Subject: [PATCH][next] netfilter: ebtables: Replace zero-length array with flexible-array member

From: "Gustavo A. R. Silva" <[email-masked]>
Date: Mon, 17 Feb 2020 15:44:23 -0600

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]

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 <[email-masked]>
---
 net/bridge/netfilter/ebtables.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
index e1256e03a9a8..78db58c7aec2 100644
--- a/net/bridge/netfilter/ebtables.c
+++ b/net/bridge/netfilter/ebtables.c
@@ -1561,7 +1561,7 @@ 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 */
-- 
2.25.0