netdev - Re: [PATCH net] udp: on peeking bad csum, drop packets even if not at head

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Date:   Mon, 21 Aug 2017 18:37:29 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Network Development <netdev@...r.kernel.org>
Cc:     David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net] udp: on peeking bad csum, drop packets even if not at head

On Mon, Aug 21, 2017 at 5:39 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> From: Willem de Bruijn <willemb@...gle.com>
>
> When peeking, if a bad csum is discovered, the skb is unlinked from
> the queue with __sk_queue_drop_skb and the peek operation restarted.
>
> __sk_queue_drop_skb only drops packets that match the queue head. With
> sk_peek_off, the skb need not be at head, causing the call to fail and
> the same skb to be found again on restart.
>
> Walk the queue to find the correct skb. Limit the walk to sk_peek_off,
> to bound cycle cost to at most twice the original skb_queue_walk in
> __skb_try_recv_from_queue.
>
> The operation may race with updates to sk_peek_off. As the operation
> is retried, it will eventually succeed.
>
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Eric just suggested an alternative that does not require looping, which
is much nicer.