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

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Date:	Mon, 10 Oct 2011 10:02:03 +0200
From:	danborkmann@...earbox.net
To:	Eric Dumazet <eric.dumazet@...il.com>
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

Hi Eric,

Quoting Eric Dumazet <eric.dumazet@...il.com>:
> 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, you're absolutely right! Here's the trivial patch:

af_packet: removed unnecessary BUG_ON assertion in tpacket_destruct_skb

If skb is NULL, then stack trace is thrown on anyway on dereference.  
Therefore,
the stack trace triggered by BUG_ON is duplicate.

Signed-off-by: Daniel Borkmann <danborkmann@...glemail.com>

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index fabb4fa..886ae50 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1170,8 +1170,6 @@ static void tpacket_destruct_skb(struct sk_buff *skb)
  	struct packet_sock *po = pkt_sk(skb->sk);
  	void *ph;

-	BUG_ON(skb == NULL);
-
  	if (likely(po->tx_ring.pg_vec)) {
  		ph = skb_shinfo(skb)->destructor_arg;
  		BUG_ON(__packet_get_status(po, ph) != TP_STATUS_SENDING);


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