Как ответ на Node, я выложу на странице руководства FreeBSD для mount (8) следующее:
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.
И раздел справочной страницы chmod (2), который ссылается на бит suid:
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).
Имейте в виду, что это угроза безопасности, и знайте, что вы делаете, когда вы его включаете, во FreeBSD, но я верю, что Linux также требует включения специального флага монтирования и изменит поведение файлов в этом каталоге.