Fuzzing for cups-filters could have higher coverage with low effort

Hello @fish98!

I hear that OpenPrinting is soon hosting Winter-of-Code participants again, maybe it is the right time to pitch this:

CUPS-Filters does a lot of media parsing, and media parsing code is diverse and sometimes complicated.

I think it would be good to fuzz it like this:

• Make the fuzzer generate the raw media files as input for the cups-filters tools (or the cfFilter* functions from filter.h).
• For seeding the fuzzer with good starting points, normal media files can be used, and ideally also a variety of crash inputs from past bugs and vulnerabilities.

I am raising this because I noticed that the existing fuzzers for cups-filters and libcupsfilters only exercise a small subset of the functionality of these libraries (PDF output helpers and dithering utilities, to be precise). You do not need to have individual fuzzers for these - If the exercised helpers are also used by any of the CUPS-filters tools, it is enough to give an input seed file to the fuzzer which reaches this code, and a modern coverage-guided fuzzer is then able to exercise it.

(P.S., I am unfortunately short on time to do anything on this myself, but I have some Podman Containerfiles that can serve as a starting point to make the cups-filters projects build again. (Although I remember I had trouble linking it statically.) I can happily share that over email if you are interested. Let me know.)

Start by reviewing the existing cups-filters and libcupsfilters projects under fuzzing/projects, then inspect cupsfilters/filter.h and the available Podman Containerfiles. The work is complete when fuzzers accept raw media files, use normal and historical crash inputs as seeds, and exercise the requested cups-filters tools or cfFilter* functions.