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

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Message-ID: <CAPDyKFqd6QKSOzZp2Cq1kMusNQYithdodk2anpWWVOiQ6w0+SQ@mail.gmail.com>
Date:   Fri, 8 May 2020 10:12:11 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Maxim Levitsky <maximlevitsky@...il.com>,
        Alex Dubov <oakad@...oo.com>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memstick: Replace zero-length array with flexible-array

On Thu, 7 May 2020 at 21:17, 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>

Applied for next, thanks!

Kind regards
Uffe


> ---
>  include/linux/memstick.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/memstick.h b/include/linux/memstick.h
> index 216a713bef7f..da4c65f9435f 100644
> --- a/include/linux/memstick.h
> +++ b/include/linux/memstick.h
> @@ -288,7 +288,7 @@ struct memstick_host {
>         int                 (*set_param)(struct memstick_host *host,
>                                          enum memstick_param param,
>                                          int value);
> -       unsigned long       private[0] ____cacheline_aligned;
> +       unsigned long       private[] ____cacheline_aligned;
>  };
>
>  struct memstick_driver {
>