Re: [PATCH net] tunnels: Accept PACKET_HOST in skb_tunnel_check_pmtu().

[Stefano Brivio <sbrivio@redhat.com>]

On Sat, 29 Mar 2025 01:33:44 +0100
Guillaume Nault <gnault@redhat.com> wrote:

> Because skb_tunnel_check_pmtu() doesn't handle PACKET_HOST packets,
> commit 30a92c9e3d6b ("openvswitch: Set the skbuff pkt_type for proper
> pmtud support.") forced skb->pkt_type to PACKET_OUTGOING for
> openvswitch packets that are sent using the OVS_ACTION_ATTR_OUTPUT
> action. This allowed such packets to invoke the
> iptunnel_pmtud_check_icmp() or iptunnel_pmtud_check_icmpv6() helpers
> and thus trigger PMTU update on the input device.
> 
> However, this also broke other parts of PMTU discovery. Since these
> packets don't have the PACKET_HOST type anymore, they won't trigger the
> sending of ICMP Fragmentation Needed or Packet Too Big messages to
> remote hosts when oversized (see the skb_in->pkt_type condition in
> __icmp_send() for example).
> 
> These two skb->pkt_type checks are therefore incompatible as one
> requires skb->pkt_type to be PACKET_HOST, while the other requires it
> to be anything but PACKET_HOST.
> 
> It makes sense to not trigger ICMP messages for non-PACKET_HOST packets
> as these messages should be generated only for incoming l2-unicast
> packets. However there doesn't seem to be any reason for
> skb_tunnel_check_pmtu() to ignore PACKET_HOST packets.

No valid reason, right.

That (bogus) check just came from the specific functionality I meant to
implement back then: PMTU discovery for paths where we forward packets
(PACKET_OTHERHOST or PACKET_OUTGOING).

But we should handle packets that are (in some sense) going to us
(PACKET_HOST) in the same way.

> Allow both cases to work by allowing skb_tunnel_check_pmtu() to work on
> PACKET_HOST packets and not overriding skb->pkt_type in openvswitch
> anymore.
> 
> Fixes: 30a92c9e3d6b ("openvswitch: Set the skbuff pkt_type for proper pmtud support.")
> Fixes: 4cb47a8644cc ("tunnels: PMTU discovery support for directly bridged IP packets")
> Signed-off-by: Guillaume Nault <gnault@redhat.com>

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>

Thanks for fixing this!

-- 
Stefano