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Date:   Thu, 13 Sep 2018 10:58:40 -0700 (PDT)
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To:     yanhaishuang@cmss.chinamobile.com
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        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2,net-next 1/2] ip_gre: fix parsing gre header in
 ipgre_err
From:   David Miller <davem@davemloft.net>
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From: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Date: Wed, 12 Sep 2018 17:21:21 +0800

> @@ -86,7 +86,7 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
>  
>  	options = (__be32 *)(greh + 1);
>  	if (greh->flags & GRE_CSUM) {
> -		if (skb_checksum_simple_validate(skb)) {
> +		if (csum_err && skb_checksum_simple_validate(skb)) {
>  			*csum_err = true;
>  			return -EINVAL;
>  		}

You want to ignore csum errors, but you do not want to elide the side
effects of the skb_checksum_simple_validate() call which are to set
skb->csum_valid and skb->csum.

Therefore, the skb_checksum_simple_validate() call still needs to be
performed. We just wont return -EINVAL in the NULL csum_err case.