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

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Message-ID: <202301052021.5AEF89CB@keescook>
Date:   Thu, 5 Jan 2023 20:22:17 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
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, Jan 06, 2023 at 09:55:30AM +0900, Vincent MAILHOL wrote:
> 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.

Good point -- I think my eyes scanned through the "union" above and I
just jumped at using __DECLARE_FLEX_ARRAY. I'll send a v2. Thanks!

-- 
Kees Cook