swap.h: 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>

diff --git a/include/linux/swap.h b/include/linux/swap.h
index b835d8dbea0e3d95ef552ba2540939ced3358cb5..e1bbf7a16b2767ab08d53f9b297d5c1c87cc992f 100644 (file)
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -275,7 +275,7 @@ struct swap_info_struct {
                                         */
        struct work_struct discard_work; /* discard worker */
        struct swap_cluster_list discard_clusters; /* discard clusters list */
-       struct plist_node avail_lists[0]; /*
+       struct plist_node avail_lists[]; /*
                                           * entries in swap_avail_heads, one
                                           * entry per node.
                                           * Must be last as the number of the