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, 4 insertions, 1 deletions

diff --git a/include/net/netfilter/nf_conntrack_helper.h b/include/net/netfilter/nf_conntrack_helper.h
index 15d746558665..29539ed1008f 100644
--- a/include/net/netfilter/nf_conntrack_helper.h
+++ b/include/net/netfilter/nf_conntrack_helper.h
@@ -66,9 +66,12 @@ struct nf_conn_help {
 	u8 expecting[NF_CT_MAX_EXPECT_CLASSES];
 
 	/* private helper information. */
-	char data[];
+	char data[32] __aligned(8);
 };
 
+#define NF_CT_HELPER_BUILD_BUG_ON(structsize) \
+	BUILD_BUG_ON((structsize) > FIELD_SIZEOF(struct nf_conn_help, data))
+
 struct nf_conntrack_helper *__nf_conntrack_helper_find(const char *name,
 						       u16 l3num, u8 protonum);