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.

Instrumenting an application with gcov

Gcov works together with gcc compiler. It can be easily enabled by passing -fprofile-arcs -ftest-coverage flags to gcc. You only need to make sure that you passed the flags to all gcc invocations during building your application. Typically, the flags can be added to CFLAGS or similar environment variables.

Picotls uses CMake for building itself. The following simple script clones and builds picotls:

Here we also enabled debug info, disabled some optimizations, and turned on AddressSanitizer which can help to catch various memory corruption issues.

For testing, we are going to use cli program which can start a simple TLS 1.3 server based on picotls. The sources of cli can be found at t/cli.c.

Gcov normally dumps code coverage data when a program finishes. cli is a very simple server which doesn’t provide a graceful way to stop itself. We just have to kill the process. But in this case Gcov won’t be able to dump code coverage data. The following patch for t/cli.c helps to overcome this problem:

The patch adds a  handler for SIGINT signal. The handler calls __gcov_flush() function which dumps gathered code coverage data. Now we can just stop cli process by Ctrl-C, and code coverage data will be dumped by the signal handler.

Don’t forget to re-build picotls after applying the patch.

Initializing lcov

Gcov doesn’t provide a good human-readable report. Once your instrumented application finishes, gcov is going to store data to .gcno files for each source file. The data is stored in a binary format.  To make it a bit more readable, you can call gcov for a source file you are interested in, for example:

$ gcov -l -o CMakeFiles/cli.dir/t/cli.c.gcno t/cli.c

The command above creates several .gcov files which contain code coverage info in more readable format:

$ tail -10 cli.c##cli.c.gcov
        -: 420:
        1: 421: if (resolve_address((struct sockaddr *)&sa, &salen, host, port, family, SOCK_STREAM, IPPROTO_TCP) != 0)
    #####: 422: exit(1);
        -: 423:
        1: 424: if (is_server) {
        1: 425: return run_server((struct sockaddr *)&sa, salen, &ctx, file, &hsprop);
        -: 426: } else {
    #####: 427: return run_client((struct sockaddr *)&sa, salen, &ctx, host, file, &hsprop);
        -: 428: }
        -: 429:}

You may have noticed that gcov marked uncovered lines with #####. Technically, you can use grep to find all untested lines:

$ find . -name "*.gcov" | xargs grep "#####"

But luckily we have lcov which can transform .gcov files in to an HTML report . Once we built our application with gcov, we can initialize lcov:

$ lcov --zerocounters --directory .
$ lcov --no-external --capture --initial \
	--directory . --output-file

The commands above create a “baseline” that contains zero coverage.

Running a picotls server and starting a fuzzer

Now it’s time to run fuzzing. First, let’s start a local picotls server:

$ echo "I am a picotls server" > message
$ ./cli -c certs/server_cert.pem -k certs/server_key.pem -i message 20101

You can get certificates and keys in tlsbunny repository. Then, let’s run a simple fuzzer for TLSPlaintext structure:

$ git clone
$ cd tlsbunny-and-docker
$ git checkout 561df07499ced89bf0e659754b0b3428a4ffd2c2
$ javac -d classes -classpath src/main/java \
$ java -classpath classes \
       -Dtlsbunny.port=20101 \
       -Dtlsbunny.threads=1 \ \

Fuzzing is not going to be fast because the fuzzer talks to the server via a socket. Furthermore, the fuzzer can set TLSPlaintext.length field greater than the actual size of content of the TLSPlaintext structure. As a result, the server can wait for additional data which in the end results to a timeout.

After the fuzzer is finished, we can stop the server:

$ kill -SIGINT `pidof cli`

Generating an HTML report with lcov

Once fuzzing is finished, we can ask lcov to generate an HTML report for us:

$ mkdir cc_report
$ lcov --no-external --capture \
	--directory . --output-file
$ lcov --add-tracefile --add-tracefile \
$ genhtml --ignore-errors source --legend --title picotls \

First two lcov calls prepare code coverage data for the report and merge it with the baseline we previously created. Finally, genhtml creates an HTML report which will be stored in cc_report directory.

The report showed an expected result: such a simple fuzzer didn’t cover much. Uncovered functions in the report may give an idea which TLS structures may be also fuzzed to improve coverage. For example, server_handle_certificate_verify function from lib/picotls.c was not covered which means that we may consider implementing a fuzzer for client authentication.


Code coverage info may be helpful in fuzzing. But if you were able to get 100% coverage in your code coverage report, it doesn’t mean that you have tested everything, and your software is free of bugs. Reaching all code during testing may not be enough. It would be really nice to reach all code with all possible memory states. Unfortunately, a code coverage report doesn’t show this kind of information.

Traditionally, fuzzing is used for testing applications written in C/C++ to catch possible memory corruption issues which most likely are going to have security implications. But it doesn’t mean that fuzzing can’t be used for testing applications written in other languages. Even if a language (such Java) doesn’t allow to access memory directly, fuzzing can still uncover bugs. For example, fuzzing may uncover an unexpected runtime exception which occurs during processing incorrect data. No matter which language is used, a good application should properly handle incorrect data and react with an expected action, for example, by throwing a documented exception. An unexpected behavior in processing incorrect data may still have security implications.

Here are a couple of useful articles which helped me a lot: