net: Add netif_get_gro_max_size helper for GRO

Add a small netif_get_gro_max_size() helper which returns the maximum IPv4
or IPv6 GRO size of the netdevice.

We later add a netif_get_gso_max_size() equivalent as well for GSO, so that
these helpers can be used consistently instead of open-coded checks.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20240923212242.15669-1-daniel@iogearbox.net
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/netdevice.h

@@ -5029,6 +5029,15 @@ void netif_set_tso_max_segs(struct net_device *dev, unsigned int segs);
 void netif_inherit_tso_max(struct net_device *to,
 			   const struct net_device *from);
 
+static inline unsigned int
+netif_get_gro_max_size(const struct net_device *dev, const struct sk_buff *skb)
+{
+	/* pairs with WRITE_ONCE() in netif_set_gro(_ipv4)_max_size() */
+	return skb->protocol == htons(ETH_P_IPV6) ?
+	       READ_ONCE(dev->gro_max_size) :
+	       READ_ONCE(dev->gro_ipv4_max_size);
+}
+
 static inline bool netif_is_macsec(const struct net_device *dev)
 {
 	return dev->priv_flags & IFF_MACSEC;

net/core/gro.c

@@ -98,7 +98,6 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
 	unsigned int headlen = skb_headlen(skb);
 	unsigned int len = skb_gro_len(skb);
 	unsigned int delta_truesize;
-	unsigned int gro_max_size;
 	unsigned int new_truesize;
 	struct sk_buff *lp;
 	int segs;
@@ -112,12 +111,8 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
 	if (p->pp_recycle != skb->pp_recycle)
 		return -ETOOMANYREFS;
 
-	/* pairs with WRITE_ONCE() in netif_set_gro(_ipv4)_max_size() */
-	gro_max_size = p->protocol == htons(ETH_P_IPV6) ?
-			READ_ONCE(p->dev->gro_max_size) :
-			READ_ONCE(p->dev->gro_ipv4_max_size);
-
-	if (unlikely(p->len + len >= gro_max_size || NAPI_GRO_CB(skb)->flush))
+	if (unlikely(p->len + len >= netif_get_gro_max_size(p->dev, p) ||
+		     NAPI_GRO_CB(skb)->flush))
 		return -E2BIG;
 
 	if (unlikely(p->len + len >= GRO_LEGACY_MAX_SIZE)) {