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========================================================
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LibFuzzer -- a library for coverage-guided fuzz testing.
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========================================================
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.. contents::
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:local:
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:depth: 1
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Introduction
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============
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libFuzzer -- library for in-process evolutionary fuzzing of other libraries.
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The typical workflow looks like the following.
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First, implement a fuzzing target function, like this::
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// fuzz_target.cc
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extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
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DoSomethingInterestingWithMyAPI(Data, Size);
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return 0;
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}
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Next, build the Fuzzer library as a static archive. Note that libFuzzer contains the `main()` function::
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svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
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clang++ -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer
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ar ruv libFuzzer.a Fuzzer*.o
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Then build the target function and the library you are going to test.
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You should use SanitizerCoverage_ and one of ASan, MSan, or UBSan.
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Link it with `libFuzzer.a`::
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clang -fsanitize-coverage=edge -fsanitize=address your_lib.cc fuzz_target.cc libFuzzer.a -o my_fuzzer
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Create a directory with the initial "seed" samlpes.
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For some input types libFuzzer will work just fine w/o any seeds,
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but for complex inputs this step is very important::
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mkdir CORPUS_DIR
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cp /some/input/samples/* CORPUS_DIR
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Finally, run the fuzzer on the `CORPUS_DIR`::
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./my_fuzzer CORPUS_DIR # -max_len=1000 -jobs=20 -more_lags=...
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As new interesting test cases are discovered they will be added to the corpus.
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If a bug is discovered by the sanitizer (ASan, etc) it will be reported as usual and the reproducer
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will be written to disk.
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Each Fuzzer process is single-threaded (unless the library starts its own
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threads). You can run the libFuzzer on the same corpus in multiple processes
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in parallel (use the flags `-jobs=N` and `-workers=N`).
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libFuzzer is similar in concept to AFL_,
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but uses in-process Fuzzing, which is more fragile and restrictive, but
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potentially much faster as it has no overhead for process start-up.
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It uses LLVM's SanitizerCoverage_ instrumentation to get in-process
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coverage-feedback
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The code resides in the LLVM repository, requires the fresh Clang compiler to build
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and is used to fuzz various parts of LLVM,
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but the Fuzzer itself does not (and should not) depend on any
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part of LLVM and can be used for other projects w/o requiring the rest of LLVM.
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Usage
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=====
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To run fuzzing pass 0 or more directories. New samples will be written into `dir1`, other directories will be read once during startup.::
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./fuzzer [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ]
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To run individual tests without fuzzing pass 1 or more files::
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./fuzzer [-flag1=val1 [-flag2=val2 ...] ] file1 [file2 ...]
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The most important flags are::
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seed 0 Random seed. If 0, seed is generated.
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runs -1 Number of individual test runs (-1 for infinite runs).
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max_len 0 Maximum length of the test input. If 0, libFuzzer tries to guess a good value based on the corpus and reports it.
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timeout 1200 Timeout in seconds (if positive). If one unit runs more than this number of seconds the process will abort.
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timeout_exitcode 77 Unless abort_on_timeout is set, use this exitcode on timeout.
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max_total_time 0 If positive, indicates the maximal total time in seconds to run the fuzzer.
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help 0 Print help.
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merge 0 If 1, the 2-nd, 3-rd, etc corpora will be merged into the 1-st corpus. Only interesting units will be taken.
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jobs 0 Number of jobs to run. If jobs >= 1 we spawn this number of jobs in separate worker processes with stdout/stderr redirected to fuzz-JOB.log.
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workers 0 Number of simultaneous worker processes to run the jobs. If zero, "min(jobs,NumberOfCpuCores()/2)" is used.
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use_traces 0 Experimental: use instruction traces
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only_ascii 0 If 1, generate only ASCII (isprint+isspace) inputs.
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artifact_prefix "" Write fuzzing artifacts (crash, timeout, or slow inputs) as $(artifact_prefix)file
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exact_artifact_path "" Write the single artifact on failure (crash, timeout) as $(exact_artifact_path). This overrides -artifact_prefix and will not use checksum in the file name. Do not use the same path for several parallel processes.
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print_final_stats 0 If 1, print statistics at exit.
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close_fd_mask 0 If 1, close stdout at startup; if 2, close stderr; if 3, close both.
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For the full list of flags run the fuzzer binary with ``-help=1``.
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Usage examples
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==============
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.. contents::
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:local:
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:depth: 1
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Toy example
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-----------
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A simple function that does something interesting if it receives the input "HI!"::
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cat << EOF >> test_fuzzer.cc
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#include <stdint.h>
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#include <stddef.h>
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extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
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if (size > 0 && data[0] == 'H')
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if (size > 1 && data[1] == 'I')
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if (size > 2 && data[2] == '!')
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__builtin_trap();
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return 0;
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}
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EOF
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# Build test_fuzzer.cc with asan and link against libFuzzer.a
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clang++ -fsanitize=address -fsanitize-coverage=edge test_fuzzer.cc libFuzzer.a
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# Run the fuzzer with no corpus.
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./a.out
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You should get an error pretty quickly::
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#0 READ units: 1 exec/s: 0
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#1 INITED cov: 3 units: 1 exec/s: 0
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#2 NEW cov: 5 units: 2 exec/s: 0 L: 64 MS: 0
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#19237 NEW cov: 9 units: 3 exec/s: 0 L: 64 MS: 0
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#20595 NEW cov: 10 units: 4 exec/s: 0 L: 1 MS: 4 ChangeASCIIInt-ShuffleBytes-ChangeByte-CrossOver-
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#34574 NEW cov: 13 units: 5 exec/s: 0 L: 2 MS: 3 ShuffleBytes-CrossOver-ChangeBit-
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#34807 NEW cov: 15 units: 6 exec/s: 0 L: 3 MS: 1 CrossOver-
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==31511== ERROR: libFuzzer: deadly signal
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...
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artifact_prefix='./'; Test unit written to ./crash-b13e8756b13a00cf168300179061fb4b91fefbed
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PCRE2
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-----
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Here we show how to use libFuzzer on something real, yet simple: pcre2_::
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COV_FLAGS=" -fsanitize-coverage=edge,indirect-calls,8bit-counters"
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# Get PCRE2
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svn co svn://vcs.exim.org/pcre2/code/trunk pcre
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# Build PCRE2 with AddressSanitizer and coverage.
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(cd pcre; ./autogen.sh; CC="clang -fsanitize=address $COV_FLAGS" ./configure --prefix=`pwd`/../inst && make -j && make install)
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# Build the fuzzing target function that does something interesting with PCRE2.
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cat << EOF > pcre_fuzzer.cc
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#include <string.h>
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#include <stdint.h>
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#include "pcre2posix.h"
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extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
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if (size < 1) return 0;
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char *str = new char[size+1];
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memcpy(str, data, size);
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str[size] = 0;
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regex_t preg;
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if (0 == regcomp(&preg, str, 0)) {
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regexec(&preg, str, 0, 0, 0);
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regfree(&preg);
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}
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delete [] str;
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return 0;
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}
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EOF
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clang++ -g -fsanitize=address $COV_FLAGS -c -std=c++11 -I inst/include/ pcre_fuzzer.cc
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# Link.
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clang++ -g -fsanitize=address -Wl,--whole-archive inst/lib/*.a -Wl,-no-whole-archive libFuzzer.a pcre_fuzzer.o -o pcre_fuzzer
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This will give you a binary of the fuzzer, called ``pcre_fuzzer``.
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Now, create a directory that will hold the test corpus::
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mkdir -p CORPUS
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For simple input languages like regular expressions this is all you need.
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For more complicated inputs populate the directory with some input samples.
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Now run the fuzzer with the corpus dir as the only parameter::
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./pcre_fuzzer ./CORPUS
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You will see output like this::
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Seed: 1876794929
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#0 READ cov 0 bits 0 units 1 exec/s 0
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#1 pulse cov 3 bits 0 units 1 exec/s 0
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#1 INITED cov 3 bits 0 units 1 exec/s 0
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#2 pulse cov 208 bits 0 units 1 exec/s 0
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#2 NEW cov 208 bits 0 units 2 exec/s 0 L: 64
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#3 NEW cov 217 bits 0 units 3 exec/s 0 L: 63
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#4 pulse cov 217 bits 0 units 3 exec/s 0
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* The ``Seed:`` line shows you the current random seed (you can change it with ``-seed=N`` flag).
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* The ``READ`` line shows you how many input files were read (since you passed an empty dir there were inputs, but one dummy input was synthesised).
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* The ``INITED`` line shows you that how many inputs will be fuzzed.
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* The ``NEW`` lines appear with the fuzzer finds a new interesting input, which is saved to the CORPUS dir. If multiple corpus dirs are given, the first one is used.
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* The ``pulse`` lines appear periodically to show the current status.
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Now, interrupt the fuzzer and run it again the same way. You will see::
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Seed: 1879995378
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#0 READ cov 0 bits 0 units 564 exec/s 0
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#1 pulse cov 502 bits 0 units 564 exec/s 0
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...
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#512 pulse cov 2933 bits 0 units 564 exec/s 512
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#564 INITED cov 2991 bits 0 units 344 exec/s 564
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#1024 pulse cov 2991 bits 0 units 344 exec/s 1024
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#1455 NEW cov 2995 bits 0 units 345 exec/s 1455 L: 49
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This time you were running the fuzzer with a non-empty input corpus (564 items).
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As the first step, the fuzzer minimized the set to produce 344 interesting items (the ``INITED`` line)
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You may run ``N`` independent fuzzer jobs in parallel on ``M`` CPUs::
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N=100; M=4; ./pcre_fuzzer ./CORPUS -jobs=$N -workers=$M
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By default (``-reload=1``) the fuzzer processes will periodically scan the CORPUS directory
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and reload any new tests. This way the test inputs found by one process will be picked up
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by all others.
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If ``-workers=$M`` is not supplied, ``min($N,NumberOfCpuCore/2)`` will be used.
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Heartbleed
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----------
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Remember Heartbleed_?
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As it was recently `shown <https://blog.hboeck.de/archives/868-How-Heartbleed-couldve-been-found.html>`_,
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fuzzing with AddressSanitizer can find Heartbleed. Indeed, here are the step-by-step instructions
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to find Heartbleed with LibFuzzer::
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wget https://www.openssl.org/source/openssl-1.0.1f.tar.gz
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tar xf openssl-1.0.1f.tar.gz
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COV_FLAGS="-fsanitize-coverage=edge,indirect-calls" # -fsanitize-coverage=8bit-counters
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(cd openssl-1.0.1f/ && ./config &&
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make -j 32 CC="clang -g -fsanitize=address $COV_FLAGS")
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# Get and build LibFuzzer
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svn co http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer
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clang -c -g -O2 -std=c++11 Fuzzer/*.cpp -IFuzzer
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# Get examples of key/pem files.
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git clone https://github.com/hannob/selftls
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cp selftls/server* . -v
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cat << EOF > handshake-fuzz.cc
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#include <openssl/ssl.h>
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#include <openssl/err.h>
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#include <assert.h>
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#include <stdint.h>
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#include <stddef.h>
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SSL_CTX *sctx;
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int Init() {
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SSL_library_init();
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SSL_load_error_strings();
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ERR_load_BIO_strings();
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OpenSSL_add_all_algorithms();
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assert (sctx = SSL_CTX_new(TLSv1_method()));
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assert (SSL_CTX_use_certificate_file(sctx, "server.pem", SSL_FILETYPE_PEM));
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assert (SSL_CTX_use_PrivateKey_file(sctx, "server.key", SSL_FILETYPE_PEM));
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return 0;
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}
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extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
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static int unused = Init();
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SSL *server = SSL_new(sctx);
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BIO *sinbio = BIO_new(BIO_s_mem());
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BIO *soutbio = BIO_new(BIO_s_mem());
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SSL_set_bio(server, sinbio, soutbio);
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SSL_set_accept_state(server);
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BIO_write(sinbio, Data, Size);
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SSL_do_handshake(server);
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SSL_free(server);
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return 0;
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}
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EOF
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# Build the fuzzer.
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clang++ -g handshake-fuzz.cc -fsanitize=address \
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openssl-1.0.1f/libssl.a openssl-1.0.1f/libcrypto.a Fuzzer*.o
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# Run 20 independent fuzzer jobs.
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./a.out -jobs=20 -workers=20
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Voila::
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#1048576 pulse cov 3424 bits 0 units 9 exec/s 24385
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=================================================================
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==17488==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x629000004748 at pc 0x00000048c979 bp 0x7fffe3e864f0 sp 0x7fffe3e85ca8
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READ of size 60731 at 0x629000004748 thread T0
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#0 0x48c978 in __asan_memcpy
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#1 0x4db504 in tls1_process_heartbeat openssl-1.0.1f/ssl/t1_lib.c:2586:3
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#2 0x580be3 in ssl3_read_bytes openssl-1.0.1f/ssl/s3_pkt.c:1092:4
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Note: a `similar fuzzer <https://boringssl.googlesource.com/boringssl/+/HEAD/FUZZING.md>`_
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is now a part of the boringssl source tree.
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Advanced features
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=================
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.. contents::
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:local:
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:depth: 1
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Dictionaries
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------------
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*EXPERIMENTAL*.
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LibFuzzer supports user-supplied dictionaries with input language keywords
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or other interesting byte sequences (e.g. multi-byte magic values).
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Use ``-dict=DICTIONARY_FILE``. For some input languages using a dictionary
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may significantly improve the search speed.
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The dictionary syntax is similar to that used by AFL_ for its ``-x`` option::
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# Lines starting with '#' and empty lines are ignored.
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# Adds "blah" (w/o quotes) to the dictionary.
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kw1="blah"
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# Use \\ for backslash and \" for quotes.
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kw2="\"ac\\dc\""
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# Use \xAB for hex values
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kw3="\xF7\xF8"
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# the name of the keyword followed by '=' may be omitted:
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"foo\x0Abar"
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Data-flow-guided fuzzing
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------------------------
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*EXPERIMENTAL*.
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With an additional compiler flag ``-fsanitize-coverage=trace-cmp`` (see SanitizerCoverageTraceDataFlow_)
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and extra run-time flag ``-use_traces=1`` the fuzzer will try to apply *data-flow-guided fuzzing*.
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That is, the fuzzer will record the inputs to comparison instructions, switch statements,
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and several libc functions (``memcmp``, ``strcmp``, ``strncmp``, etc).
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It will later use those recorded inputs during mutations.
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This mode can be combined with DataFlowSanitizer_ to achieve better sensitivity.
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AFL compatibility
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-----------------
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LibFuzzer can be used in parallel with AFL_ on the same test corpus.
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Both fuzzers expect the test corpus to reside in a directory, one file per input.
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You can run both fuzzers on the same corpus in parallel::
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./afl-fuzz -i testcase_dir -o findings_dir /path/to/program -r @@
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./llvm-fuzz testcase_dir findings_dir # Will write new tests to testcase_dir
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Periodically restart both fuzzers so that they can use each other's findings.
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How good is my fuzzer?
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----------------------
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Once you implement your target function ``LLVMFuzzerTestOneInput`` and fuzz it to death,
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you will want to know whether the function or the corpus can be improved further.
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One easy to use metric is, of course, code coverage.
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You can get the coverage for your corpus like this::
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ASAN_OPTIONS=coverage=1 ./fuzzer CORPUS_DIR -runs=0
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This will run all the tests in the CORPUS_DIR but will not generate any new tests
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and dump covered PCs to disk before exiting.
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Then you can subtract the set of covered PCs from the set of all instrumented PCs in the binary,
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see SanitizerCoverage_ for details.
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User-supplied mutators
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----------------------
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LibFuzzer allows to use custom (user-supplied) mutators,
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see FuzzerInterface.h_
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Startup initialization
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----------------------
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If the library being tested needs to be initialized, there are several options.
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The simplest way is to have a statically initialized global object::
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static bool Initialized = DoInitialization();
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Alternatively, you may define an optional init function and it will receive
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the program arguments that you can read and modify::
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extern "C" int LLVMFuzzerInitialize(int *argc, char ***argv) {
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ReadAndMaybeModify(argc, argv);
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return 0;
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}
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Try to avoid initialization inside the target function itself as
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it will skew the coverage data. Don't do this::
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extern "C" int LLVMFuzzerTestOneInput(...) {
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static bool initialized = false;
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if (!initialized) {
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...
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}
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}
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Fuzzing components of LLVM
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==========================
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.. contents::
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:local:
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:depth: 1
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clang-format-fuzzer
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-------------------
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The inputs are random pieces of C++-like text.
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Build (make sure to use fresh clang as the host compiler)::
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cmake -GNinja -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DLLVM_USE_SANITIZER=Address -DLLVM_USE_SANITIZE_COVERAGE=YES -DCMAKE_BUILD_TYPE=Release /path/to/llvm
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ninja clang-format-fuzzer
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mkdir CORPUS_DIR
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./bin/clang-format-fuzzer CORPUS_DIR
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Optionally build other kinds of binaries (asan+Debug, msan, ubsan, etc).
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Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=23052
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clang-fuzzer
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------------
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The behavior is very similar to ``clang-format-fuzzer``.
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Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=23057
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llvm-as-fuzzer
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--------------
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Tracking bug: https://llvm.org/bugs/show_bug.cgi?id=24639
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llvm-mc-fuzzer
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--------------
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This tool fuzzes the MC layer. Currently it is only able to fuzz the
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disassembler but it is hoped that assembly, and round-trip verification will be
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added in future.
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When run in dissassembly mode, the inputs are opcodes to be disassembled. The
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fuzzer will consume as many instructions as possible and will stop when it
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finds an invalid instruction or runs out of data.
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Please note that the command line interface differs slightly from that of other
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fuzzers. The fuzzer arguments should follow ``--fuzzer-args`` and should have
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a single dash, while other arguments control the operation mode and target in a
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similar manner to ``llvm-mc`` and should have two dashes. For example::
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llvm-mc-fuzzer --triple=aarch64-linux-gnu --disassemble --fuzzer-args -max_len=4 -jobs=10
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Buildbot
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--------
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We have a buildbot that runs the above fuzzers for LLVM components
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24/7/365 at http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-fuzzer .
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FAQ
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=========================
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Q. Why libFuzzer does not use any of the LLVM support?
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------------------------------------------------------
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There are two reasons.
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First, we want this library to be used outside of the LLVM w/o users having to
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build the rest of LLVM. This may sound unconvincing for many LLVM folks,
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but in practice the need for building the whole LLVM frightens many potential
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users -- and we want more users to use this code.
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Second, there is a subtle technical reason not to rely on the rest of LLVM, or
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any other large body of code (maybe not even STL). When coverage instrumentation
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is enabled, it will also instrument the LLVM support code which will blow up the
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coverage set of the process (since the fuzzer is in-process). In other words, by
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using more external dependencies we will slow down the fuzzer while the main
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reason for it to exist is extreme speed.
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Q. What about Windows then? The Fuzzer contains code that does not build on Windows.
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------------------------------------------------------------------------------------
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Volunteers are welcome.
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Q. When this Fuzzer is not a good solution for a problem?
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---------------------------------------------------------
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* If the test inputs are validated by the target library and the validator
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asserts/crashes on invalid inputs, in-process fuzzing is not applicable.
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* Bugs in the target library may accumulate w/o being detected. E.g. a memory
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corruption that goes undetected at first and then leads to a crash while
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testing another input. This is why it is highly recommended to run this
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in-process fuzzer with all sanitizers to detect most bugs on the spot.
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* It is harder to protect the in-process fuzzer from excessive memory
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consumption and infinite loops in the target library (still possible).
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* The target library should not have significant global state that is not
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reset between the runs.
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* Many interesting target libs are not designed in a way that supports
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the in-process fuzzer interface (e.g. require a file path instead of a
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byte array).
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* If a single test run takes a considerable fraction of a second (or
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|
more) the speed benefit from the in-process fuzzer is negligible.
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* If the target library runs persistent threads (that outlive
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execution of one test) the fuzzing results will be unreliable.
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Q. So, what exactly this Fuzzer is good for?
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--------------------------------------------
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This Fuzzer might be a good choice for testing libraries that have relatively
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small inputs, each input takes < 10ms to run, and the library code is not expected
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to crash on invalid inputs.
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Examples: regular expression matchers, text or binary format parsers, compression,
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network, crypto.
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Trophies
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|
========
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* GLIBC: https://sourceware.org/glibc/wiki/FuzzingLibc
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* MUSL LIBC:
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* http://git.musl-libc.org/cgit/musl/commit/?id=39dfd58417ef642307d90306e1c7e50aaec5a35c
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* http://www.openwall.com/lists/oss-security/2015/03/30/3
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* `pugixml <https://github.com/zeux/pugixml/issues/39>`_
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* PCRE: Search for "LLVM fuzzer" in http://vcs.pcre.org/pcre2/code/trunk/ChangeLog?view=markup;
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also in `bugzilla <https://bugs.exim.org/buglist.cgi?bug_status=__all__&content=libfuzzer&no_redirect=1&order=Importance&product=PCRE&query_format=specific>`_
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* `ICU <http://bugs.icu-project.org/trac/ticket/11838>`_
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* `Freetype <https://savannah.nongnu.org/search/?words=LibFuzzer&type_of_search=bugs&Search=Search&exact=1#options>`_
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* `Harfbuzz <https://github.com/behdad/harfbuzz/issues/139>`_
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* `SQLite <http://www3.sqlite.org/cgi/src/info/088009efdd56160b>`_
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* `Python <http://bugs.python.org/issue25388>`_
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|
* OpenSSL/BoringSSL: `[1] <https://boringssl.googlesource.com/boringssl/+/cb852981cd61733a7a1ae4fd8755b7ff950e857d>`_ `[2] <https://openssl.org/news/secadv/20160301.txt>`_ `[3] <https://boringssl.googlesource.com/boringssl/+/2b07fa4b22198ac02e0cee8f37f3337c3dba91bc>`_ `[4] <https://boringssl.googlesource.com/boringssl/+/6b6e0b20893e2be0e68af605a60ffa2cbb0ffa64>`_
|
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|
|
* `Libxml2
|
|
<https://bugzilla.gnome.org/buglist.cgi?bug_status=__all__&content=libFuzzer&list_id=68957&order=Importance&product=libxml2&query_format=specific>`_
|
|
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|
* `Linux Kernel's BPF verifier <https://github.com/iovisor/bpf-fuzzer>`_
|
|
|
|
* LLVM: `Clang <https://llvm.org/bugs/show_bug.cgi?id=23057>`_, `Clang-format <https://llvm.org/bugs/show_bug.cgi?id=23052>`_, `libc++ <https://llvm.org/bugs/show_bug.cgi?id=24411>`_, `llvm-as <https://llvm.org/bugs/show_bug.cgi?id=24639>`_, Disassembler: http://reviews.llvm.org/rL247405, http://reviews.llvm.org/rL247414, http://reviews.llvm.org/rL247416, http://reviews.llvm.org/rL247417, http://reviews.llvm.org/rL247420, http://reviews.llvm.org/rL247422.
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.. _pcre2: http://www.pcre.org/
|
|
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|
.. _AFL: http://lcamtuf.coredump.cx/afl/
|
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|
|
.. _SanitizerCoverage: http://clang.llvm.org/docs/SanitizerCoverage.html
|
|
.. _SanitizerCoverageTraceDataFlow: http://clang.llvm.org/docs/SanitizerCoverage.html#tracing-data-flow
|
|
.. _DataFlowSanitizer: http://clang.llvm.org/docs/DataFlowSanitizer.html
|
|
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|
.. _Heartbleed: http://en.wikipedia.org/wiki/Heartbleed
|
|
|
|
.. _FuzzerInterface.h: https://github.com/llvm-mirror/llvm/blob/master/lib/Fuzzer/FuzzerInterface.h
|