Runninig a forkbomb in Jenkins

Did you think this was the end of my forkbomb chronicles, lol? Of course not!

Of course I had to come with a part 3!

Basically, part infinity of me being bored in college, and doing random sh!t to pass time.

⚠️ Warning: THIS IS JUST A JOKE. DO NOT DO THIS IN PROFESSIONAL CONTEXTS. It can quite literally crash the system.

We had a course called Devops Lab, where Jenkins was a major part of a syllabus. After finishing the activity in a few minutes I had a lot of time to play around in the 2 hour lab.
(This experiment was performed around mid March, and was sitting in my unposted Drafts)

At that time, I'd been experimenting with forkbombs in my own laptop. Of course I had to try if I can run a forkbomb within jenkins. And see what happens.

So, here I am.

Jenkins is an open-source automation server primarily used for CI/CD (Continuous Integration and Continuous Delivery). In plain terms: every time a developer pushes code, Jenkins can automatically pull it, build it, run tests, and deploy it. It does this by running pipelines, sequences of steps defined either in a Jenkinsfile or through the UI. Each step can execute shell commands on the machine Jenkins is running on (or on connected agents).

In our Devops Lab, we were using it to build and test simple Java projects. My jenkins was configured to run inside Docker containers, mostly because I had severely low storage left on my laptop, and I wanted to save data, and I like Docker, it's easy to manage apps in docker.

In Jenkins, you can add a build step called "Execute shell", it literally just runs a bash script on the host (or container). I created a new freestyle project, named it something innocent, went to Build Steps → Add build step → Execute shell, and pasted the script in. One click on "Build Now" and we were off to the races.

#!/bin/bash
# ============================================================
# FORKBOMB — JENKINS EDITION
# For educational purposes only. 
# ============================================================

# --- SAFETY NET ---
# ulimit -u sets RLIMIT_NPROC — enforced by the kernel in
# copy_process() inside kernel/fork.c. When your UID's process
# count hits this ceiling, fork() returns EAGAIN. Bash then
# prints "retry: Resource temporarily unavailable" a few times
# before finally giving up with "Resource temporarily unavailable".
ulimit -u 50

# --- HIDE FROM THE JENKINS PROCESS REAPER ---
# Jenkins tracks child processes via their process group and
# an env variable called JENKINS_NODE_COOKIE (BUILD_ID is the
# older alias). After a build finishes, Jenkins walks the
# process table and kills anything with a matching cookie.
# Setting it to an arbitrary value orphans the processes from
# Jenkins' tracking.
export BUILD_ID=dontKillMe


# Defining the bomb
# (Jenkins' script parser choked on the classic :(){ :|:& };: syntax,
# so the named-function form was used instead)
bomb() {
  bomb | bomb &
}

# Light the fuse
bomb   

# keep the main process active so we can see the fireworks in the log
sleep 10

The output

The script runs as the Jenkins system user inside /tmp, Jenkins writes it to a randomly named temp file (you can see the path in the output: /tmp/jenkins13172110508468162848.sh) and executes it directly with /bin/bash.

Started by user [anu](http://localhost:8080/user/anu)
Running as SYSTEM
Building in workspace /var/jenkins_home/workspace/forkbomb
[forkbomb] $ /bin/bash /tmp/jenkins13172110508468162848.sh
/tmp/jenkins13172110508468162848.sh: fork: retry: Resource temporarily unavailable
/tmp/jenkins13172110508468162848.sh: fork: retry: Resource temporarily unavailable
/tmp/jenkins13172110508468162848.sh: fork: retry: Resource temporarily unavailable
/tmp/jenkins13172110508468162848.sh: fork: retry: Resource temporarily unavailable

/tmp/jenkins13172110508468162848.sh: fork: Resource temporarily unavailable
Build step 'Execute shell' marked build as failure
Finished: FAILURE

slayy!! it ran for 10 secs!!! yayayayayay

Resource temporarily unavailable means that ulimit -u 50 did its job and defused the bomb.

Anyways! I found this experiment really fun.

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The output