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Date:   Mon, 9 Nov 2020 15:29:13 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Alexander Lobakin <alobakin@pm.me>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Martin Varghese <martin.varghese@nokia.com>,
        Pravin B Shelar <pshelar@ovn.org>,
        Willem de Bruijn <willemb@google.com>,
        Guillaume Nault <gnault@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Florian Westphal <fw@strlen.de>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Yadu Kishore <kyk.segfault@gmail.com>,
        Vladimir Oltean <vladimir.oltean@nxp.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH net-next] net: skb_vlan_untag(): don't reset transport
 offset if set by GRO layer
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On Thu, 05 Nov 2020 21:29:01 +0000 Alexander Lobakin wrote:
> Similar to commit fda55eca5a33f
> ("net: introduce skb_transport_header_was_set()"), avoid resetting
> transport offsets that were already set by GRO layer. This not only
> mirrors the behavior of __netif_receive_skb_core(), but also makes
> sense when it comes to UDP GSO fraglists forwarding: transport offset
> of such skbs is set only once by GRO receive callback and remains
> untouched and correct up to the xmitting driver in 1:1 case, but
> becomes junk after untagging in ingress VLAN case and breaks UDP
> GSO offload. This does not happen after this change, and all types
> of forwarding of UDP GSO fraglists work as expected.
> 
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> ---
>  net/core/skbuff.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index c5e6c0b83a92..39c13b9cf79d 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -5441,9 +5441,11 @@ struct sk_buff *skb_vlan_untag(struct sk_buff *skb)
>  		goto err_free;
>  
>  	skb_reset_network_header(skb);
> -	skb_reset_transport_header(skb);
>  	skb_reset_mac_len(skb);
>  
> +	if (!skb_transport_header_was_set(skb))
> +		skb_reset_transport_header(skb);
> +

Patch looks fine, thanks, but I don't understand why you decided to 
move the reset?  It's not like it's not in order of headers, either.
Let's keep the series of resets identical to __netif_receive_skb_core(),
shall we?

>  	return skb;
>  
>  err_free: