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 <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_conntrack_helper.h

···
 	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);

net/netfilter/nf_conntrack_amanda.c

···
 {
 	int ret, i;
 
+	NF_CT_HELPER_BUILD_BUG_ON(0);
+
 	for (i = 0; i < ARRAY_SIZE(search); i++) {
 		search[i].ts = textsearch_prepare(ts_algo, search[i].string,
 						  search[i].len,

net/netfilter/nf_conntrack_ftp.c

···
 {
 	int i, ret = 0;
 
+	NF_CT_HELPER_BUILD_BUG_ON(sizeof(struct nf_ct_ftp_master));
+
 	ftp_buffer = kmalloc(65536, GFP_KERNEL);
 	if (!ftp_buffer)
 		return -ENOMEM;

net/netfilter/nf_conntrack_h323_main.c

···
 {
 	int ret;
 
+	NF_CT_HELPER_BUILD_BUG_ON(sizeof(struct nf_ct_h323_master));
+
 	h323_buffer = kmalloc(65536, GFP_KERNEL);
 	if (!h323_buffer)
 		return -ENOMEM;

net/netfilter/nf_conntrack_netbios_ns.c

···
 
 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);
 }

net/netfilter/nf_conntrack_pptp.c

···
 
 static int __init nf_conntrack_pptp_init(void)
 {
+	NF_CT_HELPER_BUILD_BUG_ON(sizeof(struct nf_ct_pptp_master));
+
 	return nf_conntrack_helper_register(&pptp);
 }
 

net/netfilter/nf_conntrack_sane.c

···
 {
 	int i, ret = 0;
 
+	NF_CT_HELPER_BUILD_BUG_ON(sizeof(struct nf_ct_sane_master));
+
 	sane_buffer = kmalloc(65536, GFP_KERNEL);
 	if (!sane_buffer)
 		return -ENOMEM;

net/netfilter/nf_conntrack_sip.c

···
 {
 	int i, ret;
 
+	NF_CT_HELPER_BUILD_BUG_ON(sizeof(struct nf_ct_sip_master));
+
 	if (ports_c == 0)
 		ports[ports_c++] = SIP_PORT;
 

net/netfilter/nf_conntrack_tftp.c

···
 {
 	int i, ret;
 
+	NF_CT_HELPER_BUILD_BUG_ON(0);
+
 	if (ports_c == 0)
 		ports[ports_c++] = TFTP_PORT;