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strcmp() / memcmp() token capture library

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

This Linux-only 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

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 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/ \
      /path/to/target/program [...params, including $i...]

  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.

PS. The library is Linux-only because 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. Porting to platforms with /proc/<pid>/maps
(e.g., FreeBSD) should be trivial.

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