netdev - [PATCH][next] batman-adv: Replace zero-length array with flexible-array member

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Message-ID: <20200217214300.GA14129@embeddedor>
Date:   Mon, 17 Feb 2020 15:43:00 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Marek Lindner <mareklindner@...mailbox.ch>,
        Simon Wunderlich <sw@...onwunderlich.de>,
        Antonio Quartulli <a@...table.cc>,
        Sven Eckelmann <sven@...fation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     b.a.t.m.a.n@...ts.open-mesh.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] batman-adv: 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>
---
 net/batman-adv/distributed-arp-table.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/batman-adv/distributed-arp-table.c b/net/batman-adv/distributed-arp-table.c
index 3d21dd83f8cc..b85da4b7a77b 100644
--- a/net/batman-adv/distributed-arp-table.c
+++ b/net/batman-adv/distributed-arp-table.c
@@ -88,7 +88,7 @@ struct batadv_dhcp_packet {
 	__u8 sname[64];
 	__u8 file[128];
 	__be32 magic;
-	__u8 options[0];
+	__u8 options[];
 };
 
 #define BATADV_DHCP_YIADDR_LEN sizeof(((struct batadv_dhcp_packet *)0)->yiaddr)
-- 
2.25.0