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

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Message-ID: <20200226212317.GA2172@embeddedor>
Date:   Wed, 26 Feb 2020 15:23:17 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Kevin Curtis <kevin.curtis@...site.co.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Krzysztof Halasa <khc@...waw.pl>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] WAN: 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/wan/farsync.h | 2 +-
 drivers/net/wan/wanxl.c   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wan/farsync.h b/drivers/net/wan/farsync.h
index 47b8e36f97ab..5f43568a9715 100644
--- a/drivers/net/wan/farsync.h
+++ b/drivers/net/wan/farsync.h
@@ -65,7 +65,7 @@
 struct fstioc_write {
         unsigned int  size;
         unsigned int  offset;
-        unsigned char data[0];
+	unsigned char data[];
 };
 
 
diff --git a/drivers/net/wan/wanxl.c b/drivers/net/wan/wanxl.c
index 23f93f1c815d..499f7cd19a4a 100644
--- a/drivers/net/wan/wanxl.c
+++ b/drivers/net/wan/wanxl.c
@@ -78,7 +78,7 @@ struct card {
 	struct sk_buff *rx_skbs[RX_QUEUE_LENGTH];
 	struct card_status *status;	/* shared between host and card */
 	dma_addr_t status_address;
-	struct port ports[0];	/* 1 - 4 port structures follow */
+	struct port ports[];	/* 1 - 4 port structures follow */
 };
 
 
-- 
2.25.0