On Tue, 10 May 2022 at 13:53, Christian Brauner [off-list ref] wrote:

quoted

What exactly are the attributes that systemd requires?

We keep a repo with ideas for (kernel) extensions - we should probably
publish that somewhere - but the list we used for a prototype roughly
contains:

* mount flags MOUNT_ATTR_RDONLY etc.
* time flags MOUNT_ATTR_RELATIME etc. (could probably be combined with
  mount flags. We missed the opportunity to make them proper enums
  separate from other mount flags imho.)
* propagation "flags" (MS_SHARED)
* peer group
* mnt_id of the mount
* mnt_id of the mount's parent
* owning userns

Sounds good thus far.   And hey, we don't even need a new syscall:
statx(2) could handle these fine.

There's a bit more advanced stuff systemd would really want but which I
think is misplaced in a mountinfo system call including:
* list of primary and auxiliary block device major/minor

It's when you need to return variable size arrays or list of strings
that the statx kind of interface falls down.

For that a hierarchical namespace is a much better choice, as it can
represent arbitrary levels of arrays, while doing that with a
specialized syscall is going to be cumbersome.

I just have a really hard time understanding how this belongs into the
(f)getxattr() system call family and why it would be a big deal to just
make this a separate system call.

Fragmenting syntactically equivalent interfaces is bad, unifying them
is good.  Dave's example of adding a new syscall for retrieving
multiple xattrs  is a prime example.

Thanks,
Miklos