What does it mean for a folder to have suid permission?

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                          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.
                        
Asked by Just a learner (2022 rep)
Aug 16, 2019, 01:51 PM
Last activity: Aug 17, 2019, 02:12 AM
What does it mean for a folder to have suid permission?

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