netfilter: helper: add build-time asserts for helper data size

add a 32 byte scratch area in the helper struct instead of relying
on variable sized helpers plus compile-time asserts to let us know
if 32 bytes aren't enough anymore.

Not having variable sized helpers will later allow to add BUILD_BUG_ON
for the total size of conntrack extensions -- the helper extension is
the only one that doesn't have a fixed size.

The (useless!) NF_CT_HELPER_BUILD_BUG_ON(0); are added so that in case
someone adds a new helper and copy-pastes from one that doesn't store
private data at least some indication that this macro should be used
somehow is there...

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/net/netfilter/nf_conntrack_netbios_ns.c b/net/netfilter/nf_conntrack_netbios_ns.c
index 4c8f30a3d6d2..496ce173f0c1 100644
--- a/net/netfilter/nf_conntrack_netbios_ns.c
+++ b/net/netfilter/nf_conntrack_netbios_ns.c
@@ -58,6 +58,8 @@ static struct nf_conntrack_helper helper __read_mostly = {
 
 static int __init nf_conntrack_netbios_ns_init(void)
 {
+	NF_CT_HELPER_BUILD_BUG_ON(0);
+
 	exp_policy.timeout = timeout;
 	return nf_conntrack_helper_register(&helper);
 }