netdev - Re: [net-next PATCH] net: ipv4: fix listify ip_rcv

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Date:   Wed, 11 Jul 2018 16:41:35 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        <netdev@...r.kernel.org>
Subject: Re: [net-next PATCH] net: ipv4: fix listify ip_rcv_finish in case of
 forwarding

On 11/07/18 16:01, Jesper Dangaard Brouer wrote:
> In commit 5fa12739a53d ("net: ipv4: listify ip_rcv_finish") calling
> dst_input(skb) was split-out.  The ip_sublist_rcv_finish() just calls
> dst_input(skb) in a loop.
>
> The problem is that ip_sublist_rcv_finish() forgot to remove the SKB
> from the list before invoking dst_input().  Further more we need to
> clear skb->next as other parts of the network stack use another kind
> of SKB lists for xmit_more (see dev_hard_start_xmit).
>
> A crash occurs if e.g. dst_input() invoke ip_forward(), which calls
> dst_output()/ip_output() that eventually calls __dev_queue_xmit() +
> sch_direct_xmit(), and a crash occurs in validate_xmit_skb_list().
>
> This patch only fixes the crash, but there is a huge potential for
> a performance boost if we can pass an SKB-list through to ip_forward.
>
> Fixes: 5fa12739a53d ("net: ipv4: listify ip_rcv_finish")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
Acked-by: Edward Cree <ecree@...arflare.com>

But it feels weird and asymmetric to only NULL skb->next (not ->prev), and
 to have to do this by hand rather than e.g. being able to use
 list_del_init(&skb->list).  Hopefully this can be revisited once
 sch_direct_xmit() has been changed to use the list_head rather than SKB
 special lists.