Ttdfjt

I know what it means for a file to have suid permission. It means when other users have execute permission for it, they execute as the owner of the file. But what does it imply when a folder has suid permission? I did some testing and it seems nothing special for the folder. Could anyone help to plain a little? Thanks.

I'm using Oracle Linux 7.6.

root:[~]# cat /etc/*release*
Oracle Linux Server release 7.6
NAME="Oracle Linux Server"
VERSION="7.6"
ID="ol"
VARIANT="Server"
VARIANT_ID="server"
VERSION_ID="7.6"
PRETTY_NAME="Oracle Linux Server 7.6"
ANSI_COLOR="0;31"
CPE_NAME="cpe:/o:oracle:linux:7:6:server"
HOME_URL="https://linux.oracle.com/"
BUG_REPORT_URL="https://bugzilla.oracle.com/"

ORACLE_BUGZILLA_PRODUCT="Oracle Linux 7"
ORACLE_BUGZILLA_PRODUCT_VERSION=7.6
ORACLE_SUPPORT_PRODUCT="Oracle Linux"
ORACLE_SUPPORT_PRODUCT_VERSION=7.6
Red Hat Enterprise Linux Server release 7.6 (Maipo)
Oracle Linux Server release 7.6
cpe:/o:oracle:linux:7:6:server
root:[~]#

Below is my testing on a freshly installed server.

root:[~]# pwd
/root
root:[~]# ls -lad /root
dr-xr-x---. 9 root root 4096 Aug 16 22:07 /root
root:[~]# mkdir test
root:[~]# ls -lad test
drwxr-xr-x. 2 root root 4096 Aug 16 22:07 test
root:[~]#
root:[~]# useradd a
root:[~]# passwd a
Changing password for user a.
New password:
BAD PASSWORD: The password is a palindrome
Retype new password:
passwd: all authentication tokens updated successfully.
root:[~]# chmod u+s test
root:[~]#
root:[~]# su - a
[a@localhost ~]$ cd /root/test
-bash: cd: /root/test: Permission denied
[a@localhost ~]$ cd /root
-bash: cd: /root: Permission denied
[a@localhost ~]$ logout
root:[~]#
root:[~]# ls -lad /root
dr-xr-x---. 10 root root 4096 Aug 16 22:07 /root
root:[~]# chmod o+x /root
root:[~]#
root:[~]# su - a
Last login: Fri Aug 16 22:08:54 CST 2019 on pts/0
[a@localhost ~]$ cd /root/test
[a@localhost test]$
[a@localhost test]$ pwd
/root/test
[a@localhost test]$ ls -la .
total 8
drwsr-xr-x. 2 root root 4096 Aug 16 22:07 .
dr-xr-x--x. 10 root root 4096 Aug 16 22:07 ..
[a@localhost test]$ touch file1
touch: cannot touch ‘file1’: Permission denied
[a@localhost test]$ logout
root:[~]#
root:[~]# chmod o+w test/
root:[~]#
root:[~]# su - a
Last login: Fri Aug 16 22:09:31 CST 2019 on pts/0
[a@localhost ~]$
[a@localhost ~]$ cd /root/test
[a@localhost test]$ touch file1
[a@localhost test]$ ls -la
total 8
drwsr-xrwx. 2 root root 4096 Aug 16 22:11 .
dr-xr-x--x. 10 root root 4096 Aug 16 22:07 ..
-rw-rw-r--. 1 a a 0 Aug 16 22:11 file1
[a@localhost test]$ mkdir folder1
[a@localhost test]$ ls -la
total 12
drwsr-xrwx. 3 root root 4096 Aug 16 22:11 .
dr-xr-x--x. 10 root root 4096 Aug 16 22:07 ..
-rw-rw-r--. 1 a a 0 Aug 16 22:11 file1
drwxrwxr-x. 2 a a 4096 Aug 16 22:11 folder1
[a@localhost test]$

As you can see, it seems the files and folders the user a created in /root/test didn't inherit the owner and group of it. The owner and group is a and not root. Are there any problems with my testing? I'm new in Linux.

That doesn't mean anything on your Oracle Linux or on any Linux system.

However it may have meaning on FreeBSD. Quoting from the chmod(2) manpage:

If mode ISUID (set UID) is set on a directory, and the MNT_SUIDDIR option
was used in the mount of the file system, then the owner of any new files
and subdirectories created within this directory are set to be the same
as the owner of that directory. If this function is enabled, 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 behavior does not
change the requirements for the user to be allowed to write the file, but
only the eventual owner after it has been created. Group inheritance is
not affected.

This feature is designed for use on fileservers serving PC users via ftp,
SAMBA, or netatalk. It provides security 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. For more details of the suid-
dir mount option, see mount(8).

This is not supported on other *BSD systems like NetBSD or OpenBSD.

According to the GNU manual, it means files (including subfolders) created in the directory will inherit its group and user:

On a few systems, a directory’s set-user-ID bit has a similar effect on the ownership of new subfiles and the set-user-ID bits of new subdirectories. These mechanisms let users share files more easily, by lessening the need to use chmod or chown to share new files.