[llvm-dev] libfuzzer questions

Brian Cain via llvm-dev llvm-dev at lists.llvm.org
Tue Aug 11 17:51:46 PDT 2015

On Tue, Aug 11, 2015 at 7:25 PM, Kostya Serebryany <kcc at google.com> wrote:

> ...
>> So if I'm seeing tens of thousands of distinct test files, that
>> represents tens of thousands of distinct edges?
> In the extreme case -- yes.
> However usually a single file covers more than one unique edge.
> Also, if you are running the fuzzer in parallel (-jobs=N) some edges can
> be discovered many times.
> ...

> With -fsanitize-coverage=indirect-calls it will also track indir call
> edges (uniq pairs of caller-callee).
Ok, I think the parallel jobs and unique caller/callee pairs must be where
it got amped up a bit.  I'm using "bb,indirect-calls,8bit-counters".

>  save_minimized_corpus               0 If 1, the minimized corpus is
>>> saved into the first input directory
>>> -------------
>> Ohh, ok.  I think I misunderstood this to trying to minimize the size of
>> the test case while still reproducing a crash.  Similar to how afl-tmin
>> works, I was thinking.  I'll give this a try.
>> Should I only use this option periodically or can I run it this way all
>> the time?  Do we end up spending more execution time minimizing the
>> corpus?  Will it delete redundant test cases, including ones that were
>> there before this test run started?
> You should only use this option if you want to store the minimized corpus
> somewhere,

Ok, so I can do it periodically to prune the test corpus.  That's great.

> or if the initial stage (between  "#0      READ" and "#1331   INITED")
> takes too long.
> Otherwise you should not bother since libFuzzer minimizes the corpus in
> memory on every run.
> (minimization is done with a trivial greedy algorithm, not even close to
> really minimal solution, but good enough).
> The output looks like this:
> #0      READ   cov 0 bits 0 units 1331 exec/s 0
> ...
> #1024   pulse  cov 8043 bits 13474 units 1331 exec/s 256
> #1331   INITED cov 8050 bits 13689 units 594 exec/s 221
> #2048   pulse  cov 8050 bits 13689 units 594 exec/s 341
> This means that the corpus on disk had 1331 units, they were read,
> shuffled, executed, and those that added coverage were chosen.

Hah!  this means I've been misreading this line all along.  My eyes zoomed
in on "N exec/s" and I assumed that was the throughput (and I just ignored
the "suffix" entry).  So it's "cov X" / "bits Y" / "units Z" /  "exec/s W"
?  I guess the PCRE2 use case in the docs explains some of this -- I
should've read this more closely.

Aha, of course.

> I run non-async-signal-safe code in the signal handler, bummer.
> Let me try to fix this (no promises for a quick fix, I'll be out for a
> while).
Ok, no problem.  If you don't get around to it I can probably come up w/a

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