linux-kernel - Re: [PATCH] Bluetooth: btintel: Replace zero-length array with flexible-array member

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Message-Id: <CEB64C73-3868-4D26-A610-560F2ABDAF9D@holtmann.org>
Date:   Thu, 13 Feb 2020 08:30:12 +0100
From:   Marcel Holtmann <marcel@...tmann.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Johan Hedberg <johan.hedberg@...il.com>,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Bluetooth: btintel: Replace zero-length array with
 flexible-array member

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
> inadvertenly 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/bluetooth/btintel.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)

patch has been applied to bluetooth-next tree.

Regards

Marcel