RDMA: 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")

Link: https://lore.kernel.org/r/20200213010425.GA13068@embeddedor.com
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com> # added a few more

1 files changed, 1 insertions, 1 deletions

diff --git a/include/rdma/rdmavt_mr.h b/include/rdma/rdmavt_mr.h
index 72a3856d4057..ce6c888f7fe7 100644
--- a/include/rdma/rdmavt_mr.h
+++ b/include/rdma/rdmavt_mr.h
@@ -85,7 +85,7 @@ struct rvt_mregion {
 	u8  lkey_published;     /* in global table */
 	struct percpu_ref refcount;
 	struct completion comp; /* complete when refcount goes to zero */
-	struct rvt_segarray *map[0];    /* the segments */
+	struct rvt_segarray *map[];    /* the segments */
 };
 
 #define RVT_MAX_LKEY_TABLE_BITS 23