MFC r328876:
Modify ip6_get_prevhdr() to be able use it safely.

Instead of returning pointer to the previous header, return its offset.
In frag6_input() use m_copyback() and determined offset to store next
header instead of accessing to it by pointer and assuming that the memory
is contiguous.

In rip6_input() use offset returned by ip6_get_prevhdr() instead of
calculating it from pointers arithmetic, because IP header can belong
to another mbuf in the chain.

Reported by: Maxime Villard <max at m00nbsd dot net>

Merge r280955

Preserve IPv6 fragment IDs accross reassembly and refragmentation

When forwarding fragmented IPv6 packets and filtering with PF we
reassemble and refragment. That means we generate new fragment headers
and a new fragment ID.

We already save the fragment IDs so we can do the reassembly so it's
straightforward to apply the incoming fragment ID on the refragmented
packets.

Differential Revision: https://reviews.freebsd.org/D2817
Reviewed by: gnn

Merge r278842

Factor out ip6_fragment() function, to be used in IPv6 stack and pf(4).

Differential Revision: https://reviews.freebsd.org/D2815
Reviewed by: gnn

Merge r278828, r278832

- Factor out ip6_deletefraghdr() function, to be shared between IPv6 stack and pf(4).
- Move ip6_deletefraghdr() to frag6.c. (Suggested by bz)

Differential Revision: https://reviews.freebsd.org/D2813
Reviewed by: gnn

MFC r279588:
Fix deadlock in IPv6 PCB code.

When several threads are trying to send datagram to the same destination,
but fragmentation is disabled and datagram size exceeds link MTU,
ip6_output() calls pfctlinput2(PRC_MSGSIZE). It does notify all
sockets wanted to know MTU to this destination. And since all threads
hold PCB lock while sending, taking the lock for each PCB in the
in6_pcbnotify() leads to deadlock.

RFC 3542 p.11.3 suggests notify all application wanted to receive
IPV6_PATHMTU ancillary data for each ICMPv6 packet too big message.
But it doesn't require this, when we don't receive ICMPv6 message.

Change ip6_notify_pmtu() function to be able use it directly from
ip6_output() to notify only one socket, and to notify all sockets
when ICMPv6 packet too big message received.

MFC r279684:
tcp6_ctlinput() doesn't pass MTU value to in6_pcbnotify().
Check cmdarg isn't NULL before dereference, this check was in the
ip6_notify_pmtu() before r279588.

PR: 197059
Sponsored by: Yandex LLC

MFC r266800 by vanhu:
IPv4-in-IPv6 and IPv6-in-IPv4 IPsec tunnels.
For IPv6-in-IPv4, you may need to do the following command
on the tunnel interface if it is configured as IPv4 only:
ifconfig <interface> inet6 -ifdisabled

Code logic inspired from NetBSD.
PR: kern/169438

MC r266822 by bz:
Use IPv4 statistics in ipsec4_process_packet() rather than the IPv6
version. This also unbreaks the NOINET6 builds after r266800.

MFC r268083 by zec:
The assumption in ipsec4_process_packet() that the payload may be
only IPv4 is wrong, so check the IP version before mangling the
payload header.

MFC r272394:
Do not strip outer header when operating in transport mode.
Instead requeue mbuf back to IPv4 protocol handler. If there is one extra IP-IP
encapsulation, it will be handled with tunneling interface. And thus proper
interface will be exposed into mbuf's rcvif. Also, tcpdump that listens on tunneling
interface will see packets in both directions.

PR: 194761

Merge r263091: fix mbuf flags clash that lead to failure of operation
of IPSEC and packet filters.

PR: kern/185876
PR: kern/186755

Merge r280955

Preserve IPv6 fragment IDs accross reassembly and refragmentation

When forwarding fragmented IPv6 packets and filtering with PF we
reassemble and refragment. That means we generate new fragment headers
and a new fragment ID.

We already save the fragment IDs so we can do the reassembly so it's
straightforward to apply the incoming fragment ID on the refragmented
packets.

Differential Revision: https://reviews.freebsd.org/D2817
Reviewed by: gnn

Merge r278842

Factor out ip6_fragment() function, to be used in IPv6 stack and pf(4).

Differential Revision: https://reviews.freebsd.org/D2815
Reviewed by: gnn

Merge r278828, r278832

- Factor out ip6_deletefraghdr() function, to be shared between IPv6 stack and pf(4).
- Move ip6_deletefraghdr() to frag6.c. (Suggested by bz)

Differential Revision: https://reviews.freebsd.org/D2813
Reviewed by: gnn

MFC r279588:
Fix deadlock in IPv6 PCB code.

When several threads are trying to send datagram to the same destination,
but fragmentation is disabled and datagram size exceeds link MTU,
ip6_output() calls pfctlinput2(PRC_MSGSIZE). It does notify all
sockets wanted to know MTU to this destination. And since all threads
hold PCB lock while sending, taking the lock for each PCB in the
in6_pcbnotify() leads to deadlock.

RFC 3542 p.11.3 suggests notify all application wanted to receive
IPV6_PATHMTU ancillary data for each ICMPv6 packet too big message.
But it doesn't require this, when we don't receive ICMPv6 message.

Change ip6_notify_pmtu() function to be able use it directly from
ip6_output() to notify only one socket, and to notify all sockets
when ICMPv6 packet too big message received.

MFC r279684:
tcp6_ctlinput() doesn't pass MTU value to in6_pcbnotify().
Check cmdarg isn't NULL before dereference, this check was in the
ip6_notify_pmtu() before r279588.

PR: 197059
Sponsored by: Yandex LLC

MFC r266800 by vanhu:
IPv4-in-IPv6 and IPv6-in-IPv4 IPsec tunnels.
For IPv6-in-IPv4, you may need to do the following command
on the tunnel interface if it is configured as IPv4 only:
ifconfig <interface> inet6 -ifdisabled

Code logic inspired from NetBSD.
PR: kern/169438

MC r266822 by bz:
Use IPv4 statistics in ipsec4_process_packet() rather than the IPv6
version. This also unbreaks the NOINET6 builds after r266800.

MFC r268083 by zec:
The assumption in ipsec4_process_packet() that the payload may be
only IPv4 is wrong, so check the IP version before mangling the
payload header.

MFC r272394:
Do not strip outer header when operating in transport mode.
Instead requeue mbuf back to IPv4 protocol handler. If there is one extra IP-IP
encapsulation, it will be handled with tunneling interface. And thus proper
interface will be exposed into mbuf's rcvif. Also, tcpdump that listens on tunneling
interface will see packets in both directions.

PR: 194761

Merge r263091: fix mbuf flags clash that lead to failure of operation
of IPSEC and packet filters.

PR: kern/185876
PR: kern/186755