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From: Marcel Holtmann <marcel@holtmann.org>
To: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Alexander Aring <alex.aring@gmail.com>,
	Jukka Rissanen <jukka.rissanen@linux.intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	linux-bluetooth@vger.kernel.org, linux-wpan@vger.kernel.org,
	netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH][next] 6lowpan: Replace zero-length array with flexible-array member
Date: Fri, 28 Feb 2020 14:52:05 +0100
Message-ID: <170C625F-CF47-4474-970B-EB31FA49E9A4@holtmann.org> (raw)
In-Reply-To: <20200228134758.GA29760@embeddedor>

Hi Gustavo,

> 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/6lowpan.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)

patch has been applied to bluetooth-next tree.

Regards

Marcel


  reply index

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-28 13:47 Gustavo A. R. Silva
2020-02-28 13:52 ` Marcel Holtmann [this message]
2020-02-28 14:02   ` Gustavo A. R. Silva

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