Re: [Xen-devel] [PATCH 14/14] fuzz/x86_emulate: Add an option to limit the number of instructions executed

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 09/15/2017 02:38 PM, Wei Liu wrote:
> On Fri, Aug 25, 2017 at 05:43:43PM +0100, George Dunlap wrote:
>> AFL considers a testcase to be a useful addition not only if there are
>> tuples exercised by that testcase which were not exercised otherwise,
>> but also if the *number* of times an individual tuple is exercised
>> changes significantly; in particular, if the number of the highes bit
>> changes (i.e., if it is run 1, 2-3, 4-7, 8-15, &c).
>>
>> Unfortunately, one simple way to increase these stats it to execute
>> the same (or similar) instructions multiple times.  Such long
>> testcases take exponentially longer to fuzz: the fuzzer spends more
>> time flipping bits looking for meaningful changes, and each execution
>> takes longer because it is doing more things.  So long paths which add
>> nothing to the actual code coverage but effectively "distract" the
>> fuzzer, making it less effective.
>>
>> Experiments have shown that not allowing infinite number of
>> instruction retries for the old (non-compact) format does indeed speed
>> up and increase code coverage.  However, it has also shown that on the
>> new, more compact format, having no instruction limit causes the highest
>> throughput in code coverage.
>>
>> So leave the option in, but have it default to 0 (no limit).
> 
> How does limiting the number of loops help afl produce better input?
> Wouldn't afl still try to flip bits beyond the limit (say, the >=n+1
> instructions when the limit is n)? I assume it will give up at some
> point, but when?

* Limiting the number of loops means longer testcases don't look
different than shorter testcases

* If a testcase doesn't look different than existing testcases, then AFL
will discard it rather than adding it to the queue

* Larger "queue" testcases exhibit a quadratic effect for search time:
They are longer to fuzz (since they're bigger) and each testcase is
longer to run.

* So limiting the number of loops causes the testcases in the queue to
be shorter, which in turn decreases the time to fuzz and execute test
cases in the queue, which causes higher throughput.

> I guess my point is having a limit but doesn't tell afl about it seems
> a bit sub-optimal to me. I'm not quite sure if I understand correctly
> how afl works though.

Well I think the key thing is to look at the graph.  Either 1) there's
something wrong with my methodology, or 2) for the original layout,
limiting the number of testcases helps improve AFL's performance.

 -George

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