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

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Message-ID: <CAG48ez0Jg9Eeh=RWpYh=sKhzukE3Sza2RKMmNs8o0FrHU0dj9w@mail.gmail.com>
Date:   Fri, 6 Jan 2023 14:19:34 +0100
From:   Jann Horn <jannh@...gle.com>
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>,
        Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3] ethtool: Replace 0-length array with flexible array

On Fri, Jan 6, 2023 at 5:28 AM 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>
> ---
> v3: don't use helper (vincent)
> 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[];

Stupid question: Is this syntax allowed in UAPI headers despite not
being part of standard C90 or C++? Are we relying on all C/C++
compilers for pre-C99 having gcc/clang extensions?