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>

4 files changed, 5 insertions, 5 deletions

diff --git a/net/netfilter/ipset/ip_set_bitmap_ip.c b/net/netfilter/ipset/ip_set_bitmap_ip.c
index 0a2196f591064..486959f70cf31 100644
--- a/net/netfilter/ipset/ip_set_bitmap_ip.c
+++ b/net/netfilter/ipset/ip_set_bitmap_ip.c
@@ -46,7 +46,7 @@ struct bitmap_ip {
 	u8 netmask;		/* subnet netmask */
 	struct timer_list gc;	/* garbage collection */
 	struct ip_set *set;	/* attached to this ip_set */
-	unsigned char extensions[0]	/* data extensions */
+	unsigned char extensions[]	/* data extensions */
 		__aligned(__alignof__(u64));
 };
 
diff --git a/net/netfilter/ipset/ip_set_bitmap_ipmac.c b/net/netfilter/ipset/ip_set_bitmap_ipmac.c
index 739e343efaf63..2310a316e0aff 100644
--- a/net/netfilter/ipset/ip_set_bitmap_ipmac.c
+++ b/net/netfilter/ipset/ip_set_bitmap_ipmac.c
@@ -49,7 +49,7 @@ struct bitmap_ipmac {
 	size_t memsize;		/* members size */
 	struct timer_list gc;	/* garbage collector */
 	struct ip_set *set;	/* attached to this ip_set */
-	unsigned char extensions[0]	/* MAC + data extensions */
+	unsigned char extensions[]	/* MAC + data extensions */
 		__aligned(__alignof__(u64));
 };
 
diff --git a/net/netfilter/ipset/ip_set_bitmap_port.c b/net/netfilter/ipset/ip_set_bitmap_port.c
index b49978dd810dc..e56ced66f202d 100644
--- a/net/netfilter/ipset/ip_set_bitmap_port.c
+++ b/net/netfilter/ipset/ip_set_bitmap_port.c
@@ -37,7 +37,7 @@ struct bitmap_port {
 	size_t memsize;		/* members size */
 	struct timer_list gc;	/* garbage collection */
 	struct ip_set *set;	/* attached to this ip_set */
-	unsigned char extensions[0]	/* data extensions */
+	unsigned char extensions[]	/* data extensions */
 		__aligned(__alignof__(u64));
 };
 
diff --git a/net/netfilter/ipset/ip_set_hash_gen.h b/net/netfilter/ipset/ip_set_hash_gen.h
index e52d7b7597a0d..1ee43752d6d3c 100644
--- a/net/netfilter/ipset/ip_set_hash_gen.h
+++ b/net/netfilter/ipset/ip_set_hash_gen.h
@@ -76,7 +76,7 @@ struct hbucket {
 	DECLARE_BITMAP(used, AHASH_MAX_TUNED);
 	u8 size;		/* size of the array */
 	u8 pos;			/* position of the first free entry */
-	unsigned char value[0]	/* the array of the values */
+	unsigned char value[]	/* the array of the values */
 		__aligned(__alignof__(u64));
 };
 
@@ -109,7 +109,7 @@ struct htable {
 	u8 htable_bits;		/* size of hash table == 2^htable_bits */
 	u32 maxelem;		/* Maxelem per region */
 	struct ip_set_region *hregion;	/* Region locks and ext sizes */
-	struct hbucket __rcu *bucket[0]; /* hashtable buckets */
+	struct hbucket __rcu *bucket[]; /* hashtable buckets */
 };
 
 #define hbucket(h, i)		((h)->bucket[i])