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

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Message-ID: <CA+FuTSdmRQem9d8amUdYg=pdZCDHiMVHjCCmKh-3dxoKk3th9g@mail.gmail.com>
Date:   Fri, 29 May 2020 23:14:56 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tun: correct header offsets in napi frags mode

On Fri, May 29, 2020 at 8:27 PM David Miller <davem@...emloft.net> wrote:
>
> 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?

Of course. How is this

"
    Ensure the link layer header lies in linear as eth_type_trans pulls
    ETH_HLEN. Then take the same code paths for frags as for not frags.
    Push the link layer header back just before calling napi_gro_frags.

    By pulling up to ETH_HLEN from frag0 into linear, this disables the
    frag0 optimization in the special case when IFF_NAPI_FRAGS is
    called with zero length iov[0] (and thus empty skb->linear).
"

Seemed good to add the extra clarification. I don't see a reasonable way
to avoid that consequence, especially as I cannot restore the first skb frag
(iov[1]) if it was exactly ETH_HLEN bytes and thus freed by __skb_pull_tail.