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From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To: Alexander Aring <alex.aring@gmail.com>,
	Stefan Schmidt <stefan@datenfreihafen.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>
Cc: linux-wpan@vger.kernel.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: [PATCH][next] cfg802154: Replace zero-length array with flexible-array member
Date: Fri, 28 Feb 2020 07:59:59 -0600
Message-ID: <20200228135959.GA30464@embeddedor> (raw)

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>
---
 include/net/cfg802154.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/cfg802154.h b/include/net/cfg802154.h
index 6f86073a5d7d..6ed07844eb24 100644
--- a/include/net/cfg802154.h
+++ b/include/net/cfg802154.h
@@ -214,7 +214,7 @@ struct wpan_phy {
 	/* the network namespace this phy lives in currently */
 	possible_net_t _net;
 
-	char priv[0] __aligned(NETDEV_ALIGN);
+	char priv[] __aligned(NETDEV_ALIGN);
 };
 
 static inline struct net *wpan_phy_net(struct wpan_phy *wpan_phy)
-- 
2.25.0


             reply index

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-28 13:59 Gustavo A. R. Silva [this message]
2020-02-28 15:22 ` Stefan Schmidt

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