Code coverage analysis is used in software testing to discover untested pieces of an application. Gathering code coverage data may also be useful for fuzzing. Basically it may help to figure out which parts of a program were not reached during fuzzing. This info can then be used to improve the fuzzer.
Let’s try to gather some code coverage data during fuzzing. As an example, we’re going to test picotls with tlsbunny. Picotls is an implementation of TLS 1.3 protocol written in C, and tlsbunny is a framework for building negative tests and fuzzers for TLS 1.3 implementations. We’re going to use gcov for gathering code coverage data, and lcov for creating a report.
Picotls is a TLS 1.3 implementation written in C. At the moment of writing this post, picotls implements TLS 1.3 draft 26.
I have been experimenting with TLS 1.3 in tlsbunny project. This is a framework for building negative tests and fuzzers for TLS 1.3 implementations. For example, tlsbunny has several simple fuzzers for TLS structures like TLSPlaintext, Handshake, ClientHello, etc. It would not be worse to run those fuzzers against picotls server.
Here is a brief overview which describes how TLS can be used for establishing a secure TLS connection. First, we briefly discuss what SSL/TLS protocols are. Next, we’ll talk about secure TLS protocol versions and parameters. Finally, we’ll describe how TLS can be parametrized securely.