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

[Mehdi Amini via llvm-dev]

> On Sep 21, 2016, at 9:36 AM, Kostya Serebryany via llvm-dev <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?

— 
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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