Why doesn't setuid work with mount?

Question

There is a setuid bit on the file.

user@host:~$ ls -l /bin/mount
-rwsr-xr-x 1 root root 40152 May 26 19:31 /bin/mount

Why does operating system require root access to execute the 'mount'?

user@host:~$ /bin/mount /dev/sdb1 /mnt
mount: only root can do that
user@host:~$ sudo /bin/mount /dev/sdb1 /mnt
user@host:~$ lsblk  | grep sdb

sdb      8:16   0  102M  0 disk
└─sdb1   8:17   0  101M  0 part /mnt

My /etc/fstab

# / was on /dev/sda1 during installation
UUID=026bb2d9-1c0c-4163-85a1-f83b2221eb34 /               ext4    errors=remount-ro 0       1

# swap was on /dev/sda5 during installation
UUID=aec6b039-13b1-4568-abb1-2be1f3429325 none            swap    sw              0       0

Answer 1

Unix has the concept of real and effective UIDs (and GIDs for that matter).

When you run a setuid program then the effective ID of the process is set to the owner of the file.

So in the case of mount you have an effective ID of root. But you still have a real ID of user.

Programs such as passwd or su or mount can check the real ID to see who is running it and act differently accordingly. For passwd this allows the root user to change another's password; for su it allows switching users without knowing the password.

For mount it allows users to mount their own filesystems if they are defined in /etc/fstab and have the user attribute associated with them

eg in my Debian fstab I have:

/dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0

This means a non-root user can run mount /dev/sr0 or mount /media/cdrom0 and it will attempt to mount the CD/DVD.

This is logic built into the mount program itself; it checks the real ID of the caller.