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README.tokencap

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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
+  -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.
+
+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.
+