[Bug 271991] Crash on some network packets with fresh stable

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271991

--- Comment #5 from Mark Johnston <markj@FreeBSD.org> ---
(In reply to Ivan Rozhuk from comment #3)
I don't think m_pullup() is the right layer to handle unmapped mbufs.  It gets
called very frequently, and mb_unmapped_to_ext() is expensive even when it does
nothing, since it has to visit each mbuf in the chain.

(In reply to Ivan Rozhuk from comment #4)
The patch to m_pullup() assumes that "m" is mapped, which might not be the
case.

Hmm, virtio-net does not set IFCAP_MEXTPG, so the protocol layer will map
mbufs.  This is actually rather suboptimal, and the virtio driver should learn
to handle unmapped mbufs.  e1000 should handle unmapped mbufs though.  Do you
see "NOMAP" in the interface flags as reported by ifconfig?

(In reply to Ivan Rozhuk from comment #0)
It would be useful to see output from 'p/x *n' in frame 9.

I think it wouldn't be too difficult to make bpf_filter() work with unmapped
mbufs.  m_xword() and m_xhalf() would need a bit of special logic.  And I think
ng_bpf() doesn't need to do this pullup at all unless BPF JIT is enabled, and
that's not even compiled into GENERIC.

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