linux-kernel - [PATCH][next] net: hns: Replace zero-length array with flexible-array member

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Date:   Mon, 24 Feb 2020 18:09:39 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Yisen Zhuang <yisen.zhuang@...wei.com>,
        Salil Mehta <salil.mehta@...wei.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] net: hns: 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@...eddedor.com>
---
 drivers/net/ethernet/hisilicon/hns/hns_dsaf_ppe.h | 2 +-
 drivers/net/ethernet/hisilicon/hns/hns_dsaf_rcb.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/hisilicon/hns/hns_dsaf_ppe.h b/drivers/net/ethernet/hisilicon/hns/hns_dsaf_ppe.h
index 2721f1f1ab42..0f0e16f9afc0 100644
--- a/drivers/net/ethernet/hisilicon/hns/hns_dsaf_ppe.h
+++ b/drivers/net/ethernet/hisilicon/hns/hns_dsaf_ppe.h
@@ -92,7 +92,7 @@ struct ppe_common_cb {
 	u8 comm_index;   /*ppe_common index*/
 
 	u32 ppe_num;
-	struct hns_ppe_cb ppe_cb[0];
+	struct hns_ppe_cb ppe_cb[];
 
 };
 
diff --git a/drivers/net/ethernet/hisilicon/hns/hns_dsaf_rcb.h b/drivers/net/ethernet/hisilicon/hns/hns_dsaf_rcb.h
index 3741befb914e..a9f805925699 100644
--- a/drivers/net/ethernet/hisilicon/hns/hns_dsaf_rcb.h
+++ b/drivers/net/ethernet/hisilicon/hns/hns_dsaf_rcb.h
@@ -108,7 +108,7 @@ struct rcb_common_cb {
 	u32 ring_num;
 	u32 desc_num; /*  desc num per queue*/
 
-	struct ring_pair_cb ring_pair_cb[0];
+	struct ring_pair_cb ring_pair_cb[];
 };
 
 int hns_rcb_buf_size2type(u32 buf_size);
-- 
2.25.0