Re: [PATCH net-next v3 3/5] net: gso: restore ids of outer ip headers correctly

[Paolo Abeni <pabeni@redhat.com>]

On 8/21/25 9:30 AM, Richard Gobert wrote:
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 68dc47d7e700..9941c39b5970 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -3772,10 +3772,9 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
>  	 * IPv4 header has the potential to be fragmented.
>  	 */
>  	if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) {
> -		struct iphdr *iph = skb->encapsulation ?
> -				    inner_ip_hdr(skb) : ip_hdr(skb);
> -
> -		if (!(iph->frag_off & htons(IP_DF)))
> +		if (!(ip_hdr(skb)->frag_off & htons(IP_DF)) ||
> +		    (skb->encapsulation &&
> +		     !(inner_ip_hdr(skb)->frag_off & htons(IP_DF))))
>  			features &= ~NETIF_F_TSO_MANGLEID;

FWIW, I think the above is the problematic part causing GSO PARTIAL issues.

By default UDP tunnels do not set the DF bit, and most/all devices
implementing GSO_PARTIAL clear TSO for encapsulated packet when MANGLEID
is not available.

I think the following should workaround the problem (assuming my email
client did not corrupt the diff), but I also fear this change will cause
very visible regressions in existing setups.

Note that the current status is incorrect - GSO partial devices are
mangling the outer IP ID for encapsulated packets even when the outer
header IP DF is not set.

/P
---
diff --git a/tools/testing/selftests/drivers/net/hw/tso.py
b/tools/testing/selftests/drivers/net/hw/tso.py
index 3370827409aa..b0c71a0d8028 100755
--- a/tools/testing/selftests/drivers/net/hw/tso.py
+++ b/tools/testing/selftests/drivers/net/hw/tso.py
@@ -214,8 +214,8 @@ def main() -> None:
             # name,       v4/v6  ethtool_feature
tun:(type,    partial, args)
             ("",            "4", "tx-tcp-segmentation",           None),
             ("",            "6", "tx-tcp6-segmentation",          None),
-            ("vxlan",        "", "tx-udp_tnl-segmentation",
("vxlan",  True,  "id 100 dstport 4789 noudpcsum")),
-            ("vxlan_csum",   "", "tx-udp_tnl-csum-segmentation",
("vxlan",  False, "id 100 dstport 4789 udpcsum")),
+            ("vxlan",        "", "tx-udp_tnl-segmentation",
("vxlan",  True,  "id 100 dstport 4789 noudpcsum df set")),
+            ("vxlan_csum",   "", "tx-udp_tnl-csum-segmentation",
("vxlan",  False, "id 100 dstport 4789 udpcsum df set")),
             ("gre",         "4", "tx-gre-segmentation",
("gre",    False,  "")),
             ("gre",         "6", "tx-gre-segmentation",
("ip6gre", False,  "")),
         )