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Code Coverage

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For projects written in C/C++, you can generate code coverage reports using Clang source-based code coverage. This page walks you through the basic steps. For more details, see Clang's documentation.

Code coverage reports generation for other languages is not supported yet.

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Pull the latest Docker images

Docker images get regularly updated with a newer version of build tools, build configurations, scripts, and other changes. We recommend you pull the most recent images by running the following command:

$ python infra/helper.py pull_images

Build fuzz targets

Code coverage report generation requires a special build configuration to be used. To create a code coverage build for your project, run these commands:

$ python infra/helper.py build_image $PROJECT_NAME
$ python infra/helper.py build_fuzzers --sanitizer=coverage $PROJECT_NAME

Establish access to GCS

To get a good understanding of fuzz testing quality, you should generate code coverage reports by running fuzz targets against the corpus aggregated by OSS-Fuzz. Set up gsutil and ensure that you have access to the corpora by doing the following:

Install the gsutil tool.
Check whether you have access to the corpus for your project:

$ gsutil ls gs://${PROJECT_NAME}-corpus.clusterfuzz-external.appspot.com/

If you see an authorization error from the command above, run this:

$ gcloud auth login

and try again. Once gsutil works, you can run the report generation.

Generate code coverage reports

Full project report

If you want to generate a code coverage report using the corpus aggregated on OSS-Fuzz, run this command:

$ python infra/helper.py coverage $PROJECT_NAME

If you want to generate a code coverage report using the corpus you have locally, copy the corpus into the build/corpus/$PROJECT_NAME/<fuzz_target_name>/ directories for each fuzz target, then run this command:

$ python infra/helper.py coverage --no-corpus-download $PROJECT_NAME

Single fuzz target

You can generate a code coverage report for a particular fuzz target by using the --fuzz-target argument:

$ python infra/helper.py coverage --fuzz-target=<fuzz_target_name> $PROJECT_NAME

In this mode, you can specify an arbitrary corpus location for the fuzz target (instead of the corpus downloaded from OSS-Fuzz) by using --corpus-dir:

$ python infra/helper.py coverage --fuzz-target=<fuzz_target_name> \
    --corpus-dir=<my_local_corpus_dir> $PROJECT_NAME

Additional arguments for `llvm-cov`

You may want to use some of the options provided by the llvm-cov tool, like -ignore-filename-regex=. You can pass these to the helper script after --:

$ python infra/helper.py coverage $PROJECT_NAME -- \
    -ignore-filename-regex=.*code/to/be/ignored/.* <other_extra_args>

If you want to specify particular source files or directories to show in the report, list their paths at the end of the extra arguments sequence:

$ python infra/helper.py coverage zlib -- \
    <other_extra_args> /src/zlib/inftrees.c /src/zlib_uncompress_fuzzer.cc /src/zlib/zutil.c

If you want OSS-Fuzz to use extra arguments when generating code coverage reports for your project, add the arguments into your project.yaml file as follows:

coverage_extra_args: -ignore-filename-regex=.*crc.* -ignore-filename-regex=.*adler.* <other_extra_args>

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