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

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Date:   Mon, 24 Feb 2020 10:43:46 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Claudiu Manoil <claudiu.manoil@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Fugang Duan <fugang.duan@....com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] freescale: 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/freescale/enetc/enetc.h    | 2 +-
 drivers/net/ethernet/freescale/enetc/enetc_hw.h | 2 +-
 drivers/net/ethernet/freescale/fec.h            | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/freescale/enetc/enetc.h b/drivers/net/ethernet/freescale/enetc/enetc.h
index dd4a227ffc7a..9938f7a5fc0a 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc.h
+++ b/drivers/net/ethernet/freescale/enetc/enetc.h
@@ -163,7 +163,7 @@ struct enetc_int_vector {
 	char name[ENETC_INT_NAME_MAX];
 
 	struct enetc_bdr rx_ring ____cacheline_aligned_in_smp;
-	struct enetc_bdr tx_ring[0];
+	struct enetc_bdr tx_ring[];
 };
 
 struct enetc_cls_rule {
diff --git a/drivers/net/ethernet/freescale/enetc/enetc_hw.h b/drivers/net/ethernet/freescale/enetc/enetc_hw.h
index 62554f28ce07..da134e211c1a 100644
--- a/drivers/net/ethernet/freescale/enetc/enetc_hw.h
+++ b/drivers/net/ethernet/freescale/enetc/enetc_hw.h
@@ -588,7 +588,7 @@ struct tgs_gcl_data {
 	__le32		bth;
 	__le32		ct;
 	__le32		cte;
-	struct gce	entry[0];
+	struct gce	entry[];
 };
 
 struct enetc_cbd {
diff --git a/drivers/net/ethernet/freescale/fec.h b/drivers/net/ethernet/freescale/fec.h
index f79e57f735b3..bd898f5b4da5 100644
--- a/drivers/net/ethernet/freescale/fec.h
+++ b/drivers/net/ethernet/freescale/fec.h
@@ -584,7 +584,7 @@ struct fec_enet_private {
 	int pps_enable;
 	unsigned int next_counter;
 
-	u64 ethtool_stats[0];
+	u64 ethtool_stats[];
 };
 
 void fec_ptp_init(struct platform_device *pdev, int irq_idx);
-- 
2.25.0