linux-kernel - Re: [PATCH][next] hwspinlock: hwspinlock_internal.h: Replace zero-length array with flexible-array member

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Message-ID: <CADBw62qjK=0_UKmOyvvYszLkjgVi8LQTwAg1Vqm07-89fOU=fg@mail.gmail.com>
Date:   Fri, 20 Mar 2020 09:17:57 +0800
From:   Baolin Wang <baolin.wang7@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-remoteproc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] hwspinlock: hwspinlock_internal.h: Replace
 zero-length array with flexible-array member

On Fri, Mar 20, 2020 at 5:38 AM Gustavo A. R. Silva
<gustavo@...eddedor.com> wrote:
>
> 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>

Looks good to me. Thanks.
Reviewed-by: Baolin Wang <baolin.wang7@...il.com>

> ---
>  drivers/hwspinlock/hwspinlock_internal.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/hwspinlock/hwspinlock_internal.h b/drivers/hwspinlock/hwspinlock_internal.h
> index 9eb6bd020dc7..29892767bb7a 100644
> --- a/drivers/hwspinlock/hwspinlock_internal.h
> +++ b/drivers/hwspinlock/hwspinlock_internal.h
> @@ -56,7 +56,7 @@ struct hwspinlock_device {
>         const struct hwspinlock_ops *ops;
>         int base_id;
>         int num_locks;
> -       struct hwspinlock lock[0];
> +       struct hwspinlock lock[];
>  };
>
>  static inline int hwlock_to_id(struct hwspinlock *hwlock)
> --
> 2.23.0
>


-- 
Baolin Wang