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Subject: Re: [PATCH v2,net-next 1/2] ip_gre: fix parsing gre header in
 ipgre_err
From:   Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
In-Reply-To: <20180913.105840.140151724801067072.davem@davemloft.net>
Date:   Fri, 14 Sep 2018 10:09:51 +0800
Cc:     kuznet@ms2.inr.ac.ru, jbenc@redhat.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
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 <20180913.105840.140151724801067072.davem@davemloft.net>
To:     David Miller <davem@davemloft.net>
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> On 2018??9??14??, at ????1:58, David Miller <davem@davemloft.net> wrote:
> 
> 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.
> 
> 

How about doing like this:

--- a/net/ipv4/gre_demux.c
+++ b/net/ipv4/gre_demux.c
@@ -86,13 +86,14 @@ 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 (!skb_checksum_simple_validate(skb)) {
+                       skb_checksum_try_convert(skb, IPPROTO_GRE, 0,
+                                                null_compute_pseudo);
+               } else if (csum_err) {
                        *csum_err = true;
                        return -EINVAL;
                }

-               skb_checksum_try_convert(skb, IPPROTO_GRE, 0,
-                                        null_compute_pseudo);

Thanks for reviewing.