netdev - Re: [PATCH net] net: xdp: pull ethernet header off packet after computing skb->protocol

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Date:   Mon, 17 Aug 2020 11:48:58 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     Jason@...c4.com
Cc:     brouer@...hat.com, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH net] net: xdp: pull ethernet header off packet after
 computing skb->protocol

From: "Jason A. Donenfeld" <Jason@...c4.com>
Date: Mon, 17 Aug 2020 09:48:10 +0200

> On 8/17/20, Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>> On Sun, 16 Aug 2020 15:29:37 -0700 (PDT)
>> David Miller <davem@...emloft.net> wrote:
>>
>>> From: "Jason A. Donenfeld" <Jason@...c4.com>
>>> Date: Sat, 15 Aug 2020 09:29:30 +0200
>>>
>>> > When an XDP program changes the ethernet header protocol field,
>>> > eth_type_trans is used to recalculate skb->protocol. In order for
>>> > eth_type_trans to work correctly, the ethernet header must actually be
>>> > part of the skb data segment, so the code first pushes that onto the
>>> > head of the skb. However, it subsequently forgets to pull it back off,
>>> > making the behavior of the passed-on packet inconsistent between the
>>> > protocol modifying case and the static protocol case. This patch fixes
>>> > the issue by simply pulling the ethernet header back off of the skb
>>> > head.
>>> >
>>> > Fixes: 297249569932 ("net: fix generic XDP to handle if eth header was
>>> > mangled")
>>> > Cc: Jesper Dangaard Brouer <brouer@...hat.com>
>>> > Cc: David S. Miller <davem@...emloft.net>
>>> > Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
>>>
>>> Applied and queued up for -stable, thanks.
>>>
>>> Jesper, I wonder how your original patch was tested because it pushes a
>>> packet
>>> with skb->data pointing at the ethernet header into the stack.  That
>>> should be
>>> popped at this point as per this fix here.
>>
>> I think this patch is wrong, because eth_type_trans() also does a
>> skb_pull_inline(skb, ETH_HLEN).
> 
> Huh, wow. That's one unusual and confusing function. But indeed it
> seems like I'm the one who needs to reevaluate testing methodology
> here. I'm very sorry for the noise and hassle.

I've reverted this change from my tree.