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Date:   Mon, 31 Jul 2023 20:09:59 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     Yue Haibing <yuehaibing@huawei.com>
Cc:     <davem@davemloft.net>, <dsahern@kernel.org>, <edumazet@google.com>,
        <pabeni@redhat.com>, <yoshfuji@linux-ipv6.org>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <simon.horman@corigine.com>
Subject: Re: [PATCH v2] ip6mr: Fix skb_under_panic in ip6mr_cache_report()
Message-ID: <20230731200959.2019cb9c@kernel.org>
In-Reply-To: <20230728121703.29572-1-yuehaibing@huawei.com>
References: <20230728121703.29572-1-yuehaibing@huawei.com>
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On Fri, 28 Jul 2023 20:17:03 +0800 Yue Haibing wrote:
>  #ifdef CONFIG_IPV6_PIMSM_V2
> +	int nhoff = skb_network_offset(pkt);
>  	if (assert == MRT6MSG_WHOLEPKT || assert == MRT6MSG_WRMIFWHOLE)
> -		skb = skb_realloc_headroom(pkt, -skb_network_offset(pkt)
> -						+sizeof(*msg));
> +		skb = skb_realloc_headroom(pkt, -nhoff + sizeof(*msg));

These changes look unnecessary. You can leave this code be (as ugly as
it is)...

>  	else
>  #endif
>  		skb = alloc_skb(sizeof(struct ipv6hdr) + sizeof(*msg), GFP_ATOMIC);
> @@ -1073,7 +1073,7 @@ static int ip6mr_cache_report(const struct mr_table *mrt, struct sk_buff *pkt,
>  		   And all this only to mangle msg->im6_msgtype and
>  		   to set msg->im6_mbz to "mbz" :-)
>  		 */
> -		skb_push(skb, -skb_network_offset(pkt));
> +		__skb_pull(skb, nhoff);

.. and just replace the push here with:

  __skb_pull(skb, skb_network_offset(pkt));
-- 
pw-bot: cr