[PATCH net v2 0/2] ip_tunnel: fix PMTU ICMP reply routing

[PATCH net v2 0/2] ip_tunnel: fix PMTU ICMP reply routing

[Laika Price via B4 Relay]

---
Changes in v2:
- Fix incorrect PMTU exceptions test
- Link to v1: https://patch.msgid.link/20260613-master-v1-1-df796e8e2d74@gmail.com

To: David Ahern <dsahern@kernel.org>
To: Ido Schimmel <idosch@nvidia.com>
To: "David S. Miller" <davem@davemloft.net>
To: Eric Dumazet <edumazet@google.com>
To: Jakub Kicinski <kuba@kernel.org>
To: Paolo Abeni <pabeni@redhat.com>
To: Simon Horman <horms@kernel.org>
To: Shuah Khan <shuah@kernel.org>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org

---
Laika Price (2):
      [net] ip_tunnel: drop stale dst from generated PMTU ICMP replies
      [net] selftests: pmtu: fix incorrect PMTU exception generation

 net/ipv4/ip_tunnel_core.c           | 2 ++
 tools/testing/selftests/net/pmtu.sh | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)
---
base-commit: 2a2974b5145cdf2f4db134be1a2157e9ca4a1cf0
change-id: 20260613-master-a299166b9069

Best regards,
--  
Laika Price <laikabcprice@gmail.com>

[PATCH net v2 1/2] ip_tunnel: drop stale dst from generated PMTU ICMP replies

[Laika Price via B4 Relay]

From: Laika Price <laikabcprice@gmail.com>

iptunnel_pmtud_build_icmp(...) and iptunnel_pmtud_build_icmpv6(...) take
in an sk_buff, modify it to create a PMTU ICMP error reply, and return it.
As part of these modifications, the source/destination ethernet and IP
addresses are swapped around which makes the sk_buff's current dst invalid.

If the stale dst is left, the packet can skip input routing and be
forwarded using the original output device. This was observed when sending
packets to a VXLAN over a WireGuard tunnel - the ICMP reply was generated
but it was sent over the VXLAN instead of to the WireGuard tunnel.

Drop the stale dst after building the PMTU reply so that the packet is
routed using its new headers when it is reinjected.

Signed-off-by: Laika Price <laikabcprice@gmail.com>
---
 net/ipv4/ip_tunnel_core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index d3c677e9b..949150e43 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -267,6 +267,7 @@ static int iptunnel_pmtud_build_icmp(struct sk_buff *skb, int mtu)
 
 	eth_header(skb, skb->dev, ntohs(eh.h_proto), eh.h_source, eh.h_dest, 0);
 	skb_reset_mac_header(skb);
+	skb_dst_drop(skb);
 
 	return skb->len;
 }
@@ -370,6 +371,7 @@ static int iptunnel_pmtud_build_icmpv6(struct sk_buff *skb, int mtu)
 
 	eth_header(skb, skb->dev, ntohs(eh.h_proto), eh.h_source, eh.h_dest, 0);
 	skb_reset_mac_header(skb);
+	skb_dst_drop(skb);
 
 	return skb->len;
 }

-- 
2.54.0

[PATCH net v2 2/2] selftests: pmtu: fix incorrect PMTU exception generation

[Laika Price via B4 Relay]

From: Laika Price <laikabcprice@gmail.com>

pmtu_ipv4_br_vxlan4_exception generates PMTU exceptions by pinging an IP
on the other side of a tunnel. This was incorrect as it would return upon
the first ICMP Fragmentation Needed due to the -w flag being used in
conjunction with || return 1.

This patch updates pmtu_ipv4_br_vxlan4_exception to be in line with how
PMTU exceptions are generated in other tests such as in test_pmtu_ipvX

    run_cmd ${ns_a} ${ping} -q -M want -i 0.1 -w 1 -s 1800 ${dst1}
    run_cmd ${ns_a} ${ping} -q -M want -i 0.1 -w 1 -s 1800 ${dst2}

Signed-off-by: Laika Price <laikabcprice@gmail.com>
---
 tools/testing/selftests/net/pmtu.sh | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/pmtu.sh b/tools/testing/selftests/net/pmtu.sh
index a3323c21f..9498d9f53 100755
--- a/tools/testing/selftests/net/pmtu.sh
+++ b/tools/testing/selftests/net/pmtu.sh
@@ -1456,8 +1456,8 @@ test_pmtu_ipvX_over_bridged_vxlanY_or_geneveY_exception() {
 	mtu "${ns_a}" ${type}_a $((${ll_mtu} + 1000))
 	mtu "${ns_b}" ${type}_b $((${ll_mtu} + 1000))
 
-	run_cmd ${ns_c} ${ping} -q -M want -i 0.1 -c 10 -s $((${ll_mtu} + 500)) ${dst} || return 1
-	run_cmd ${ns_a} ${ping} -q -M want -i 0.1 -w 1  -s $((${ll_mtu} + 500)) ${dst} || return 1
+	run_cmd ${ns_c} ${ping} -q -M want -i 0.1 -w 1 -s $((${ll_mtu} + 500)) ${dst}
+	run_cmd ${ns_a} ${ping} -q -M want -i 0.1 -w 1 -s $((${ll_mtu} + 500)) ${dst}
 
 	# Check that exceptions were created
 	pmtu="$(route_get_dst_pmtu_from_exception "${ns_c}" ${dst})"

-- 
2.54.0

Re: [PATCH net v2 0/2] ip_tunnel: fix PMTU ICMP reply routing

[Jakub Kicinski]

On Sat, 13 Jun 2026 16:38:27 +0100 Laika Price wrote:
> Disregard v2 of this series.
> 
> Apologies, I'm new to kernel development as did not realise that I should
> squash commits that would cause the kernel to not build / fail tests. I am
> sending in a v3 with these squashed.
> 
> Sorry for the noise.

I'm not sure what build failure you're talking about.
Please observe the 24h cooldown between submitting new versions 
of a patch.