linux-kernel - Re: [PATCH] treewide: Replace zero-length array with flexible-array

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Message-ID: <CADRPPNQy5PE8vDLCu+XeZpXDab=OnrLcUjWG5F0=uy00QXOQnw@mail.gmail.com>
Date:   Fri, 22 May 2020 16:24:37 -0500
From:   Li Yang <leoyang.li@....com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] treewide: Replace zero-length array with flexible-array

On Thu, May 7, 2020 at 1:49 PM Gustavo A. R. Silva
<gustavoars@...nel.org> 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]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> 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 <gustavoars@...nel.org>
> ---
>  include/linux/fsl/bestcomm/bestcomm.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Applied for next.  Thanks.

Regards,
Leo

>
> diff --git a/include/linux/fsl/bestcomm/bestcomm.h b/include/linux/fsl/bestcomm/bestcomm.h
> index a0e2e6b19b57..154e541ce57e 100644
> --- a/include/linux/fsl/bestcomm/bestcomm.h
> +++ b/include/linux/fsl/bestcomm/bestcomm.h
> @@ -27,7 +27,7 @@
>   */
>  struct bcom_bd {
>         u32     status;
> -       u32     data[0];        /* variable payload size */
> +       u32     data[]; /* variable payload size */
>  };
>
>  /* ======================================================================== */
>