Linux does (since version 3.6) have the ability to prevent users from creating hardlinks to files they don't own. (See man 5 proc under "/proc/sys/fs/protected_

Linux does (since version 3.6) have the ability to prevent users from creating hardlinks to files they don't own. (See man 5 proc under "/proc/sys/fs/protected_hardlinks".) I think FreeBSD has a similar sysctl option. The linked article does mention it but warns "If you're not using systemd, the vanilla Linux kernel does not enable these protections by default".