[PATCH] af_packet: tpacket_destruct_skb, deref skb after BUG_ON assertion

[PATCH] af_packet: tpacket_destruct_skb, deref skb after BUG_ON assertion

[danborkmann]

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@iogearbox.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);

Re: [PATCH] af_packet: tpacket_destruct_skb, deref skb after BUG_ON assertion

[Eric Dumazet]

Le dimanche 09 octobre 2011 à 17:19 +0200, danborkmann@iogearbox.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@iogearbox.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

Re: [PATCH] af_packet: tpacket_destruct_skb, deref skb after BUG_ON assertion

[danborkmann]

Hi Eric,

Quoting Eric Dumazet <eric.dumazet@gmail.com>:
> Le dimanche 09 octobre 2011 à 17:19 +0200, danborkmann@iogearbox.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@iogearbox.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@googlemail.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);

Re: [PATCH] af_packet: tpacket_destruct_skb, deref skb after BUG_ON assertion

[Eric Dumazet]

Le lundi 10 octobre 2011 à 10:02 +0200, danborkmann@iogearbox.net a
écrit :

> Thanks, you're absolutely right! Here's the trivial patch:
> 
> af_packet: removed unnecessary BUG_ON assertion in tpacket_destruct_skb
> 

OK but a proper title should be 

[PATCH] af_packet: remove unnecessary BUG_ON() 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@googlemail.com>
> 

missing "---" separator

> 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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