Re: NetBSD syzbot broken?

["'Aleksandr Nogikh' via syzkaller-netbsd-bugs" <syzkaller-netbsd-bugs%googlegroups.com@localhost>]

On Thu, Apr 11, 2024 at 12:44 PM Aleksandr Nogikh <nogikh%google.com@localhost> wrote:
>
> On Thu, Apr 11, 2024 at 3:36 AM Taylor R Campbell <riastradh%netbsd.org@localhost> wrote:
> >
> > > Date: Wed, 3 Apr 2024 20:05:54 +0200
> > > From: Aleksandr Nogikh <nogikh%google.com@localhost>
> > >
> > > That is indeed due to some problem with the host qemu, but that makes it
> > > even more mysterious. I didn't update anything on the host and I think
> > > nobody else did that either. And it's now unfortunately impossible to
> > > pinpoint the exact moment when it began to fail with "failed to create the
> > > VM Instance:" -- syzbot cleans up old logs and in some of these logs there
> > > are actually ordinary NetBSD compilation errors. Could a NetBSD host have
> > > updated the qemu package itself?
> >
> > Not sure what you mean by this -- is syzbot running NetBSD guests
> > under a NetBSD host?
> >
> > If so, this message is pretty weird:
> >
> >    Could not access KVM kernel module: No such file or directory
> >    qemu-system-x86_64: failed to initialize kvm: No such file or directory
> >
> > I would expect to see something about nvmm, not kvm, if that were the
> > case; qemu under a NetBSD host has never supported `-accel kvm' or
> > anything like that -- only `-accel nvmm'.  If there haven't been any
> > recent changes about this, surely this would have failed all along
> > because NetBSD as a host has never had kvm.
> >
> > But maybe I misunderstood your question?  I'm fuzzy on how syzbot
> > operates at a high level -- I've only dug into the issues it reports.
> > Certainly I'd be astonished if a NetBSD _guest_ changed any kind of
> > package installation on the _host_.
>
>
> Ah, I'm sorry, I have confused it with the FreeBSD setup on syzbot.
> NetBSD fuzzing indeed runs on a Linux host.
>
> >
> > > In the syzkaller code, we do have an option to distinguish whether we need
> > > to explicitly specify the raw format or not:
> > > https://github.com/google/syzkaller/blob/51c4dcff83b0574620c280cc5130ef59cc4a2e32/vm/qemu/qemu.go#L459
> > >
> > > We can set UseNewQemuImageOptions=true for NetBSD and it should hopefully
> > > be working again. But I still don't understand why it has popped up..
> >
> > It seems like if a file is supposed to be interpreted as a raw image,
> > it would be prudent to say `format=raw'.  How do I find how the image
> > (/syzkaller/managers/ci2-netbsd/latest.tmp/image) is created and what
> > format it is supposed to be in?
>
> I think it's easier to just use the new arguments format here. I've
> sent a PR: https://github.com/google/syzkaller/pull/4672
>
> But the main problem is that it tries to use kvm acceleration, but
> fails to: `qemu-system-x86_64: failed to initialize kvm: No such file
> or directory`.
>
> It's still unclear whether it didn't use kvm for netbsd builds before,
> but somehow began to, or for some reason we lost the nested
> virtualization support on our GCE instance.

I've sent https://github.com/google/syzkaller/commit/95ed9ece851c5ce0f8db8fbe8c852457b4c36a85,
let's see if it changes anything.

>
> >
> > Conceivably if something changed about that somehow (e.g., maybe
> > NetBSD newfs or mkimage started doing putting in different formatting
> > that might confuse qemu's format detection) that could break this.
> > But it seems unlikely.

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