Re: [Xen-devel] [PATCH v3 09/12] fuzz/x86_emulate: Make input more compact

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 10/10/2017 05:59 PM, Andrew Cooper wrote:
> On 10/10/17 17:20, George Dunlap wrote:
>> At the moment, AFL reckons that for any given input, 87% of it is
>> completely irrelevant: that is, it can change it as much as it wants
>> but have no impact on the result of the test; and yet it can't remove
>> it.
>>
>> This is largely because we interpret the blob handed to us as a large
>> struct, including CR values, MSR values, segment registers, and a full
>> cpu_user_regs.
>>
>> Instead, modify our interpretation to have a "set state" stanza at the
>> front.  Begin by reading a 16-bit value; if it is lower than a certain
>> threshold, set some state according to what byte it is, and repeat.
>> Continue until the byte is above a certain threshold.
>>
>> This allows AFL to compact any given test case much smaller; to the
>> point where now it reckons there is not a single byte of the test file
>> which becomes irrelevant.  Testing have shown that this option both
>> allows AFL to reach coverage much faster, and to have a total coverage
>> higher than with the old format.
>>
>> Make this an option (rather than a unilateral change) to enable
>> side-by-side performance comparison of the old and new formats.
>>
>> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> 
> I am still of the opinion that this is a waste of effort, which would be
> better spent actually removing the irrelevant state in the first place;
> not building an obfuscation algorithm.
> 
> I'm not going to nack the patch because that is probably over the top,
> but I'm not in favour if this change going in.

Did you look at the evidence I presented, demonstrating that this
significantly increases the effectiveness of AFL?

 -George

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