linux-kernel - Re: [PATCH v3] ethtool: Replace 0-length array with flexible array

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Message-ID: <CAMZ6RqKyvGwh7iOVvtONGYA21==Lj6p9JECstCBFSWRRCPAVZQ@mail.gmail.com>
Date:   Fri, 6 Jan 2023 14:38:18 +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 v3] ethtool: Replace 0-length array with flexible array

On Fri. 6 Jan 2023 at 13:28, 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

Side comment, this link does not mention the __DECLARE_FLEX_ARRAY().
It could be good to add a reference to the helper here. But of course,
this is not a criticism of this patch.

> 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>

Reviewed-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>

> ---
> v3: don't use helper (vincent)

You may want to double check your other patches as well. At least this
one is also using the helper when not needed:

  https://lore.kernel.org/netdev/20230105223642.never.980-kees@kernel.org/T/#u

> v2: https://lore.kernel.org/lkml/20230105233420.gonna.036-kees@kernel.org
> ---
>  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..3135fa0ba9a4 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];
> +       __u32                           rule_locs[];
>  };
>
>
> --
> 2.34.1
>