linux-kernel - [PATCH][next] 6lowpan: Replace zero-length array with flexible-array member

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Message-ID: <20200228134758.GA29760@embeddedor>
Date:   Fri, 28 Feb 2020 07:47:58 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Alexander Aring <alex.aring@...il.com>,
        Jukka Rissanen <jukka.rissanen@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     linux-bluetooth@...r.kernel.org, linux-wpan@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] 6lowpan: 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>
---
 include/net/6lowpan.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/6lowpan.h b/include/net/6lowpan.h
index a71378007e61..c80539be1542 100644
--- a/include/net/6lowpan.h
+++ b/include/net/6lowpan.h
@@ -138,7 +138,7 @@ struct lowpan_dev {
 	struct lowpan_iphc_ctx_table ctx;
 
 	/* must be last */
-	u8 priv[0] __aligned(sizeof(void *));
+	u8 priv[] __aligned(sizeof(void *));
 };
 
 struct lowpan_802154_neigh {
-- 
2.25.0