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README.tokencap.md

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strcmp() / memcmp() token capture library

(See ../docs/README for the general instruction manual.)

This companion library allows you to instrument strcmp(), memcmp(), and related functions to automatically extract syntax tokens passed to any of these libcalls. The resulting list of tokens may be then given as a starting dictionary to afl-fuzz (the -x option) to improve coverage on subsequent fuzzing runs.

This may help improving coverage in some targets, and do precisely nothing in others. In some cases, it may even make things worse: if libtokencap picks up syntax tokens that are not used to process the input data, but that are a part of - say - parsing a config file... well, you're going to end up wasting a lot of CPU time on trying them out in the input stream. In other words, use this feature with care. Manually screening the resulting dictionary is almost always a necessity.

As for the actual operation: the library stores tokens, without any deduping, by appending them to a file specified via AFL_TOKEN_FILE. If the variable is not set, the tool uses stderr (which is probably not what you want).

Similarly to afl-tmin, the library is not "proprietary" and can be used with other fuzzers or testing tools without the need for any code tweaks. It does not require AFL-instrumented binaries to work.

To use the library, you need to make sure that your fuzzing target is compiled with -fno-builtin and is linked dynamically. If you wish to automate the first part without mucking with CFLAGS in Makefiles, you can set AFL_NO_BUILTIN=1 when using afl-gcc. This setting specifically adds the following flags:

  -fno-builtin-strcmp -fno-builtin-strncmp -fno-builtin-strcasecmp
  -fno-builtin-strcasencmp -fno-builtin-memcmp -fno-builtin-strstr
  -fno-builtin-strcasestr

The next step is simply loading this library via LD_PRELOAD. The optimal usage pattern is to allow afl-fuzz to fuzz normally for a while and build up a corpus, and then fire off the target binary, with libtokencap.so loaded, on every file found by AFL in that earlier run. This demonstrates the basic principle:

  export AFL_TOKEN_FILE=$PWD/temp_output.txt

  for i in <out_dir>/queue/id*; do
    LD_PRELOAD=/path/to/libtokencap.so \
      /path/to/target/program [...params, including $i...]
  done

  sort -u temp_output.txt >afl_dictionary.txt

If you don't get any results, the target library is probably not using strcmp() and memcmp() to parse input; or you haven't compiled it with -fno-builtin; or the whole thing isn't dynamically linked, and LD_PRELOAD is having no effect.

Portability hints: There is probably no particularly portable and non-invasive way to distinguish between read-only and read-write memory mappings. The __tokencap_load_mappings() function is the only thing that would need to be changed for other OSes.

Current supported OSes are: Linux, Darwin, FreeBSD (thanks to @devnexen)