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From: Geert Uytterhoeven <geert@linux-m68k.org>
To: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: linux-m68k <linux-m68k@lists.linux-m68k.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] zorro: Replace zero-length array with flexible-array member
Date: Tue, 3 Mar 2020 10:57:19 +0100	[thread overview]
Message-ID: <CAMuHMdXCWtgGKM1Uqtps4CYbGsfBZtoQ8e4t8VrMoL29nP4V3w@mail.gmail.com> (raw)
In-Reply-To: <20200227183748.GA31018@embeddedor>

Hi Gustavo,

Thanks for your patch!

On Thu, Feb 27, 2020 at 7:35 PM Gustavo A. R. Silva
<gustavo@embeddedor.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")

Note that in practice, this "undefined behavior" may lead to corrupting
the next member, when the zero-length array is written to.

> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
i.e. will queue in the m68k for-v5.7 branch.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

      reply	other threads:[~2020-03-03  9:57 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-27 18:37 Gustavo A. R. Silva
2020-03-03  9:57 ` Geert Uytterhoeven [this message]

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