netdev - Re: [PATCH v2] ethtool: Replace 0-length array with flexible array

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Date:   Fri, 6 Jan 2023 09:55:30 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Kees Cook <keescook@...omium.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        kernel test robot <lkp@...el.com>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Sean Anderson <sean.anderson@...o.com>,
        Alexandru Tachici <alexandru.tachici@...log.com>,
        Amit Cohen <amcohen@...dia.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] ethtool: Replace 0-length array with flexible array

On Fri. 6 Jan 2023 at 08:34, Kees Cook <keescook@...omium.org> wrote:
> Zero-length arrays are deprecated[1]. Replace struct ethtool_rxnfc's
> "rule_locs" 0-length array with a flexible array. Detected with GCC 13,
> using -fstrict-flex-arrays=3:
>
> net/ethtool/common.c: In function 'ethtool_get_max_rxnfc_channel':
> net/ethtool/common.c:558:55: warning: array subscript i is outside array bounds of '__u32[0]' {aka 'unsigned int[]'} [-Warray-bounds=]
>   558 |                         .fs.location = info->rule_locs[i],
>       |                                        ~~~~~~~~~~~~~~~^~~
> In file included from include/linux/ethtool.h:19,
>                  from include/uapi/linux/ethtool_netlink.h:12,
>                  from include/linux/ethtool_netlink.h:6,
>                  from net/ethtool/common.c:3:
> include/uapi/linux/ethtool.h:1186:41: note: while referencing
> 'rule_locs'
>  1186 |         __u32                           rule_locs[0];
>       |                                         ^~~~~~~~~
>
> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays
>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Jakub Kicinski <kuba@...nel.org>
> Cc: Andrew Lunn <andrew@...n.ch>
> Cc: kernel test robot <lkp@...el.com>
> Cc: Oleksij Rempel <linux@...pel-privat.de>
> Cc: Sean Anderson <sean.anderson@...o.com>
> Cc: Alexandru Tachici <alexandru.tachici@...log.com>
> Cc: Amit Cohen <amcohen@...dia.com>
> Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>
> Cc: Vincent Mailhol <mailhol.vincent@...adoo.fr>
> Cc: netdev@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> v2: resend, this time without missing netdev CC. :)
> ---
>  include/uapi/linux/ethtool.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/uapi/linux/ethtool.h b/include/uapi/linux/ethtool.h
> index 58e587ba0450..9b97b3e0ec1f 100644
> --- a/include/uapi/linux/ethtool.h
> +++ b/include/uapi/linux/ethtool.h
> @@ -1183,7 +1183,7 @@ struct ethtool_rxnfc {
>                 __u32                   rule_cnt;
>                 __u32                   rss_context;
>         };
> -       __u32                           rule_locs[0];
> +       __DECLARE_FLEX_ARRAY(__u32,     rule_locs);

Can't this simply be a C99 flexible array member?

        __u32                           rule_locs[];

As far as I understand, __DECLARE_FLEX_ARRAY() is a hack to allow the
declaration of a flexible array within unions (which otherwise do not
accept flexible array members). However, ethtool_rxnfc being a struct,
I do not see the need for __DECLARE_FLEX_ARRAY() here.

Yours sincerely,
Vincent Mailhol