2025-06-26: Exam Embedded Computing 1

NAME:

All of the questions below have multiple answers that might be True or False. Multiple answers might be True, just as multiple answers might be False. Please mark the checkbox in the respective column, to the best of your knowledge.

Each of your correct marks adds one point to the total. Each incorrect mark subtracts one point! An unchecked answer counts as incorrect, thus subtracting one point - as does checking both boxes.

Pointers Considered Harmful

Say you compile and run the program below, which is obviously buggy: it accesses memory that it never allocated. How large can the resulting suffering become?

int main()
{
    int* ip = (int*)666;
    *ip = 666;
    return 0;
}

Answer	True	False
It could happen that address 666 (which is likely to contain OS data because that address is very low) has the status bits of your internal SSD, and overwriting these could turn your SSD into an expensive brick.
The OS intercepts a hardware exception from the Memory Management Unit (MMU) which says, “At 666 there is no memory allocated for the current process”. As a result the OS terminates the process.
The incident is logged, you are logged out of your computer immediately, and you can never log in again.
A fellow process on the system might have rightfully allocated memory at position 666, and it is that process that falls victim to your mistake.

Doing Nothing, But Loudly

Say you compile and run the program below, which is not exactly buggy in the sense of the program above, but might behave in a way that is not nice to others (you will agree that there are actually two “mights”, but there is still some room for misbehaviour).

If that program misbehaves, what is going to happen?

#include <stdint.h>

int main()
{
    uint64_t what_is_it;                               // <-- to be or not to be!
    uint64_t what_is_it_supposed_to_be;                // <-- ...

    while (what_is_it)
        what_is_it = what_is_it_supposed_to_be;
    return 0;
}

Answer	True	False
The system will come to a halt because the process never yields the CPU.
The next mouse-move or key-press interrupts the CPU (and thus the misbehaving process). From within the interrupt service routine, the OS notices that a process went nuts and kills it.
The fan (if any) might try to cool the CPU.
The OS tries hard to be nice to everybody and does not kill the offender. Rather, the offender receives a penalty - less CPU cake - in favor of those who behave.

Permissions: Device Access (For Example)

User jfasch wants to control some peripheral using an IO pin (read and write access on /dev/gpiochip0 is required to use an IO pin). Given the following situation, what would be the options?

$ ls -l /dev/gpiochip0
crw-rw---- 1 root gpio 254, 0 Jun 15 22:52 /dev/gpiochip0

$ id
uid=1001(jfasch) gid=1001(jfasch) groups=1001(jfasch)

Answer	True	False
Simply do request and use the required IO.
Become root, and then request and use the required IO.
Ask the administrator to make jfasch member of the sudo group, and then request and use the required IO.
Ask the administrator to make jfasch member of the sudo group, re-login, become root, and then request and use the required IO.
Ask the administrator to make jfasch member of the gpio group, re-login, and then as jfasch request and use the required IO.

Permissions: File Access (Simple)

The file /etc/passwd contains user records of the form

jfasch:x:1001:1001::/home/jfasch:/bin/bash

, where the last field (fields are separated with ‘:’) contains the login shell.

Given that /etc/passwd has permissions like

$ ls -l /etc/passwd
-rw-r--r-- 1 root root 1623 Dec 30  2020 /etc/passwd

And given that jfasch has credentials like

$ id
uid=1001(jfasch) gid=1001(jfasch) groups=1001(jfasch)

Would user jfasch be able to determine his login shell from reading /etc/passwd?

Answer	True	False
Yes

Would user jfasch be able to set his login shell to e.g. /bin/zsh by modifying his login shell field in /etc/passwd?

Answer	True	False
Yes

Permissions: File Access (Involving Directory Access)

Given a directory with the following permissions (and the permissions on its contained files),

$ ls -dl ~/some-directory/
drwxr-xr-x 2 jfasch jfasch 4096 Jun 25 19:26 /home/jfasch/some-directory/
$ ls -l ~/some-directory/
total 4
-rw-r--r-- 1 jfasch jfasch 1623 Jun 25 19:31 my-bitcoins.money

Would anybody on the system be able to read /home/jfasch/some-directory/my-bitcoins.money?

Answer	True	False
Yes

What are jfasch’s options to not disclose the contents of /home/jfasch/some-directory/my-bitcoins.money to anybody on the system?

Answer	True	False
chmod 666 ~/some-directory
chmod 700 ~/some-directory
chmod 770 ~/some-directory (provided that nobody else but jfasch is member of group jfasch)
chmod go-w ~/some-directory/my-bitcoins.money

Object Oriented Programming: Polymorphic Use Of Types

When (and why) would you design an interface and provide one or more implementations of it?

Answer	True	False
Always. Even if I do not yet see that there could be an alternative implementation of the class that I’m about to write, I design an interface and make the class implement it - because in the future there could be.
I’m about to implement code for one particular sensor device (a temperature sensor, say), and on top of that I intend to use that device as a basis for a data logger. To be able to unit test the data logger code without real hardware, I design an interface for the sensor, and provide two implementations of it: real hardware and mockup.
Always. I do this for performance. Indirect function calls (brought about by the dynamic dispatch mechanism of virtual methods) are way faster than direct calls.
I implement a data logger framework - routines for CSV writing, cloud communication, and whatnot. That code should not depend on a particular sensor type (e.g. DS18B20 which is a Onewire device), but rather I want to sell the code to companies who bring their own sensor types.

SSH Keys, And Crypto Keys In General

What could happen if I disclose my (SSH) keys - especially the private key - to a malicious person? (This could happen if I leave it on an unencrypted filesystem on a laptop that I lose, or when I have it on a system where multipe users are logged in and I don’t get the access permissions right.)

Answer	True	False
Nothing.
The malicous finder could impersonate as myself on Github where I maintain a project that many others depend on, and inject a trojan horse kind of security hole.
If I am able to use that key pair to login to my home server (by punching a hole like TCP/22 into my firewall), the finder could take over my home server and attack all other devices in my home network.