net: mip6: 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]

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: David S. Miller <davem@davemloft.net>

diff --git a/include/net/mip6.h b/include/net/mip6.h
index f1c28971c362f50ad3d59e57f34a7bad9fc13b31..67cd7e50804c7d7e58188e1a1e5876e437dac65e 100644 (file)
--- a/include/net/mip6.h
+++ b/include/net/mip6.h
@@ -25,7 +25,7 @@ struct ip6_mh {
        __u8    ip6mh_reserved;
        __u16   ip6mh_cksum;
        /* Followed by type specific messages */
-       __u8    data[0];
+       __u8    data[];
 } __packed;
 
 #define IP6_MH_TYPE_BRR                0   /* Binding Refresh Request */