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

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Date:   Wed, 25 Mar 2020 11:05:36 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] acpi: nfit.h: Replace zero-length array with
 flexible-array member

On Thu, Mar 19, 2020 at 9:15 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
> 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>

Dan,

I'm assuming that you will take care of this one or please let me know
otherwise.

Cheers!

> ---
>  drivers/acpi/nfit/nfit.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/acpi/nfit/nfit.h b/drivers/acpi/nfit/nfit.h
> index 24241941181c..af09143ce403 100644
> --- a/drivers/acpi/nfit/nfit.h
> +++ b/drivers/acpi/nfit/nfit.h
> @@ -144,32 +144,32 @@ struct nfit_spa {
>         unsigned long ars_state;
>         u32 clear_err_unit;
>         u32 max_ars;
> -       struct acpi_nfit_system_address spa[0];
> +       struct acpi_nfit_system_address spa[];
>  };
>
>  struct nfit_dcr {
>         struct list_head list;
> -       struct acpi_nfit_control_region dcr[0];
> +       struct acpi_nfit_control_region dcr[];
>  };
>
>  struct nfit_bdw {
>         struct list_head list;
> -       struct acpi_nfit_data_region bdw[0];
> +       struct acpi_nfit_data_region bdw[];
>  };
>
>  struct nfit_idt {
>         struct list_head list;
> -       struct acpi_nfit_interleave idt[0];
> +       struct acpi_nfit_interleave idt[];
>  };
>
>  struct nfit_flush {
>         struct list_head list;
> -       struct acpi_nfit_flush_address flush[0];
> +       struct acpi_nfit_flush_address flush[];
>  };
>
>  struct nfit_memdev {
>         struct list_head list;
> -       struct acpi_nfit_memory_map memdev[0];
> +       struct acpi_nfit_memory_map memdev[];
>  };
>
>  enum nfit_mem_flags {
> --
> 2.23.0
>