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, 2 insertions, 2 deletions

diff --git a/net/ipv4/netfilter/ip_tables.c b/net/ipv4/netfilter/ip_tables.c
index 10b91ebdf213..c2670eaa74e6 100644
--- a/net/ipv4/netfilter/ip_tables.c
+++ b/net/ipv4/netfilter/ip_tables.c
@@ -1211,7 +1211,7 @@ struct compat_ipt_replace {
 	u32			underflow[NF_INET_NUMHOOKS];
 	u32			num_counters;
 	compat_uptr_t		counters;	/* struct xt_counters * */
-	struct compat_ipt_entry	entries[0];
+	struct compat_ipt_entry	entries[];
 };
 
 static int
@@ -1562,7 +1562,7 @@ compat_do_ipt_set_ctl(struct sock *sk,	int cmd, void __user *user,
 struct compat_ipt_get_entries {
 	char name[XT_TABLE_MAXNAMELEN];
 	compat_uint_t size;
-	struct compat_ipt_entry entrytable[0];
+	struct compat_ipt_entry entrytable[];
 };
 
 static int