Advent of Code 2023 Day1

My challenge to myself: implement the Advent of Code challenges entirely in eBPF. Having done these challenges before I know the input is essentially a file. I want to simply be able to cat the input file, and have eBPF tell me the answer. None of the problem-solving logic is to be in user space - all I'm allowing myself to do in user space is load the BPF programs, initialize and load maps, and output the results.

I've used a kernel v6.5.0 as supplied in Ubuntu 23.10

Install & set up environment

Similar to learning-ebpf examples:

# Clone the repo
git clone --recurse-submodules https://github.com/lizrice/ebpf-advent
cd ebpf-advent

# Start a Lima VM
limactl start advent-ebpf.yaml

# You might want multiple shells. For each, just run
limactl shell advent-ebpf

# You'll need to be root 
sudo -s

# Install libbpf
cd libbpf/src
make install 
cd ../..

Building and running the code

Run any of make p1, make p2 or make p2a to get an executable called day1. Run this (as root) in one terminal and in another run cat advent.example or cat advent.full. You could also use a third terminal to run bpftool prog trace to see tracing / debugging output.

Filtering interesting events

The kprobe attached to vfs_open() lets us ignore files that we're not interested in, being read by any other executables.

In the kprobe attached to vfs_read() we can get the address of the buffer that data will be read into, but it won't be populated at that point. Parsing the contents of the buffer is triggered by the kretprobe for vfs_read().

File parsing

cat reads into a 128k buffer. In this challenge (at least for the puzzle input I was given) the input file is 21760 bytes long. I copied the buffer memory section by section into a local buffer size ADVENT_BUFFER_LEN. Since this buffer lives on the stack it can't be arbitrarily large (in fact I had to adjust the size for the different examine_char implementations.) By using a combination of loops and recursively calling the tail call buffer_read I've been able to parse enough characters to solve this challenge.

Day 1 Part 1

The challenge here is to find the first and last digits in each line, combine those into a two-digit number and then add up all these numbers. The code is in day1p1.bpf.c.

Lines could very easily be split across the arbitrary ADVENT_BUFFER_LEN boundary.

Day 1 Part 2

In part 2, you also have to account for digits that might be spelled out as words (for example two, instead of 2).

There are two solutions here in day1p2.bpf.c and day1p2a.bpf.c.

The first is the straightforward way. The second version uses an FSM to parse the digits. This uses less stack space, so I can use a larger size for ADVENT_BUFFER_LEN (which would allow for parsing a bigger file if necessary).

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If you want to learn more about eBPF, you might want to check out my repo and book Learning eBPF