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

[ Upstream commit 6daf14140129d30207ed6a0a69851fa6a3636bda ]

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 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>
Stable-dep-of: 542ed8145e6f ("netfilter: nft_set_hash: unaligned atomic read on struct nft_set_ext")
Signed-off-by: Sasha Levin <[email protected]>

net/bridge/netfilter/ebtables.c

@@ -1581,7 +1581,7 @@ struct compat_ebt_entry_mwt {
 		compat_uptr_t ptr;
 	} u;
 	compat_uint_t match_size;
-	compat_uint_t data[0] __attribute__ ((aligned (__alignof__(struct compat_ebt_replace))));
+	compat_uint_t data[] __aligned(__alignof__(struct compat_ebt_replace));
 };
 
 /* account for possible padding between match_size and ->data */