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Date:   Sun, 16 Aug 2020 15:29:37 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     Jason@...c4.com
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, brouer@...hat.com
Subject: Re: [PATCH net] net: xdp: pull ethernet header off packet after
 computing skb->protocol

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.

Thanks.

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