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Date:   Tue, 8 Aug 2023 17:54:30 +0200
From:   Florian Westphal <fw@strlen.de>
To:     "GONG, Ruiqi" <gongruiqi@huaweicloud.com>
Cc:     Pablo Neira Ayuso <pablo@netfilter.org>,
        Jozsef Kadlecsik <kadlec@netfilter.org>,
        Florian Westphal <fw@strlen.de>,
        Roopa Prabhu <roopa@nvidia.com>,
        Nikolay Aleksandrov <razor@blackwall.org>,
        Kees Cook <keescook@chromium.org>,
        netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
        netdev@vger.kernel.org, linux-hardening@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Wang Weiyang <wangweiyang2@huawei.com>,
        Xiu Jianfeng <xiujianfeng@huawei.com>
Subject: Re: [PATCH] netfilter: ebtables: fix fortify warnings
Message-ID: <20230808155430.GB9741@breakpoint.cc>
References: <20230808014821.241688-1-gongruiqi@huaweicloud.com>
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GONG, Ruiqi <gongruiqi@huaweicloud.com> wrote:
> From: "GONG, Ruiqi" <gongruiqi1@huawei.com>
> 
> When compiling with gcc 13 and CONFIG_FORTIFY_SOURCE=y, the following
> warning appears:
> 
> In function ‘fortify_memcpy_chk’,
>     inlined from ‘size_entry_mwt’ at net/bridge/netfilter/ebtables.c:2118:2:
> ./include/linux/fortify-string.h:592:25: error: call to ‘__read_overflow2_field’
> declared with attribute warning: detected read beyond size of field (2nd parameter);
> maybe use struct_group()? [-Werror=attribute-warning]
>   592 |                         __read_overflow2_field(q_size_field, size);
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> The compiler is complaining:
> 
> memcpy(&offsets[1], &entry->watchers_offset,
>                        sizeof(offsets) - sizeof(offsets[0]));
> 
> where memcpy reads beyong &entry->watchers_offset to copy
> {watchers,target,next}_offset altogether into offsets[]. Silence the
> warning by wrapping these three up via struct_group().
> 
> Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
> ---
>  include/uapi/linux/netfilter_bridge/ebtables.h | 14 ++++++++------
>  net/bridge/netfilter/ebtables.c                |  3 +--
>  2 files changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/include/uapi/linux/netfilter_bridge/ebtables.h b/include/uapi/linux/netfilter_bridge/ebtables.h
> index a494cf43a755..e634da196d08 100644
> --- a/include/uapi/linux/netfilter_bridge/ebtables.h
> +++ b/include/uapi/linux/netfilter_bridge/ebtables.h
> @@ -182,12 +182,14 @@ struct ebt_entry {
>  	unsigned char sourcemsk[ETH_ALEN];
>  	unsigned char destmac[ETH_ALEN];
>  	unsigned char destmsk[ETH_ALEN];
> -	/* sizeof ebt_entry + matches */
> -	unsigned int watchers_offset;
> -	/* sizeof ebt_entry + matches + watchers */
> -	unsigned int target_offset;
> -	/* sizeof ebt_entry + matches + watchers + target */
> -	unsigned int next_offset;
> +	struct_group(offsets,
> +		/* sizeof ebt_entry + matches */

This is an UAPI header, I think you need to use __struct_group here.