blk-mq: 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/blk-mq.h b/include/linux/blk-mq.h
index f389d7c724bd2b25a8a6477060079282da7ff88e..b45148ba3291208fe83bcdd60d9c784ba725d4be 100644 (file)
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -173,7 +173,7 @@ struct blk_mq_hw_ctx {
         * blocking (BLK_MQ_F_BLOCKING). Must be the last member - see also
         * blk_mq_hw_ctx_size().
         */
-       struct srcu_struct      srcu[0];
+       struct srcu_struct      srcu[];
 };
 
 /**