[Bug 272965] 14.0-ALPHA1 too: armv7 'Alignment Fault' on read panic during udp_input for kyua's sys/netinet6/exthdr:exthdr ; other udp_input related panics

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=272965

--- Comment #10 from Michal Meloun <mmel@FreeBSD.org> ---
(In reply to Warner Losh from comment #8)
Unfortunately, I can't agree with that. Aligning the input mbuf is only
papering over the real problem and can only work for trivial cases.  It doesn't
help for vlans (unless HW strips header), for various protocol encapsulations
and most importantly it doesn't work at all for IPv6 :(

The real problem is that our network stack uses undefined behavior. We can
alias a byte buffer to a structure only if the given buffer pointer is conform
with the desired structure alignment, otherwise the behavior is undefined. This
is, strictly speaking , required for every architecture. The only way we can
reduce the required alignment is by using the packed attribute for the affected
structures...

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