netdev - Re: [PATCH] af_packet: tpacket_destruct_skb, deref skb after BUG

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Message-ID: <1318193866.21116.3.camel@edumazet-laptop>
Date:	Sun, 09 Oct 2011 22:57:46 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	danborkmann@...earbox.net
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] af_packet: tpacket_destruct_skb, deref skb after
 BUG_ON assertion

Le dimanche 09 octobre 2011 à 17:19 +0200, danborkmann@...earbox.net a
écrit :
> This tiny patch derefs the skb only after BUG_ON(skb==NULL) was evaluated
> and not before. Patched against latest Linus tree.
> 
> Thanks,
> Daniel
> 
> Signed-off-by: Daniel Borkmann <danborkmann@...earbox.net>
> 
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index fabb4fa..d9d833b 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -1167,11 +1167,12 @@ ring_is_full:
> 
>   static void tpacket_destruct_skb(struct sk_buff *skb)
>   {
> -	struct packet_sock *po = pkt_sk(skb->sk);
> +	struct packet_sock *po;
>   	void *ph;
> 
>   	BUG_ON(skb == NULL);
> 
> +	po = pkt_sk(skb->sk);
>   	if (likely(po->tx_ring.pg_vec)) {
>   		ph = skb_shinfo(skb)->destructor_arg;
>   		BUG_ON(__packet_get_status(po, ph) != TP_STATUS_SENDING);
> 
> 

Well, to be honest, this BUG_ON(!skb) is absolutely useless for two
reasons.

1) If skb happens to be NULL, the NULL dereference is trapped and stack
trace dumped as well.

2) Of course, tpacket_destruct_skb() being an skb destructor, skb cannot
be NULL at this point by design.

Please remove the BUG_ON() instead of trying to move it ;)

Thanks

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