hcl/hclsyntax/fuzz/README.md

hclsyntax fuzzing utilities

This directory contains helper functions and corpuses that can be used to fuzz-test the hclsyntax parsers using go-fuzz.

To fuzz, first install go-fuzz and its build tool in your GOPATH:

$ make tools

Now you can fuzz one or all of the parsers:

$ make fuzz-config FUZZ_WORK_DIR=/tmp/hcl2-fuzz-config
$ make fuzz-expr  FUZZ_WORK_DIR=/tmp/hcl2-fuzz-expr
$ make fuzz-template  FUZZ_WORK_DIR=/tmp/hcl2-fuzz-template
$ make fuzz-traversal FUZZ_WORK_DIR=/tmp/hcl2-fuzz-traversal

In all cases, set FUZZ_WORK_DIR to a directory where go-fuzz can keep state as it works. This should ideally be in a ramdisk for efficiency, and should probably not be on an SSD to avoid thrashing it.

Understanding the result

A small number of subdirectories will be created in the work directory.

If you let go-fuzz run for a few minutes (the more minutes the better) it may detect "crashers", which are inputs that caused the parser to panic. Details about these are written to $FUZZ_WORK_DIR/crashers:

$ ls /tmp/hcl2-fuzz-config/crashers
7f5e9ec80c89da14b8b0b238ec88969f658f5a2d
7f5e9ec80c89da14b8b0b238ec88969f658f5a2d.output
7f5e9ec80c89da14b8b0b238ec88969f658f5a2d.quoted

The base file above (with no extension) is the input that caused a crash. The .output file contains the panic stack trace, which you can use as a clue to figure out what caused the crash.

A good first step to fixing a detected crasher is to copy the failing input into one of the unit tests in the hclsyntax package and see it crash there too. After that, it's easy to re-run the test as you try to fix it. The file with the .quoted extension contains a form of the input that is quoted in Go syntax for easy copy-paste into a test case, even if the input contains non-printable characters or other inconvenient symbols.

Rebuilding for new Upstream Code

An archive file is created for go-fuzz to use on the first run of each of the above, as a .zip file created in this directory. If upstream code is changed these will need to be deleted to cause them to be rebuilt with the latest code:

$ make clean