Re: [Xen-devel] [PATCH 2/4] tools/fuzz: add AFL stub program for x86 insn emulator fuzzer

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Jan 24, 2017 at 05:25:29PM +0000, Andrew Cooper wrote:
> On 24/01/17 17:22, Ian Jackson wrote:
> > Wei Liu writes ("Re: [PATCH 2/4] tools/fuzz: add AFL stub program for x86 
> > insn emulator fuzzer"):
> >> On Tue, Jan 24, 2017 at 05:05:16PM +0000, Ian Jackson wrote:
> >>> If you use read you ought to put it in a loop to cope with potential
> >>> partial reads.
> >> Does fread have better properties than read?
> > Yes.  fread is not allowed to give a short read for no particular
> > reason, nor to return with EINTR.
> >
> >> I think to deal with short read we would need to stat the file to
> >> get actual size for both fread and read, right?
> > No.  In both cases, it is possible to detect whether a short read is
> > due to EOF.  With read, that would have to be in a loop.  With fread
> > you can test feof.
> >
> > You don't need to test with stat unless you want to support files
> > bigger than your static buffer.
> 
> It is expected that AFL data will eventually become longer than the
> fuzzer wishes.  You should ignore extraneous data.
> 

That's not necessarily true. It depends on how you run the fuzzer.

But yes, ignoring extraneous data is sensible.

> Also, remember that AFL prefers to pass data by pipe.

It supports both using file or STDIN.

Using file would be easier for manual retest.

Wei.

> 
> ~Andrew

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