[systemd-devel] sysext merge of /usr/local

[Itxaka Serrano Garcia]

Hey again,

I'm back with more annoying questions about sysext :D

According to the docs, sysext only "extends" the existing usr/opt/etc with
the sysext contents but we are seeing a different thing here:

root at cos-recovery:~# stat /usr/local/file2
  File: /usr/local/file2
  Size: 0         Blocks: 0          IO Block: 4096   regular empty file
Device: 0,47 Inode: 171         Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2024-06-10 14:56:50.725904625 +0000
Modify: 2024-06-10 14:56:50.725904625 +0000
Change: 2024-06-10 14:56:50.725904625 +0000
 Birth: 2024-06-10 14:56:50.725904625 +0000
root at cos-recovery:~# systemd-sysext refresh
Using extensions 'work'.
Merged extensions into '/usr'.
root at cos-recovery:~# stat /usr/local/file2
stat: cannot statx '/usr/local/file2': No such file or directory
root at cos-recovery:~# systemd-sysext unmerge
Unmerged '/usr'.
root at cos-recovery:~# stat /usr/local/file2
  File: /usr/local/file2
  Size: 0         Blocks: 0          IO Block: 4096   regular empty file
Device: 0,47 Inode: 171         Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2024-06-10 14:56:50.725904625 +0000
Modify: 2024-06-10 14:56:50.725904625 +0000
Change: 2024-06-10 14:56:50.725904625 +0000
 Birth: 2024-06-10 14:56:50.725904625 +0000

Weirdly enough, any files under /usr/ that we create are NOT overridden
when we load any sysext, it only seems to happen in the dirs inside /usr

It's true that we are using USI here and on a "normal" non-USI/UKI system
this behaviour doesn't seem to happen.

The main difference I can see is that the root fs in the failure case is a
tmpfs while in the working case it's an ext4 fs, but for example our
/usr/local is mounted to a ext4 disk:

/dev/mapper/vda3 on /usr/local type ext4 (rw,relatime)


If I do create an overlay on /usr then it seems to work as expected but its
too late for us, we lose anything mounted there and hidden dirs.

Is this a bug or its expected behaviour? Is there anything we could be
missing that triggers this behaviour? Any pointers on why this would react
like this?


Cheers,
Itxaka
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