Re: [Qemu-devel] [PATCH v3 17/22] fuzz: add support for fork-based fuzzing.

[Oleinik, Alexander]

On Thu, 2019-09-19 at 13:54 +0100, Stefan Hajnoczi wrote:
> On Wed, Sep 18, 2019 at 11:19:44PM +0000, Oleinik, Alexander wrote:
> > diff --git a/exec.c b/exec.c
> > index 235d6bc883..d3838f4ea4 100644
> > --- a/exec.c
> > +++ b/exec.c
> > @@ -2295,7 +2295,9 @@ static void ram_block_add(RAMBlock
> > *new_block, Error **errp, bool shared)
> >          qemu_ram_setup_dump(new_block->host, new_block-
> > >max_length);
> >          qemu_madvise(new_block->host, new_block->max_length,
> > QEMU_MADV_HUGEPAGE);
> >          /* MADV_DONTFORK is also needed by KVM in absence of
> > synchronous MMU */
> > +#ifndef CONFIG_FUZZ /* This conflicts with fork-based fuzzing */
> >          qemu_madvise(new_block->host, new_block->max_length,
> > QEMU_MADV_DONTFORK);
> > +#endif
> >          ram_block_notify_add(new_block->host, new_block-
> > >max_length);
> >      }
> >  }
> 
> I didn't check Makefile changes but regular softmmu binaries should
> continue to work with --enable-fuzzing so we cannot use #ifdef here.
> Perhaps this should be a runtime check similar to qtest_enabled()

Yes - I'll add a runtime check. The makefile requires a make clean
between softmmu and fuzzer builds, since the ".o"s for the fuzzer build
are compiled with fsanitize=fuzzer(must be linked against libfuzzer). 
I can see that the #ifndef CONFIG_FUZZ is a proper softmmu builds with
--enable-fuzzer are currently broken due to the ifndef CONFIG_FUZZ
check.