Challenge 2: Reissue

ChallengesChallenge 2 of 5120 minutesChallenge

Reissue

The booking hall issues tickets. Each ticket names a holder from the roster, a voided ticket keeps its place in the rack so it can be reissued later, and a stub is written on whatever space the hall has spare. The hall also holds a duty notice that nothing prints.

Your goal is to get the duty notice printed.

Hands-on lab

There are no milestones on this page and no hints. Read How These Challenges Work first.

Finish Bug Classes and Primitives and The Heap first.

Work inside the Ubuntu Desktop 24.04.4 LTS x86-64 course VM with GCC 13.x, GNU binutils 2.42 and GDB 15.x. Leave address randomization on. This challenge depends on it.

Get the files

Download reissue-v1.0.0.tar into ~/underdunn-course/downloads and verify the release digest:

cd ~/underdunn-course/downloads
sha256sum reissue-v1.0.0.tar

0324675524b16c7320c401b3e6289f6b03f72917080bc94348cab555ce50c4ff  reissue-v1.0.0.tar

Don’t continue if it differs. Download a fresh copy and verify it again.

mkdir -p ~/underdunn-course/labs/reissue
tar -xf reissue-v1.0.0.tar -C ~/underdunn-course/labs/reissue
cd ~/underdunn-course/labs/reissue/reissue-v1.0.0
sha256sum -c CHECKSUMS.sha256
sh build.sh

Every listed file has to report OK.

This build is not silent. It prints one warning, and that warning is about a real bug. It is not about the other one.

Safety note

Scope, and one run in thirty

Build and run only the program you build from this archive, inside the isolated course VM. The result is one line on standard output. Do not brute force anything.

One of the values you need is different on every run, and about one run in thirty it arrives in a form the hall’s own input path cannot carry. If something that worked a moment ago stops working, run it again before you change it. If it works on the next run you have found this, and it belongs in your own checks.

The flag

Paste exactly what the hall printed, with no surrounding whitespace:

Check your flag

Enter the complete flag, including UNDERDUNN{ and the closing brace. The check happens in your browser; the flag is not sent anywhere.

Check flag

Reset and cleanup

The hall keeps nothing between runs. For a full reset of the files:

cd ~/underdunn-course/labs/reissue
rm -rf -- reissue-v1.0.0
tar -xf ~/underdunn-course/downloads/reissue-v1.0.0.tar -C .
cd reissue-v1.0.0
sha256sum -c CHECKSUMS.sha256
sh build.sh

Keep the verified archive and any solver or modified source you want to revisit. Before a reset, copy those files outside the directory being replaced.