[llvm-dev] fuzzer crash (but not the good kind)

Thu Dec 3 12:15:50 PST 2015

I am able to build the fuzzer, but not to reproduce the crash.

First: you seem to be using a not-the-most-recent libfuzzer source, please
update.
You will need to change LLVMFuzzerTestOneInput to return 0.

Second, for some reason, the fuzzing is very slow, about 1 exec/s.
You probably want to fix that too.

Third: does this code have threads? dlopen?
Running it for a short period does not show either, but probably the fuzzer
discovers some such input that triggers dlopen in a non-main thread.
I guess the sanitizer coverage has a race in this case.

So, just drop 8bit-counters. (This flag is far from optimal, we may
actually remove it completely at some point).

hth,
--kcc

On Thu, Dec 3, 2015 at 11:14 AM, Brian Cain <brian.cain at gmail.com> wrote:

> Ah, yes -- you need to clone with --recursive.
>
> I will try the workaround though.
> On Dec 3, 2015 1:12 PM, "Kostya Serebryany" <kcc at google.com> wrote:
>
>>
>>
>> On Wed, Dec 2, 2015 at 7:17 PM, Brian Cain <brian.cain at gmail.com> wrote:
>>
>>> Kostya,
>>>
>>> Here's the git repo: https://bitbucket.org/ebadf/fuzzpy
>>>
>>> I've only tested it on arm7 and x86_64 linux, I expect there's a good
>>> chance it may not work on other OSs.
>>>
>>> If you can build it successfully ("./build.sh", requires clang and
>>> clang++ in your path), then you should run the "testemail" case like so:
>>>
>>>
>> Does not build for me out of the box:
>>
>> ./build.sh: line 70: ./configure: No such file or directory
>>
>> I wonder if a smaller test possible here.
>> Meanwhile, here is a workaround for you.
>> Instead of
>> SANITIZE_COV_OPTS="-fsanitize-coverage=bb,indirect-calls,8bit-counters"
>> try using
>> SANITIZE_COV_OPTS="-fsanitize-coverage=edge,indirect-calls"
>>
>>
>> while true; do ITERS=1000 ./run.sh tests/build/testemail
>>> tests/testemail/inputs/; done
>>>
>>> Let me know if you have any challenges building or running the test case.
>>>
>>>
>>> On Tue, Dec 1, 2015 at 7:26 PM, Kostya Serebryany <kcc at google.com>
>>> wrote:
>>>
>>>> Hi Brian,
>>>> Yes, looks like a bug in sanitizer coverage, please send the
>>>> reproducer.
>>>>
>>>> On Tue, Dec 1, 2015 at 5:22 PM, Brian Cain <brian.cain at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>> Kostya,
>>>>>
>>>>> I think I've found what looks like a reproducible bug in libFuzzer.
>>>>> The code under test is built with ASan and the first ASan CHECK failure
>>>>> shows fuzzer in the stack trace.  (see below)
>>>>>
>>>>> One of the factors that may be unique in my testing is that each
>>>>> iteration can take a very long time to execute (tens or hundreds of
>>>>> seconds).
>>>>>
>>>>> Let me know if you need more info, I think it shouldn't take much test
>>>>> time to reproduce this.
>>>>>
>>>>> ================== Job 2 exited with exit code 256 ============
>>>>> Flag: verbosity 3
>>>>> Flag: use_traces 1
>>>>> Flag: timeout 100
>>>>> Flag: max_len 16384
>>>>> Seed: 3259211893
>>>>> PreferSmall: 0
>>>>> #0      READ   units: 4975 exec/s: 0
>>>>> #1      pulse  cov: 32410 bits: 30791 indir: 714 units: 4975 exec/s: 0
>>>>> NEW0: 32410 L 13869
>>>>> ==31301==AddressSanitizer CHECK failed:
>>>>> /home/brian/src/fuzzpy/llvm_src/llvm/projects/compiler-rt/lib/sanitizer_common/sanitizer_coverage_libcdep.cc:467
>>>>> "((n % 16)) == ((0))" (0x1, 0x0)
>>>>>     #0 0x11d3b7 in __asan::AsanCheckFailed(char const*, int, char
>>>>> const*, unsigned long long, unsigned long long)
>>>>> /home/brian/src/fuzzpy/llvm_src/llvm/projects/compiler-rt/lib/asan/asan_rtl.cc:67:3
>>>>>     #1 0x122f1f in __sanitizer::CheckFailed(char const*, int, char
>>>>> const*, unsigned long long, unsigned long long)
>>>>> /home/brian/src/fuzzpy/llvm_src/llvm/projects/compiler-rt/lib/sanitizer_common/sanitizer_common.cc:159:5
>>>>>     #2 0x134317 in
>>>>> __sanitizer::CoverageData::Update8bitCounterBitsetAndClearCounters(unsigned
>>>>> char*)
>>>>> /home/brian/src/fuzzpy/llvm_src/llvm/projects/compiler-rt/lib/sanitizer_common/sanitizer_coverage_libcdep.cc:467:5
>>>>>     #3 0x1b7b53 in fuzzer::Fuzzer::PrepareCoverageBeforeRun()
>>>>> /home/brian/src/fuzzpy/llvm_src//llvm/lib/Fuzzer/FuzzerLoop.cpp:264:5
>>>>>     #4 0x1b501b in fuzzer::Fuzzer::RunOne(std::vector<unsigned char,
>>>>> std::allocator<unsigned char> > const&)
>>>>> /home/brian/src/fuzzpy/llvm_src//llvm/lib/Fuzzer/FuzzerLoop.cpp:212:3
>>>>>     #5 0x1b6be3 in fuzzer::Fuzzer::ShuffleAndMinimize()
>>>>> /home/brian/src/fuzzpy/llvm_src//llvm/lib/Fuzzer/FuzzerLoop.cpp:195:11
>>>>>     #6 0x14477b in fuzzer::FuzzerDriver(std::vector<std::string,
>>>>> std::allocator<std::string> > const&, fuzzer::UserSuppliedFuzzer&)
>>>>> /home/brian/src/fuzzpy/llvm_src//llvm/lib/Fuzzer/FuzzerDriver.cpp:303:3
>>>>>     #7 0x14183f in fuzzer::FuzzerDriver(int, char**,
>>>>> fuzzer::UserSuppliedFuzzer&)
>>>>> /home/brian/src/fuzzpy/llvm_src//llvm/lib/Fuzzer/FuzzerDriver.cpp:201:10
>>>>>     #8 0x141427 in fuzzer::FuzzerDriver(int, char**, int (*)(unsigned
>>>>> char const*, unsigned int))
>>>>> /home/brian/src/fuzzpy/llvm_src//llvm/lib/Fuzzer/FuzzerDriver.cpp:196:10
>>>>>     #9 0x1873e3 in main
>>>>> /home/brian/src/fuzzpy/llvm_src//llvm/lib/Fuzzer/FuzzerMain.cpp:19:10
>>>>>     #10 0xb6c86775 in __libc_start_main
>>>>> /build/buildd/glibc-2.21/csu/libc-start.c:289
>>>>>
>>>>> DEATH:
>>>>> artifact_prefix='./'; Test unit written to
>>>>> ./crash-ec9fa023e9db127e2589d0ab4c506055e4174611
>>>>>
>>>>>
>>>>> --
>>>>> -Brian
>>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> -Brian
>>>
>>
>>
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