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>

6 files changed, 9 insertions, 9 deletions

diff --git a/include/linux/netfilter/ipset/ip_set.h b/include/linux/netfilter/ipset/ip_set.h
index 5448c8b443db..ab192720e2d6 100644
--- a/include/linux/netfilter/ipset/ip_set.h
+++ b/include/linux/netfilter/ipset/ip_set.h
@@ -98,7 +98,7 @@ struct ip_set_counter {
 
 struct ip_set_comment_rcu {
 	struct rcu_head rcu;
-	char str[0];
+	char str[];
 };
 
 struct ip_set_comment {
diff --git a/include/linux/netfilter/x_tables.h b/include/linux/netfilter/x_tables.h
index 1b261c51b3a3..5da88451853b 100644
--- a/include/linux/netfilter/x_tables.h
+++ b/include/linux/netfilter/x_tables.h
@@ -264,7 +264,7 @@ struct xt_table_info {
 	unsigned int stacksize;
 	void ***jumpstack;
 
-	unsigned char entries[0] __aligned(8);
+	unsigned char entries[] __aligned(8);
 };
 
 int xt_register_target(struct xt_target *target);
@@ -464,7 +464,7 @@ struct compat_xt_entry_match {
 		} kernel;
 		u_int16_t match_size;
 	} u;
-	unsigned char data[0];
+	unsigned char data[];
 };
 
 struct compat_xt_entry_target {
@@ -480,7 +480,7 @@ struct compat_xt_entry_target {
 		} kernel;
 		u_int16_t target_size;
 	} u;
-	unsigned char data[0];
+	unsigned char data[];
 };
 
 /* FIXME: this works only on 32 bit tasks
@@ -494,7 +494,7 @@ struct compat_xt_counters {
 struct compat_xt_counters_info {
 	char name[XT_TABLE_MAXNAMELEN];
 	compat_uint_t num_counters;
-	struct compat_xt_counters counters[0];
+	struct compat_xt_counters counters[];
 };
 
 struct _compat_xt_align {
diff --git a/include/linux/netfilter_arp/arp_tables.h b/include/linux/netfilter_arp/arp_tables.h
index e98028f00e47..7d3537c40ec9 100644
--- a/include/linux/netfilter_arp/arp_tables.h
+++ b/include/linux/netfilter_arp/arp_tables.h
@@ -67,7 +67,7 @@ struct compat_arpt_entry {
 	__u16 next_offset;
 	compat_uint_t comefrom;
 	struct compat_xt_counters counters;
-	unsigned char elems[0];
+	unsigned char elems[];
 };
 
 static inline struct xt_entry_target *
diff --git a/include/linux/netfilter_bridge/ebtables.h b/include/linux/netfilter_bridge/ebtables.h
index 162f59d0d17a..2f5c4e6ecd8a 100644
--- a/include/linux/netfilter_bridge/ebtables.h
+++ b/include/linux/netfilter_bridge/ebtables.h
@@ -85,7 +85,7 @@ struct ebt_table_info {
 	/* room to maintain the stack used for jumping from and into udc */
 	struct ebt_chainstack **chainstack;
 	char *entries;
-	struct ebt_counter counters[0] ____cacheline_aligned;
+	struct ebt_counter counters[] ____cacheline_aligned;
 };
 
 struct ebt_table {
diff --git a/include/linux/netfilter_ipv4/ip_tables.h b/include/linux/netfilter_ipv4/ip_tables.h
index e9e1ed74cdf1..b394bd4f68a3 100644
--- a/include/linux/netfilter_ipv4/ip_tables.h
+++ b/include/linux/netfilter_ipv4/ip_tables.h
@@ -76,7 +76,7 @@ struct compat_ipt_entry {
 	__u16 next_offset;
 	compat_uint_t comefrom;
 	struct compat_xt_counters counters;
-	unsigned char elems[0];
+	unsigned char elems[];
 };
 
 /* Helper functions */
diff --git a/include/linux/netfilter_ipv6/ip6_tables.h b/include/linux/netfilter_ipv6/ip6_tables.h
index 78ab959c4575..8225f7821a29 100644
--- a/include/linux/netfilter_ipv6/ip6_tables.h
+++ b/include/linux/netfilter_ipv6/ip6_tables.h
@@ -43,7 +43,7 @@ struct compat_ip6t_entry {
 	__u16 next_offset;
 	compat_uint_t comefrom;
 	struct compat_xt_counters counters;
-	unsigned char elems[0];
+	unsigned char elems[];
 };
 
 static inline struct xt_entry_target *