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

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

[Guillaume Nault]

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.

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>
---
 net/ipv4/ip_tunnel_core.c | 2 +-
 net/openvswitch/actions.c | 6 ------
 2 files changed, 1 insertion(+), 7 deletions(-)

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index a3676155be78..364ea798511e 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -416,7 +416,7 @@ int skb_tunnel_check_pmtu(struct sk_buff *skb, struct dst_entry *encap_dst,
 
 	skb_dst_update_pmtu_no_confirm(skb, mtu);
 
-	if (!reply || skb->pkt_type == PACKET_HOST)
+	if (!reply)
 		return 0;
 
 	if (skb->protocol == htons(ETH_P_IP))
diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
index 704c858cf209..61fea7baae5d 100644
--- a/net/openvswitch/actions.c
+++ b/net/openvswitch/actions.c
@@ -947,12 +947,6 @@ static void do_output(struct datapath *dp, struct sk_buff *skb, int out_port,
 				pskb_trim(skb, ovs_mac_header_len(key));
 		}
 
-		/* Need to set the pkt_type to involve the routing layer.  The
-		 * packet movement through the OVS datapath doesn't generally
-		 * use routing, but this is needed for tunnel cases.
-		 */
-		skb->pkt_type = PACKET_OUTGOING;
-
 		if (likely(!mru ||
 		           (skb->len <= mru + vport->dev->hard_header_len))) {
 			ovs_vport_send(vport, skb, ovs_key_mac_proto(key));
-- 
2.39.2

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

[Stefano Brivio]

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

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

[Aaron Conole]

Guillaume Nault <gnault@redhat.com> writes:

> 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.
>
> 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>
> ---

Thanks, Guillaume.

Reviewed-by: Aaron Conole <aconole@redhat.com>

I did manage to test this with two hosts over the weekend, and it
appears to work for at least one forwarding case that I encountered.

Tested-by: Aaron Conole <aconole@redhat.com>

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

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Sat, 29 Mar 2025 01:33:44 +0100 you 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.
> 
> [...]

Here is the summary with links:
  - [net] tunnels: Accept PACKET_HOST in skb_tunnel_check_pmtu().
    https://git.kernel.org/netdev/net/c/8930424777e4

You are awesome, thank you!
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