netfilter: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

Lastly, fix checkpatch.pl warning
WARNING: __aligned(size) is preferred over __attribute__((aligned(size)))
in net/bridge/netfilter/ebtables.c

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/include/net/netfilter/nf_tables.h b/include/net/netfilter/nf_tables.h
index d913cdb6a27b..01f48b5e6bf2 100644
--- a/include/net/netfilter/nf_tables.h
+++ b/include/net/netfilter/nf_tables.h
@@ -224,7 +224,7 @@ int nft_validate_register_store(const struct nft_ctx *ctx,
  */
 struct nft_userdata {
 	u8			len;
-	unsigned char		data[0];
+	unsigned char		data[];
 };
 
 /**
@@ -565,7 +565,7 @@ struct nft_set_ext_tmpl {
 struct nft_set_ext {
 	u8	genmask;
 	u8	offset[NFT_SET_EXT_NUM];
-	char	data[0];
+	char	data[];
 };
 
 static inline void nft_set_ext_prepare(struct nft_set_ext_tmpl *tmpl)
@@ -1375,7 +1375,7 @@ struct nft_trans {
 	int				msg_type;
 	bool				put_net;
 	struct nft_ctx			ctx;
-	char				data[0];
+	char				data[];
 };
 
 struct nft_trans_rule {