the directory that I invoked bootc install to-filesystem on would be used, since it requires that you mount the EFI partition boot/efi in advance. Instead what happens is it uses the first available partition on the same disk that has its type set to EFI System.