bpf: udp: Avoid calling reuseport's bpf_prog from udp_gro

When the commit a6024562 ("udp: Add GRO functions to UDP socket")
added udp[46]_lib_lookup_skb to the udp_gro code path, it broke
the reuseport_select_sock() assumption that skb->data is pointing
to the transport header.

This patch follows an earlier __udp6_lib_err() fix by
passing a NULL skb to avoid calling the reuseport's bpf_prog.

Fixes: a6024562 ("udp: Add GRO functions to UDP socket")
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/ipv4/udp.c

@@ -503,7 +503,11 @@ static inline struct sock *__udp4_lib_lookup_skb(struct sk_buff *skb,
 struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
 				 __be16 sport, __be16 dport)
 {
-	return __udp4_lib_lookup_skb(skb, sport, dport, &udp_table);
+	const struct iphdr *iph = ip_hdr(skb);
+
+	return __udp4_lib_lookup(dev_net(skb->dev), iph->saddr, sport,
+				 iph->daddr, dport, inet_iif(skb),
+				 inet_sdif(skb), &udp_table, NULL);
 }
 EXPORT_SYMBOL_GPL(udp4_lib_lookup_skb);
 

net/ipv6/udp.c

@@ -243,7 +243,7 @@ struct sock *udp6_lib_lookup_skb(struct sk_buff *skb,
 
 	return __udp6_lib_lookup(dev_net(skb->dev), &iph->saddr, sport,
 				 &iph->daddr, dport, inet6_iif(skb),
-				 inet6_sdif(skb), &udp_table, skb);
+				 inet6_sdif(skb), &udp_table, NULL);
 }
 EXPORT_SYMBOL_GPL(udp6_lib_lookup_skb);