(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)