[PATCH net-next v3 1/5] net: core: use the packet's L3 protocol for the GSO size limit

[Wang Zhan <wang.zhan@smartx.com>]

gso_features_check() compares the frame length against
netif_get_gso_max_size(), which picks the IPv4 or the IPv6 limit from
skb->protocol.  A tag which is already inside the frame replaces that field
with the VLAN ethertype, as skb_vlan_push() does, and an IPv6 packet is
then measured against the IPv4 limit and segmented although the device
could send it as one TSO frame.

Let the limit lookup take the protocol as an argument, and pass the L3
protocol, so where the tag sits does not decide which limit applies.  The
helper cannot look behind the tag itself: it lives in netdevice.h, which
cannot include if_vlan.h because that header includes netdevice.h.

Fixes: e609c959a9396 ("net: Fix gso_features_check to check for both dev->gso_{ipv4_,}max_size")
Assisted-by: LLM
Signed-off-by: Wang Zhan <wang.zhan@smartx.com>

---
v3:
- new patch: the L3 protocol fix split out of the resegmentation patch
v2: https://lore.kernel.org/20260918084651.3022878-4-wang.zhan@smartx.com/
v1: https://lore.kernel.org/20260917063854.2011613-4-wang.zhan@smartx.com/
---
 include/linux/netdevice.h | 4 ++--
 net/core/dev.c            | 3 ++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index d037faff7c44b..1c28861fcbe2c 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -5562,10 +5562,10 @@ netif_get_gro_max_size(const struct net_device *dev, const struct sk_buff *skb)
 }
 
 static inline unsigned int
-netif_get_gso_max_size(const struct net_device *dev, const struct sk_buff *skb)
+netif_get_gso_max_size(const struct net_device *dev, __be16 protocol)
 {
 	/* pairs with WRITE_ONCE() in netif_set_gso(_ipv4)_max_size() */
-	return skb->protocol == htons(ETH_P_IPV6) ?
+	return protocol == htons(ETH_P_IPV6) ?
 	       READ_ONCE(dev->gso_max_size) :
 	       READ_ONCE(dev->gso_ipv4_max_size);
 }
diff --git a/net/core/dev.c b/net/core/dev.c
index f660fccfc0dbc..ffa9b0c27788c 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3843,7 +3843,8 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
 	if (gso_segs > READ_ONCE(dev->gso_max_segs))
 		return features & ~NETIF_F_GSO_MASK;
 
-	if (unlikely(skb->len >= netif_get_gso_max_size(dev, skb)))
+	if (unlikely(skb->len >=
+		     netif_get_gso_max_size(dev, vlan_get_protocol(skb))))
 		return features & ~NETIF_F_GSO_MASK;
 
 	if (!skb_shinfo(skb)->gso_type) {
-- 
2.47.3