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>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/kernel/net/netfilter/ipset/ip_set_bitmap_ipmac.c b/kernel/net/netfilter/ipset/ip_set_bitmap_ipmac.c
index 760bbf5..3d25d29 100644
--- a/kernel/net/netfilter/ipset/ip_set_bitmap_ipmac.c
+++ b/kernel/net/netfilter/ipset/ip_set_bitmap_ipmac.c
@@ -52,7 +52,7 @@ struct bitmap_ipmac {
 #ifdef HAVE_TIMER_SETUP
 	struct ip_set *set;	/* attached to this ip_set */
 #endif
-	unsigned char extensions[0]	/* MAC + data extensions */
+	unsigned char extensions[]	/* MAC + data extensions */
 		__aligned(__alignof__(u64));
 };