[srett]

Yes I tried to do research on this on the past. Linux, as in "just the kernel", doesn't care about user name formats as far as I can tell. However, the Linux philosophy goes "try to adhere to POSIX where it makes sense, break it where we think it sucks", which isn't exactly a clear statement, but lead me to conclude that it might make sense to check what posix does, if anything, because everything else is user space, e.g. GNU (coreutils, adduser) and thus isn't tied to Linux. systemd should call into libc and let it deal with the username, so if any of the nsswitch backends think 0day is a valid username and returns a uidnumber, just use it and don't try to be extra clever.

[digi_owl]

Best i recall, the kernel will offer POSIX compliant interfaces, but then also offer parallel interfaces that fix potential problems.

Similarly they will retain previously introduced, but flawed, interfaces etc until they are damn sure that they are no longer in use.