[llvm-dev] -sanitizer-coverage-prune-blocks=true and LibFuzzer (original) (raw)

Kostya Serebryany via llvm-dev llvm-dev at lists.llvm.org
Wed Sep 21 12:56:02 PDT 2016

On Wed, Sep 21, 2016 at 12:32 PM, Mehdi Amini <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> wrote: Exciting! (btw, I'd prefer 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.

— Mehdi

I can reproduce this too, but if i either increase FUZZERTESTINGSECONDS 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 execspersec units actualcov actualbits 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 -printcoverage=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> 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 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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