netfilter: conntrack: helper: Replace -EEXIST by -EBUSY

[ Upstream commit 54416fd ]

The helper registration return value is passed-through by module_init
callbacks which modprobe confuses with the harmless -EEXIST returned
when trying to load an already loaded module.

Make sure modprobe fails so users notice their helper has not been
registered and won't work.

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Fixes: 12f7a50 ("netfilter: add user-space connection tracking helper infrastructure")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: Phil Sutter

net/netfilter/nf_conntrack_helper.c

@@ -368,7 +368,7 @@ int nf_conntrack_helper_register(struct nf_conntrack_helper *me)
 			    (cur->tuple.src.l3num == NFPROTO_UNSPEC ||
 			     cur->tuple.src.l3num == me->tuple.src.l3num) &&
 			    cur->tuple.dst.protonum == me->tuple.dst.protonum) {
-				ret = -EEXIST;
+				ret = -EBUSY;
 				goto out;
 			}
 		}
@@ -379,7 +379,7 @@ int nf_conntrack_helper_register(struct nf_conntrack_helper *me)
 		hlist_for_each_entry(cur, &nf_ct_helper_hash[h], hnode) {
 			if (nf_ct_tuple_src_mask_cmp(&cur->tuple, &me->tuple,
 						     &mask)) {
-				ret = -EEXIST;
+				ret = -EBUSY;
 				goto out;
 			}
 		}