netdev - Re: [PATCH] netfilter: nft_bitwise: Adjust parentheses to fix memcmp size argument

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Date:   Wed, 14 Aug 2019 23:56:16 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        kbuild test robot <lkp@...el.com>
Subject: Re: [PATCH] netfilter: nft_bitwise: Adjust parentheses to fix
 memcmp size argument

On Wed, 2019-08-14 at 09:58 -0700, Nathan Chancellor wrote:
> clang warns:
> 
> net/netfilter/nft_bitwise.c:138:50: error: size argument in 'memcmp'
> call is a comparison [-Werror,-Wmemsize-comparison]
>         if (memcmp(&priv->xor, &zero, sizeof(priv->xor) ||
>                                       ~~~~~~~~~~~~~~~~~~^~
> net/netfilter/nft_bitwise.c:138:6: note: did you mean to compare the
> result of 'memcmp' instead?
>         if (memcmp(&priv->xor, &zero, sizeof(priv->xor) ||
>             ^
>                                                        )
> net/netfilter/nft_bitwise.c:138:32: note: explicitly cast the argument
> to size_t to silence this warning
>         if (memcmp(&priv->xor, &zero, sizeof(priv->xor) ||
>                                       ^
>                                       (size_t)(
> 1 error generated.
> 
> Adjust the parentheses so that the result of the sizeof is used for the
> size argument in memcmp, rather than the result of the comparison (which
> would always be true because sizeof is a non-zero number).
> 
> Fixes: bd8699e9e292 ("netfilter: nft_bitwise: add offload support")
> Link: https://github.com/ClangBuiltLinux/linux/issues/638
> Reported-by: kbuild test robot <lkp@...el.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  net/netfilter/nft_bitwise.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/nft_bitwise.c b/net/netfilter/nft_bitwise.c
[]
> @@ -135,8 +135,8 @@ static int nft_bitwise_offload(struct nft_offload_ctx *ctx,
>  {
>  	const struct nft_bitwise *priv = nft_expr_priv(expr);
>  
> -	if (memcmp(&priv->xor, &zero, sizeof(priv->xor) ||
> -	    priv->sreg != priv->dreg))
> +	if (memcmp(&priv->xor, &zero, sizeof(priv->xor)) ||
> +	    priv->sreg != priv->dreg)

This code should use memchr_inv and not compare against a
static uninitialized struct.

Perhaps linux should introduce and use memcchr like bsd. 
or just add something like #define memcchr memchr_inv