linux-kernel - Re: [PATCH] auxdisplay: charlcd: replace zero-length array with flexible-array member

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Message-ID: <CANiq72nt19x2Z6ErT8a1_2c0sKfjkv_yUFMZS2mf3HWp3RvnDQ@mail.gmail.com>
Date:   Wed, 12 Feb 2020 21:49:47 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] auxdisplay: charlcd: replace zero-length array with
 flexible-array member

Hi Gustavo,

On Wed, Feb 12, 2020 at 8:49 PM 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
> inadvertenly introduced[3] to the codebase from now on.

s:inadvertenly:inadvertently:

> 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>

I saw the discussion regarding this -- thanks! Do you want me to
handle this or will you push everything centrally? If the latter, have
my

Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>

Cheers,
Miguel