Devices, Filesystems and FHS →hardMultiple ChoiceObjective-mapped

LPIC-1 Devices, Filesystems and FHS Practice Question

Exhibit

$ ls -la /usr/local/bin/myapp
-rwsr-xr-x 1 root root 12345 Jan 1 00:00 /usr/local/bin/myapp
$ sudo -u user1 /usr/local/bin/myapp
Error: Permission denied
$ id user1
uid=1001(user1) gid=1001(user1) groups=1001(user1)
$ getfacl /usr/local/bin/myapp
# file: usr/local/bin/myapp
# owner: root
# group: root
user::rwx
group::r-x
other::r-x

Refer to the exhibit. A user 'user1' tries to run the setuid binary 'myapp' but gets permission denied. What is the most likely reason?

⚠ Common exam trap

The trap here is that candidates often focus on file permissions (e.g., missing execute bit or group membership) and overlook the filesystem mount option 'nosuid', which silently disables setuid/setgid behavior regardless of the binary's permission bits.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The binary has the setuid bit set, but the filesystem is mounted with 'nosuid'

When a filesystem is mounted with the 'nosuid' option, the kernel ignores setuid and setgid bits on executables within that filesystem. Even if 'myapp' has the setuid bit set, the nosuid mount flag prevents the effective UID from changing to the file owner, so 'user1' still runs the binary with their own privileges and may receive 'Permission denied' if the binary requires root-level access.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
The binary is missing the setgid bit
Why it's wrong here
Missing setgid does not cause permission denied; it just doesn't set group.
✗
The binary is not executable by others
Why it's wrong here
The permissions show r-x for others, so user1 has execute permission.
✓
The binary has the setuid bit set, but the filesystem is mounted with 'nosuid'
Why this is correct
If the filesystem is mounted with 'nosuid', setuid bits are ignored, so the program runs with the user's privileges, and the error might be due to insufficient permissions for the operation.
✗
The user does not belong to the root group
Why it's wrong here
The setuid bit elevates to owner (root), so group membership is not required for execution.

Go deeper

LPIC-1 Devices, Filesystems and FHS Practice Question

Exhibit

Why each option matters

Option-by-option breakdown

Related to this question