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In *nix systems the User Id Number (UID) and the Group Id Number (GID) are integers used for identifying uniquely users and groups. Take a look at /etc/passwd and /etc/group files (follow the links for more details about these files):

simo@xps:~$ cat /etc/passwd

We can infer:

User UID GID (primary)
root 0 0
simo 1000 1000
debian-tor 121 133

Their counterparts in the group file:

simo@xps:~$ cat /etc/group

Moreover you can see that the user “simo” belong also to the “vboxusers” group.

Real (U|G)ID vs Effective (U|G)ID

Every running process has at least 4 ID numbers associated with it:

  • the Real UID (RUID) identifies the user who launched the process.
  • the Real GID (RGID) identifies the primary group of the user that launched the process.
  • the Effective UID (EUID) and the Effective GID (EGID) are used to determine what resources the process can access.

These information can be found programmatically:

simo@xps:~/example$ cat ids.c
int main()
 uid_t real_uid = getuid();
 uid_t effect_uid = geteuid();
 gid_t real_gid = getgid();
 gid_t effect_gid = getegid();
 printf("ruid=%d euid=%d\n", real_uid, effect_uid);
 printf("rgid=%d egid=%d\n", real_gid, effect_gid);

Usually the IDs have the same value when you run a program, but sometimes happens that an operating system needs to run programs with temporarily elevated privileges in order to perform a specific task.

The setuid (set user id) is a permission bit, that allows the users to exec a program with the permissions of its owner. The setgid (set group id) is a bit that allows the user to exec a program with the permissions of the group owner.

The s(u|g)id bit on executables only changes the E(U|G)ID the executable will run as, and not the real(U|G)ID. Let’s take a closer look:

$ gcc -Wall ids.c -o example
$ sudo chown root.root example
$ ls -l
total 16
-rw-rw-r-- 1 simo simo 294  gen 17 16:21 ids.c
-rwxrwxr-x 1 root root 8816 gen 17 16:28 example
$ ./example
ruid=1000 euid=1000
rgid=1000 egid=1000
$ sudo chmod 6771 example
$ ls -l
total 16
-rw-rw-r-- 1 simo simo 294 gen 17 16:21 ids.c
-rwsrws--x 1 root root 8816 gen 17 16:29 example
$ ./example
ruid=1000 euid=0
rgid=1000 egid=0
  1. I compiled the example;
  2. I changed the owner and the group from “simo” to “root”;
  3. I ran the program and I got the same ids;
  4. I set the setuid and the setgid, pay attention to the s here
    -rwsrws--x 1 root root 8816 gen 17 16:29 example
  5. Ids changes accordingly!

Hint: in addition to the restriction on s(u|g)id interpreted scripts (any executable text file beginning with “#!”), some shells (like bash) as an extra safety measure will set the EUID back to the RUID; in this case, you will need to wrap the call to the script within a C program and setuid(…) before executing the script.