[RFC] How to test panic handlers, without crashing the kernel

[Michael Kelley]

From: Guilherme G. Piccoli <gpiccoli at igalia.com> Sent: Monday, March 4, 2024 1:43 PM
> 
> On 04/03/2024 18:12, John Ogness wrote:
> > [...]
> >> The second question is how to simulate a panic context in a
> >> non-destructive way, so we can test the panic notifiers in CI, without
> >> crashing the machine.
> >
> > I'm wondering if a "fake panic" can be implemented that quiesces all the
> > other CPUs via NMI (similar to kdb) and then calls the panic
> > notifiers. And finally releases everything back to normal. That might
> > produce a fairly realistic panic situation and should be fairly
> > non-destructive (depending on what the notifiers do and how long they
> > take).
> >
> 
> Hi Jocelyn / John,
> 
> one concern here is that the panic notifiers are kind of a no man's
> land, so we can have very simple / safe ones, while others are
> destructive in nature.
> 
> An example of a good behaving notifier that is destructive is the
> Hyper-V one, that destroys an essential host-guest interface (called
> "vmbus connection"). What happens if we trigger this one just for
> testing purposes in a debugfs interface? Likely the guest would die...
> 
> [+CCing Michael Kelley here since he seems interested in panic and is
> also expert in Hyper-V, just in case my example is bogus.]

The Hyper-V example is valid. After hv_panic_vmbus_unload()
is called, the VM won't be able to do any disk, network, or graphics
frame buffer I/O. There's no recovery short of restarting the VM.

Michael

[I have retired from Microsoft.  I'm still occasionally contributing
to Linux kernel work with email mhklinux at outlook.com.]

> 
> So, maybe the problem could be split in 2: the non-notifiers portion of
> the panic path, and the the notifiers; maybe restricting the notifiers
> you'd run is a way to circumvent the risks, like if you could pass a
> list of the specific notifiers you aim to test, this could be
> interesting. Let's see what the others think and thanks for your work in
> the DRM panic notifier =)
> 
> Cheers,
> 
> 
> Guilherme