bpf: udp: ipv6: Avoid running reuseport's bpf_prog from __udp6_lib_err

__udp6_lib_err() may be called when handling icmpv6 message. For example,
the icmpv6 toobig(type=2).  __udp6_lib_lookup() is then called
which may call reuseport_select_sock().  reuseport_select_sock() will
call into a bpf_prog (if there is one).

reuseport_select_sock() is expecting the skb->data pointing to the
transport header (udphdr in this case).  For example, run_bpf_filter()
is pulling the transport header.

However, in the __udp6_lib_err() path, the skb->data is pointing to the
ipv6hdr instead of the udphdr.

One option is to pull and push the ipv6hdr in __udp6_lib_err().
Instead of doing this, this patch follows how the original
commit 538950a ("soreuseport: setsockopt SO_ATTACH_REUSEPORT_[CE]BPF")
was done in IPv4, which has passed a NULL skb pointer to
reuseport_select_sock().

Fixes: 538950a ("soreuseport: setsockopt SO_ATTACH_REUSEPORT_[CE]BPF")
Cc: Craig Gallek <kraig@google.com>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Craig Gallek <kraig@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/ipv6/udp.c

@@ -515,7 +515,7 @@ int __udp6_lib_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	struct net *net = dev_net(skb->dev);
 
 	sk = __udp6_lib_lookup(net, daddr, uh->dest, saddr, uh->source,
-			       inet6_iif(skb), inet6_sdif(skb), udptable, skb);
+			       inet6_iif(skb), inet6_sdif(skb), udptable, NULL);
 	if (!sk) {
 		/* No socket for error: try tunnels before discarding */
 		sk = ERR_PTR(-ENOENT);