netdev - Re: [PATCH] packet: uses kfree

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Date:	Wed, 6 Apr 2016 13:27:11 -0400
From:	Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:	Weongyo Jeong <weongyo.linux@...il.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH] packet: uses kfree_skb() for drop.

On Wed, Apr 6, 2016 at 12:54 PM, Weongyo Jeong <weongyo.linux@...il.com> wrote:
> consume_skb() isn't for drop or error cases.  kfree_skb() is more proper
> one.
> Signed-off-by: Weongyo Jeong <weongyo.linux@...il.com>
> ---
>  net/packet/af_packet.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 1ecfa71..a75d5bf 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -2141,7 +2141,7 @@ drop_n_restore:
>                 skb->len = skb_len;
>         }
>  drop:
> -       consume_skb(skb);
> +       kfree_skb(skb);

This does show an inconsistency between packet_rcv and tpacket_rcv,
which calls kfree_skb.

A comment at consume_skb mentions that kfree_skb is intended for drops
that signal a failure condition, and indeed, that makes it a useful
way to track errors (e.g., with perf record -a -g -e skb:kfree_skb).

This drop path is not always an error path, though. These network taps
will legitimately drop references to any packets not destined to them.
To be precise, only the drop_n_acct label cases are delivery errors
(drops after the filter accepted the packet). Changing unconditionally
to kfree_skb does pollute that useful counter with false positives. A
pedantic solution is to change both functions to only call kfree_skb
on drop_n_acct and consume_skb otherwise.

This shorthand change does at least makes packet_rcv and tpacket_rcv more alike.