net: switchdev: 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/net/switchdev/switchdev.c b/net/switchdev/switchdev.c
index 3a1d428..6063076 100644 (file)
--- a/net/switchdev/switchdev.c
+++ b/net/switchdev/switchdev.c
@@ -29,7 +29,7 @@ struct switchdev_deferred_item {
        struct list_head list;
        struct net_device *dev;
        switchdev_deferred_func_t *func;
-       unsigned long data[0];
+       unsigned long data[];
 };
 
 static struct switchdev_deferred_item *switchdev_deferred_dequeue(void)