netdev - Re: [PATCH net] tun: correct header offsets in napi frags mode

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Message-Id: <20200529.172719.1001521060083156258.davem@davemloft.net>
Date:   Fri, 29 May 2020 17:27:19 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     willemdebruijn.kernel@...il.com
Cc:     netdev@...r.kernel.org, willemb@...gle.com
Subject: Re: [PATCH net] tun: correct header offsets in napi frags mode

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Thu, 28 May 2020 13:05:32 -0400

> Temporarily pull ETH_HLEN to make control flow the same for frags and
> not frags. Then push the header just before calling napi_gro_frags.
 ...
>  	case IFF_TAP:
> -		if (!frags)
> -			skb->protocol = eth_type_trans(skb, tun->dev);
> +		if (frags && !pskb_may_pull(skb, ETH_HLEN)) {
> +			err = -ENOMEM;
> +			goto drop;
> +		}
> +		skb->protocol = eth_type_trans(skb, tun->dev);
 ...
>  		/* Exercise flow dissector code path. */
> -		u32 headlen = eth_get_headlen(tun->dev, skb->data,
> -					      skb_headlen(skb));
> +		skb_push(skb, ETH_HLEN);
> +		headlen = eth_get_headlen(tun->dev, skb->data,
> +					  skb_headlen(skb));

I hate to be a stickler on wording in the commit message, but the
change is not really "pulling" the ethernet header from the SKB.

Instead it is invoking pskb_may_pull() which just makes sure the
header is there in the linear SKB data area.

Can you please refine this description and resubmit?

Thank you.