Re: [Xen-devel] [PATCH 01/14] fuzz/x86_emulate: Remove redundant AFL hook

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 22/09/17 16:47, George Dunlap wrote:
> On Fri, Aug 25, 2017 at 6:37 PM, Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 25/08/17 17:43, George Dunlap wrote:
> > > You don't need __AFL_INIT if you have __AFL_LOOP.
> > >
> > > Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
> > Really?  Is that covered in any documentation?
> >
> > I got the contrary impression from whichever version of AFL I was using
> > when I put this in, and a quick look over the afl-fuzz source doesn't
> > appear to equate them in any way.
> Trying to answer the feof() question, I dug into llvm mode a bit more.
> They are independent features but they can be used together: that is,
> if you compile with afl-clang-fast, you always get a forkserver.  If
> you specify the location with __AFL_INIT, that's where the forkserver
> get started; otherwise, it's somewhere before main() is called.
>
> When __AFL_LOOP(N) is present, it will execute the loop N times before
> calling exit(); at which point, the forkserver will fork another
> instance (either before main(), or at __AFL_INIT).
>
> So you don't *need* __AFL_INIT; and the amount of time it will save
> isn't much (the initialization every 1000 iterations), but I suppose
> we might as well keep it.
The purpose of c/s 63092064eb was very deliberately to cause the 
mprotect() remapping the stack as executable to be not repeated.
~Andrew

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