[Bug 259157] Test case sys.netgraph.hub.loop panics RISC-V kernel

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=259157

Mitchell Horne <mhorne@freebsd.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                URL|                            |https://reviews.freebsd.org
                   |                            |/D32580
             Status|Open                        |In Progress

--- Comment #5 from Mitchell Horne <mhorne@freebsd.org> ---
(In reply to Lutz Donnerhacke from comment #4)

The expected behaviour for a page fault in the kernel is to panic, with a
couple exceptions. Certainly, anything that overflows the thread's kernel stack
in this way will cause this.

I managed to identify why this panics riscv, and not amd64. Turns out, the
recursion is avoided when the architecture defines GET_STACK_USAGE(), as it
allows for an early return from ng_snd_item(). The test also causes panics on
32-bit arm, but we did not notice because the armv7 CI is broken :(

I created a patch which implements the missing macro for remaining archs, and
it 'fixes' the panics in this test. See:
https://reviews.freebsd.org/D32580

Still, I wonder if the test is flawed? If it is expected that the test might
overflow the kernel stack then it is not really safe for CI. Maybe the test
needs rethinking, or additional safeguards.

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