[llvm-dev] -sanitizer-coverage-prune-blocks=true and LibFuzzer

Mehdi Amini via llvm-dev
2016-09-21T19:58:38Z

> On Sep 21, 2016, at 12:56 PM, Kostya Serebryany <kcc at google.com>
wrote:
>
>
>
> On Wed, Sep 21, 2016 at 12:32 PM, Mehdi Amini <mehdi.amini at apple.com
<mailto:mehdi.amini at apple.com>> wrote:
>
>> On Sep 21, 2016, at 9:36 AM, Kostya Serebryany via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>
>> Exciting!
>>
>> (btw, I'd prefer libfuzzer at googlegroups.com <mailto:libfuzzer
at googlegroups.com> for such discussions, please start new topics there)
>
> You mean a LLVM library has a separate mailing-list? Why?
>
> Because the topic is very separate.

Can you clarify?

I thought is was about the development/debug/evolution/usability of
http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer/
<http://llvm.org/svn/llvm-project/llvm/trunk/lib/Fuzzer/>

—
Mehid




>
>
> —
> Mehdi
>
>
>>
>> I can reproduce this too, but if i either increase
FUZZER_TESTING_SECONDS to 600 or change seed=1 to seed=2 the problem is gone.
>> Looks like one of the binaries got simply unlucky with a particular
seed.
>> You can observe it like this:
>> for S in 1 2 3 4 5 6; do ./target-asan-8bit-prune-build/fuzzer -seed=$S
-runs=10000000 2>&1 | grep DONE &  done
>> #10000000       DONE   cov: 60 bits: 91 indir: 1 units: 59 exec/s:
625000
>> #10000000       DONE   cov: 60 bits: 91 indir: 1 units: 57 exec/s:
588235
>> #10000000       DONE   cov: 253 bits: 901 indir: 12 units: 467 exec/s:
526315
>> #10000000       DONE   cov: 63 bits: 95 indir: 1 units: 64 exec/s:
476190
>> #10000000       DONE   cov: 252 bits: 923 indir: 12 units: 491 exec/s:
454545
>> #10000000       DONE   cov: 253 bits: 880 indir: 12 units: 471 exec/s:
384615
>>
>> Similar things happen with other binaries:
>> for S in 1 2 3 4 5 6; do ./target-asan-8bit-nopru-build/fuzzer -seed=$S
-runs=10000000 2>&1 | grep DONE &  done
>> #10000000       DONE   cov: 103 bits: 190 indir: 1 units: 62 exec/s:
526315
>> #10000000       DONE   cov: 443 bits: 1730 indir: 12 units: 529 exec/s:
357142
>> #10000000       DONE   cov: 443 bits: 1695 indir: 12 units: 509 exec/s:
344827
>> #10000000       DONE   cov: 443 bits: 1682 indir: 12 units: 500 exec/s:
333333
>> #10000000       DONE   cov: 444 bits: 1675 indir: 12 units: 501 exec/s:
277777
>> #10000000       DONE   cov: 401 bits: 1443 indir: 12 units: 341 exec/s:
263157
>>
>> I've also tried building with trace-pc-guard (the new thing) and
results are similar.
>> name    cov     bits    execs   execs_per_sec   units   actual_cov
actual_bits
>> asan-8bit-nopru 401     1443    19790806        324439  340     401
1441
>> asan-8bit-prune 256     897     26528866        434899  485     447
1651
>> asan-edge-nopru 447     0       35589496        583434  137     447
719
>> asan-edge-prune 256     0       37576436        616007  137     447
719
>> asan-trac-nopru 401     1443    12566606        206009  340     401
1441
>> asan-trac-prune 256     891     16295346        267136  480     447
1640
>>
>> Conclusions:
>> * testing a fuzzing engine is not trivial :(
>> * testing it on a very short run with a single seed may be misleading
>>
>>
>> BTW, I am also looking into more automation of libFuzzer testing.
>> With trace-pc-guard we now have libFuzzer's flag -print_coverage=1
that will print all the covered lines.
>> My hope is that this feature can be used for more detailed analysis of
coverage differences.
>>
>> --kcc
>>
>>
>> On Wed, Sep 21, 2016 at 6:00 AM, Jonas Wagner <jonas.wagner at
epfl.ch <mailto:jonas.wagner at epfl.ch>> wrote:
>> Hello,
>>
>> Is this reproducible?
>> Fuzzing is a probabilistic business and one or even two runs don't
prove much.
>>
>> I've reproduced the behavior on two different machines. Attached is
a script to do so. To use the script,
>>
>> - create an empty folder and copy both prune-blocks.sh and
ff-http-parser.sh in there
>> - ensure clang and clang++ are in your $PATH
>> - cd /path/to/prune-blocks.sh
>> - ./prune-blocks.sh
>>
>> Let me know how it goes.
>>
>>
>> Note that I am going to change all of these coverage options soon.
>> The new thing will be
http://clang.llvm.org/docs/SanitizerCoverage.html#tracing-pcs-with-guards
<http://clang.llvm.org/docs/SanitizerCoverage.html#tracing-pcs-with-guards>
>> It will replace regular (boolean) and 8-bit-counters coverage.
>>
>> Yay, sounds exciting! I've done a couple experiments to measure the
performance and effect of the different coverage options in the recent past. If
you're interested, I'd be happy to discuss off-list; simply send me an
email.
>>
>> Best,
>> Jonas
>>
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