Like all Unix-like systems, Linux implements the traditional protection bits controlling who can access files in a filesystem (and what access they have). Fewer users, perhaps, are aware of a set of additional permission bits hidden away behind the chattr and lsattr commands. Among other things, these bits can make a file append-only, mark a file to be excluded from backups, cause a file's data to be automatically overwritten on deletion, or make a file immutable. The implementation of many of these features is incomplete at best, so perhaps it's not surprising that immutable files can still be changed in certain limited circumstances. Darrick Wong has posted a patch set changing this behavior, implementing a user-visible behavioral change that he describes as "an extraordinary way to destroy everything".