Re: NetBSD syzbot broken?

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

> 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_.

> 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?

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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