Subject: Re: [PATCH v2] netfilter: ebtables: fix fortify warnings

From: "Gustavo A. R. Silva" <[email-masked]>
Date: Tue, 8 Aug 2023 10:32:50 -0600

On Tue, Aug 08, 2023 at 09:30:38PM +0800, GONG, Ruiqi wrote:
> From: "GONG, Ruiqi" <[email-masked]>
> 
> 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 <[email-masked]>
> ---
> 
> v2: fix HDRTEST error by replacing struct_group() with __struct_group(),
> since it's a uapi header.
> 
>  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..b0caad82b693 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(/* no tag */, offsets, /* no attrs */,
> +		/* 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;
> +	);
>  	unsigned char elems[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
>  };
>  
> diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
> index 757ec46fc45a..5ec66b1ebb64 100644
> --- a/net/bridge/netfilter/ebtables.c
> +++ b/net/bridge/netfilter/ebtables.c
> @@ -2115,8 +2115,7 @@ static int size_entry_mwt(const struct ebt_entry *entry, const unsigned char *ba
>  		return ret;
>  
>  	offsets[0] = sizeof(struct ebt_entry); /* matches come first */
> -	memcpy(&offsets[1], &entry->watchers_offset,
> -			sizeof(offsets) - sizeof(offsets[0]));
> +	memcpy(&offsets[1], &entry->offsets, sizeof(offsets) - sizeof(offsets[0]));
							^^^^^^^^^^^^
You now can replace this ____________________________________|
with just `sizeof(entry->offsets)`

With that change you can add my
Reviewed-by: Gustavo A. R. Silva <[email-masked]>

Thank you
--
Gustavo

>  
>  	if (state->buf_kern_start) {
>  		buf_start = state->buf_kern_start + state->buf_kern_offset;
> -- 
> 2.41.0
>