Accidentally stumbled over a file type borg does not support yet: a whiteout (used by overlay fs)

Infos:

• "mknod filename w" on macOS
• https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html#whiteouts-and-opaque-directories
• https://lwn.net/Articles/312641/ there is also some info about white-out
• https://www.man7.org/linux/man-pages/man2/rename.2.html
• https://docs.python.org/3/library/stat.html#stat.S_ISWHT

Linux: has a hack avoiding a new file type, using a char dev 0:0 for file whiteouts and an xattr for directory whiteouts
macOS: has whiteouts
FreeBSD: has support for whiteouts, separate file type DT_WHT
OpenBSD: no whiteouts
NetBSD: ?

Is there a need for backing up / restoring whiteouts?

• for a mounted overlay fs, it should be backed up as visible to the user (no whiteouts visible)
• can one access the layers individually, so that WHT files would become visible (on BSD)?