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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew

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