Node の回答のフォローアップとして、mount(8) の FreeBSD man ページから以下を投稿します。
suiddir
A directory on the mounted file system will respond to
the SUID bit being set, by setting the owner of any new
files to be the same as the owner of the directory. New
directories will inherit the bit from their parents.
Execute bits are removed from the file, and it will not
be given to root.
This feature is designed for use on fileservers serving
PC users via ftp, SAMBA, or netatalk. It provides secu-
rity holes for shell users and as such should not be used
on shell machines, especially on home directories. This
option requires the SUIDDIR option in the kernel to work.
Only UFS file systems support this option. See chmod(2)
for more information.
そして、suid ビットを参照する chmod(2) man ページのセクション:
4000 (the setuid bit). Executable files with this bit set will
run with effective uid set to the uid of the file owner.
Directories with this bit set will force all files and sub-
directories created in them to be owned by the directory
owner and not by the uid of the creating process, if the
underlying file system supports this feature: see chmod(2)
and the suiddir option to mount(8).
これはセキュリティ上のリスクであり、FreeBSD で有効にしたときに何をしているのかを知っておいてください。ただし、Linux も特別なマウント フラグを有効にする必要があり、そのディレクトリ内のファイルの動作が変わると思います。